Measuring Safety in Single- and Twin-engine Helicopters · 2 FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • AUGUST 1991 egories and recommends certain operational limitations

F L I G HT SAFE TY FOUN D A TI O N • F L I G H T S A F E T Y D I G E S T • AUGUST 1991 1

Safety has always been a paramount concernin aviation. Safety is not an absolute; rather itis a relative measure of the risk involved whenflying in an aircraft. Several methods are usedto measure safety, but some can be misleadingand create a perception of a low level of safetyin helicopters. Misconceptions about helicop-ter safety can cause overly restrictive regula-tions and prohibit the use of safe aircraft. Thus,realistic measurement of helicopter safety iscrucial to helicopter operators and the flyingpublic.

There is a question of whether an occupant issafer in a single-engine or a twin-engine heli-copter. Some say that two engines have to bebetter than one, arguing that since there are somany twin-engine aircraft used in commercialfixed-wing operations, therefore, helicoptersalso need two engines. However, the facts donot support applying fixed-wing thinking tohelicopters. Helicopters are unique and theyare operated in difficult environments; there-fore, they should be considered differently fromfixed-wing airplanes. One must consider all

Measuring Safety in Single- andTwin-engine Helicopters

Accurate measurement of helicopter safety is crucial to boththe flying public and the operators of rotary-wing aircraft.

The author questions the veracity of some safetystatistics and he challenges recent ICAO amendments

that would restrict operations in single-engine helicopters infavor of twin-engine helicopters.

byRoy G. Fox

Chief Safety EngineerBell Helicopter Textron Inc.

causes of accidents and injuries, not just me-chanical components such as engines or tailrotor blades.

Accident data from the United States (U.S.),the United Kingdom (U.K.), and Canada wereanalyzed to determine the risk to occupants ofsingle- and twin-engine helicopters. These threenations account for about 82 percent of allknown non-Soviet bloc civil helicopters.

Why Measure Safety?

Many important equipment decisions madeby businesses, government agencies and indi-viduals are based on the perceived safety ofan aircraft. Decisions to buy, use, repair, im-prove, insure, and sell or replace an aircraftare related to perceived safety. Likewise, gov-ernment operational prohibitions are based ona perceived deterioration of safety. For example,recent Amendment 1 to ICAO (InternationalCivil Aviation Organization) Annex 6, Part III1

establishes three helicopter performance cat-

F L I G H T SAFE TY FOUN D A TI O N • F L I G H T S A F E T Y D I G E S T • AUGUST 19912

egories and recommends certain operationallimitations. The categories are:

• Performance Class 1 includes multi-en-gine helicopters that are capable of con-tinuing normal operations with one en-gine inoperative regardless of when theengine fails.

• Performance Class 2 includes multi-en-gine helicopters that are capable of con-tinuing flight after one engine fails ex-cept that a forced landing would berequired following an engine failurebetween takeoff and a specified pointand from a specified point to landing.

• Performance Class 3 refers to single-engine helicopter operations; a forcedlanding is required after engine fail-ure.

The ICAO amendment would encourage theprohibition in member countries (states) againstthe use of Performance Class 3 (single-engine)helicopters for IFR (instrument flight rules)flights, night flying, flights out of sight of land,flights with cloud ceilings of lower than 600feet or visibility less than 1,500 meters, andflights to elevated structures (heliports). [ICAOdoes not regulate; it recommends that individualstates adopt its criteria into their own regulations.The United States and many other countries havenot adopted the recommendations of ICAO Amend-ment 1 to Annex 6 Part III. — Ed.]

Because single-engine helicopters account forthree out of four helicopters in the world, thisaction will have a drastic effect upon the heli-copter community and upon the public ben-efit derived from helicopter use. Some single-engine helicopter operations will no longer beperformed because of the higher costs involvedif twin-engine helicopters are mandated. Mostmulti-engine helicopter operations are conductedin Performance Class 2. Because the accidentdata do not discriminate between performanceclasses, the safety comparisons of PerformanceClasses 2 and 3 from the available data areaccomplished in this discussion by looking atthe differences between single-engine (Perfor-mance Class 3) and multi-engine (Performance

Class 2 ) operations.

The performance class restrictions on helicop-ter operations in accordance with the ICAOAmendment 1 change includes the recommendedprohibition of single-engine helicopter opera-tions involving transport of passengers, cargoor mail for remuneration or hire. This prohibi-tion is based upon a perceived belief that twin-engine helicopters are always safer than single-engine helicopters in all environments. Accu-rate helicopter safety measurements are criti-cal for perceived safety and actual safety to beaccurately differentiated. Such accuracy alsoallows prioritized correction of safety prob-lems and the evaluation of desirable and un-desirable aspects of different aircraft configu-rations. Of personal importance to an indi-vidual, this can allow a person to determinehis risk of flying in a specific type aircraft.

Why Worry About Safety?

Why do people worry about safety in the firstplace? The primary reason is that no one wantsto suffer injury or death. Because we do notwant to think about our own injury or death,many of us tell ourselves, “I am not ever go-ing to be in an accident, therefore I won’t haveto worry about being injured or killed.” Avia-tion accident prevention is based on this con-cept: “If I can prevent the emergency, I won’thave to worry about my pain and my death.”This human coping mechanism works well forthe average individual; but management (avia-tion and regulatory) must first determine theactual risk and subsequently manage the risk.Safety is the management of risk.

Helicopters Respond DifferentlyTo A Power Loss Than Do Air-

planes

If an engine power loss occurs, the resultingemergency landings are significantly differentfor airplanes as compared to helicopters. Tomaintain control of an airplane, its airspeedmust stay above the stalling speed of the winguntil ground contact. This means that the


airplane’s airspeed at ground contact will betypically 60 to 100 knots. This high speed re-quires a shallow approach angle and a long,cleared landing site. Any obstructions (trees,buildings, fences or ground irregularities) canbe impacted by an aircraft with significant crashforces and cause injuries.

Conversely, helicopters require little more roomthan the size of the aircraft for an unpowered,emergency landing. This is because the heli-copter can descend under control after enginefailure in a condition known as autorotation,whereby the pilot decreases the pitch of themain rotor blades to allow them to be rotatedby the air flowing upwards through the rotorarc, or disc, similar to the action of wind on awindmill. The spinning main rotor acts some-what like a parachute and a near-constant de-scent rate is maintained. The pilot retains fullcontrol and is able to select the most appropri-ate landing site. A few feet above the ground,the pilot flares the aircraft and increases thepitch of the rotor blades, which increases lift.This allows the descent to be slowed just be-fore ground contact to allow a gentle touch-down at little or no forward speed when ac-complished properly (Figure 1).

Helicopters Have DifferentMissions And Uses

Using U.S. National Transportation Safety Board(NTSB) accident data for 1982 through 1985for U.S.-registered helicopters, the mission underway at the time of theaccident was determined2,and is shown in Table 1.This shows that single-piston, single-turbine andtwin-turbine helicoptersare used in the same mis-sions but in varying de-grees. Single-engine, pis-ton-powered helicoptershave a concentration inrelatively high-risk areasof flight training, personaland agricultural workwhere low cost is a driv-ing factor. These uses are

Graphic not available

major contributors to the safety record for single-piston helicopters. If twin-turbine helicoptersperformed similar missions and were oper-ated as the single-engine, piston-powered he-licopters, the twin-turbine helicopter accidentrate could rise significantly.

Helicopter Fleet Is Mixed

The U.S. Airmen and Aircraft Registry of Au-gust 1990 shows the distribution of helicop-ters (Table 2). There were 34 military surplustwin-piston helicopters on the registry that werenot included. However, the number of aircrafton the registry can be misleading because itincludes many aircraft that are wrecked, be-ing salvaged for parts, under repair, stored orused as static (nonflying) aircraft. Flight hoursare a better indicator of actual aircraft usage.

Figure 1. Helicopter autorotation

Table 1. Helicopter Missions at Time of Accident (NTSB Data 1982-1985 percent of accidents)

Single- Single- Twin- AllType of Operations piston turbine turbine Helicopters

Personal 26.2 24.4 16.0 24.9Business 9.4 23.6 32.0 14.9Instruction 21.3 2.0 8.0 14.4Executive/Corporate 0 5.6 16.0 2.4Agricultural 29.8 8.8 4.0 21.9Observation/Survey 5.1 5.2 0 5.0Public-use 1.1 4.0 8.0 2.3Ferry 1.9 4.4 16.0 3.2Positioning 0.4 0.4 0 0.4Other Work 4.8 21.6 0 10.6


Flight hours by model series were extractedfrom the U.S. Federal Aviation Administra-tion (FAA) General Aviation Activities and Avi-onics Survey annual reports for 1984 to 1988.If the FAA estimated flight hours for a modelfor two or more years of the five-year period,those flight hours were used. The accidents ofthat model series were used if flight hoursoccurred in the year of the accident. If no hoursor one year of flight hours were estimated bythe FAA reports, the accidents and flights hoursfor those affected models were deleted fromthe study. The data is considered by the au-thor to be the best available and is thereforeused in this discussion.

The usable models with their flight hours werethen arranged in groups: single-piston, single-turbine, twin-turbine helicopters and the mostcommon helicopter, the Bell Model 206. TheModel 206 flew 45 percent of all helicopterflight hours during the 1984 through 1988 time

period. The single-turbine en-gine Model 206 is also includedin the generic single-engine data.

The Canadian, U.K. and U.S.helicopter fleet flight-hoursshown in Table 3 indicate thatthese helicopter fleets are alsovaried. The Canadian accidentand flight hour data from theTransportation Safety Board ofCanada and Canadian AviationStatistics Centre were for theperiod 1982 through 1987. Theaccident data and flight hours

from the U.K. Civil Aviation Authority werefor the period 1980 through 1987. The mixtureof flying in the U.K. fleet is significantly dif-ferent than in Canada and the United States.This helps to explain why attitudes about he-licopter operations vary among ICAO states.The most common helicopter flying in the U.K.during the period was the Sikorsky S-61 twinturbine which accounted for 28.2 percent ofthe U.K. flight hours, whereas that for the Model206 was 12.3 percent.

Disregarding homebuilt and experimental he-licopters, it is estimated that of approximately15,200 rotorcraft in the world (excluding theSoviet bloc states), that 12,511, or 82 percent,are in the United States, the United Kingdomand Canada. The helicopter data are presentedby configuration groups of single-piston (SP),single-turbine (ST) and twin-turbine (TT).

Table 2. U.S.-Registered Helicopters by Engine Type (U.S.FAA Data)

Number ofHelicopters Flight Hours

(11-31-90 Flown Flight HoursType of Engine Registry) 1984-1988 (%)

Single-piston 5,5371 2,961,252 25.9Single-turbine 3,642 7,035,846 61.5Twin-turbine 1,108 1,442,116 12.6Total helicopters 10,121 11,439,214 100Aircraft with mostflight hours: Bell Model 206single-turbine only 2,092 5,215,001 45.6

Table 3. U.S., U.K., and Canada Civil Helicopters by Engine Type(flight-hours flown)

U.S./U.K./U.S. U.K. Canada Canada

Engine Type (1984-88) (1980-87) (1982-87) Combined Percent

Single-piston 2,961,252 91,737 190,894 3,243,883 21.3Single-turbine 7,035,846 239,548 2,078,376 9,353,770 61.5Twin-turbine 1,442,116 932,474 242,696 2,617,286 17.2Total helicopters 11,439,214 1,263,759 2,511,966 15,214,939 100.0Bell Model 206 single turbine only 5,215,001 155,648 1,471,675 8,703,602 45.0


The Next Step Is Measuring Safety

Now that we have some indication of the heli-copter activities, the next step is to measuresafety, or determine relative risk. There arevarious methods in use. Using the total num-ber of accidents that have occurred for a par-ticular model may be misleading because itdoes not account for fleet size and subsequentexposure through the years. Accidents peramount of exposure is a more appropriate methodto determine relative risk.

Accidents Per Fleet Ratio Is aSlight Improvement

One means of addressing the effects of fleetsize is to determine the ratio of accidents tothe size of the fleet in existence at the time ofthe comparison. For example, this ratio is de-termined by counting the number of accidentsthat have occurred involving a specific modelsince its introduction in the United States. Thistotal accident history number is then dividedby the latest “estimated” number of active he-licopters of that model in the United States.The ratio technique is inaccurate and mislead-ing; it disregards the changing fleet size overthe years by using only the latest year ’s activefleet; looks at models in different periods oftheir service life; and disregards the differentamount of flying done by various models. Also,the number of accidents will increase as a modelfleet is utilized. In Figure 2, the Bell Model 47,which is the oldest civil helicopter model, sug-gests what may happen to the other models asthey mature in the future. The number of acci-dents from 1958 through 1963 was estimatedfrom accident trends before and after that pe-riod. Since the number of “active” helicoptersis seldom known, the actual numbers of civilaircraft delivered with a U.S. registry numberwere used. (The last Model 47 was deliveredin 1973 in the United States.)

The total number of accidents grows each yearand far exceeds the number of aircraft deliv-ered. Obviously, the ratio of total accidents tothe number of aircraft in an existing fleet isgoing to be different depending upon whenthat ratio is calculated. If the ratio is deter-

mined within two years of model introduc-tion, it will probably be low. Five, 10 or 15years later, the ratio will continue to increaseregardless of the true safety of the model. Alsoshown in Figure 2 is the annual accident rateper 100,000 flight hours. Note that the acci-dent-to-fleet-number ratio continues to climbto about 160 percent as of 1985 even thoughthe accident rate is decreasing during the lastthree years. This disparity will be present forall other models and is dependent on when inthe model’s life cycle the ratio is computed.

Accidents Per Departure AreMore Meaningful

After comparing vastly different types of air-craft, it became apparent that some aircrafttypes were spending the majority of their flighttime in the more hazardous flight phases oftakeoffs and landings. Therefore, the accidentrate per departure (or mission) was decidedupon as a means to measure safety. This ap-proach answers the question “Is the likelihood

of this mission failing greater or lesser for trans-portation method A versus method B?” Thisapproach is not concerned with how long A orB takes to accomplish the mission. For example,if the mission is to transport a passenger frompoint X to point Y, any one of the followingmethods of travel can qualify for the task: jetaircraft, helicopter, train, automobile, boat andwalking. The number of accidents that oc-curred from the time of departure for eachflight from any point X to the arrival at thecorresponding point Y would then be deter-


Figure 2. Bell Model 47 accident/fleet ratio


mined for each means of travel. This becomesthe accident rate per departure for that meansof transportation.

Helicopters can perform some missions thatno other means of transportation can achieve.These unique missions include those that in-volve hovering or very slow flight, and allowvery short flight times that result in a largenumber of takeoffs and landings. Because largeairplanes spend the vast majority of their flighttime in cruise flight rather than in takeoffsand landings, their exposure to flight hazardsis not directly comparable. A study was donein 1981 that included a look at U.S. FederalAviation Regulation (FAR) Part 135 unsched-uled air-taxi helicopter safety related to fixed-wing air carriers3; the basic purpose was forrelating duty time to number of daily land-ings, but the data is applicable to this discus-sion as well. The surveyed helicopter opera-tors flew 603 single- and twin-turbine heli-copters during the subject period (1977 through1979). The percentage of singles versus twinsis not available; however, the percentage ofsingle turbines vs. twin turbines is availablefor 1983, which is the closest period for whichthat type of data is available. The 1983 U.S.registry indicates a mix of 83 percent singleturbines and 17 percent twin turbines. Themix in the helicopter survey group was esti-mated to be similar.

Air Travel Methods Are Compared

The accident rate per flight hour for the com-bined turbine helicopter fleet compared to theair carriers is shown in Figure 3A. This illus-trates that the helicopter accident rate per flighthour was slightly better than that of commuter(now regional) air carriers. To account for timespent in the more hazardous phases of flight(takeoff and approach/landing), the accidentrate in Figure 3B is based on number of depar-tures (takeoffs). The resulting helicopter acci-dent rate per 100,000 departures was 71 per-cent lower than FAR Part 135 commuter aircarriers, and was much closer to the FAR Part121 certificated air carriers.

Figure 3C for the fatal accident rate per depar-

ture shows that the helicopter rate was 69 per-cent lower than that for the commuter air car-riers4. Figure 3D shows comparable data forfatalities per departure. In this latter case, thehelicopter rate is 71 percent lower than com-muter air carriers and 79 percent lower thancertificated air carriers. These results indicatethat the helicopter industry in general is saferthan many persons believed, considering theamount of time rotary-wing aircraft spend incritical phases of flight.

The offshore oil industry in the Gulf of Mexicopresents an example of the safe operation ofturbine helicopters as shown in Table 4. In1990, there were 1,855,345 takeoffs and land-ings, about 1,500,000 of which occurred at off-shore platforms. There were 3,958,525 passen-gers moved by helicopter in the Gulf duringthe period. Of the 619 helicopters operatingin the Gulf of Mexico, 138 (22 percent) are IFRequipped, and single-turbine helicopters ac-count for 349 (56 percent) of the total helicop-ter fleet. This significant usage of single-tur-bine helicopters indicates that single turbinesare being operated safely from elevated plat-forms and over water.


Figure 3. Accident and fatality rate comparison(1977-1979)


The use of departure exposure is accurate fordetermining the risk to mission accomplish-ment, but it is not accurate for determiningsafety. Safety is related to “freedom from harm,injury or loss” and should be counted in termsof time of individual occupant exposure.

Counting Accidents PerPatient Transport

This recent safety measurement variation isused by the emergency medical services (EMS)community. It is the number of EMS aircraftaccidents that occur divided by the number ofpatients transported during the same time pe-riod. This approach uses the EMS primary func-tion of moving patients as the basis for com-parison with the safety of other modes of movingpatients. This approach is appropriate onlyfor comparing completions of medical trans-port missions, not for comparing safety of thecrew and patient; it is used to compare with

Table 4. Gulf of Mexico Helicopter Safety Data

AccidentsAccidents per

per 100,000Fleet No. of Flight 100,000 Flight-

Year Size Accidents Hours Departures Departures hours

1987 708 17 691,655 2,101,850 0.80 2.46

1988 599 10 455,330 1,384,000 0.65 2.20

1989 608 9 515,770 1,885,571 0.48 1.74

1990 619 9 533,761 1,855,345 0.49 1.69

non-patient-carrying aircraft. Figure 45 showsthe annual EMS helicopter accident rates per100,000 patients transported. Since many ofthe EMS helicopter accidents occurred with-out a patient aboard (e.g., en route to pickup,returning after transport or repositioning), thisis a mission-oriented measurement (similar toa per-departure measurement method), ratherthan per human exposure method.

Accidents Per Passenger Mile IsAnother Measurement Method

Accidents per passenger mile is another per-mission measurement with an adjustment forthe distance traveled. Fixed-wing scheduledair carriers and fixed- and rotary-wing air taxioperators record passenger-carried informa-tion from revenue flights; most general avia-tion and helicopters do not. Thus, compari-sons are seldom made in this area. Limitationsof per-mission measurements are easily notedby comparing the safety of an 80-knot aircraftwith a 400-knot aircraft, both having the samenumber of passengers and accidents per pas-senger mile. Some analysts interpret this asthe same level of occupant safety. However,the slower machine is in the air five times aslong as the faster aircraft for the same dis-tance. Therefore, using another measurementmethod, the slower aircraft experiences onlyone fifth of the accident rate per flight hour ofthe faster aircraft. This dichotomy results be-cause the primary concern is per mission andnot related to per human or occupant safety;accidents per passenger mile is only meaning-ful if the primary concern is mission comple-tion of moving a passenger a given distance,

Figure 4. EMS accidents per patient transport



not the safety of the occupants.

Accidents Per Flight Hours Is theMost Common Method

The most common method of determining safetyon the basis of accidents per flight hours isaccident rate per 100,000 flight hours. Thisaccident rate per hour is the number of acci-dents of a given model for a specific period oftime divided by the hours flown by those air-craft over that time period. Accident rate per100,000 flight hours is a good method to de-termine the aircraft damage cost expected in amodel fleet or the likelihood of aircraft dam-age. Table 5 presents the accident rates per

100,000 flight hours for U.S. general aviationfixed-wing and rotary-wing aircraft in descend-ing order.

Helicopter accident rates for the 1980s fromthe Unites States, the United Kingdom and

Canada are shown in Table 6. The time periodbreakdowns are similar to those in Table 3.

Airworthiness vs. OperationalIssues Judged

The causes of accidents resulting in serious(major/fatal) occupant injury were determined6

using NTSB data from 1982 through 1986 forsingle-turbine and twin-turbine civil helicop-ters as shown in Figure 5. Engine materialfailure (MF) initiated the crashes that caused14.8 percent of the serious injuries to occu-pants of single-turbine helicopters, as com-pared to only 3.4 percent for the serious inju-ries to occupants of twin-turbine helicopteraccidents. If only this one piece of informationis considered, the obvious conclusion is thattwo engines are better than one.

However, next consider only the cause factorof material failures other-than engine (non-engine MF). In this case, only 11.0 percent ofthe seriously injured occupants were involvedin single-turbine helicopter crashes initiatedby non-engine material failures as comparedwith 31.0 percent of those in twin-turbine he-licopter crashes. This is an indicator of thedetrimental effects of complexity and moreparts. If one were to consider only this lastpiece of information, the obvious approachshould be that one engine is better than two —a reversal of the conclusion in the previousparagraph.

Actually, the total material failures, engine andnon-engine, should be considered together, whichyields percentages of seriously injured occu-

pants due to all types of MF-causedaccidents of 25.8 percent for occu-pants in single turbines and 34.4percent for occupants in twins. Thisis consistent with more parts andcomplexity being present in twins.Because causes of deaths and in-juries cannot be limited only tothose that are engine-related, it isessential that all other factors beconsidered — both material fail-ure and nonmaterial failure (i.e.,human error).

Table 6. 1980s U.S., U.K., and Canadian Accident Rates(all causes) (accidents per 100,000 flight-hours)

UnitedCanada Kingdom United States

Type (1982-87) (1980-87) (1984-88)

Single-piston 33.53 73.79 17.83

Single-turbine (all) 9.86 17.12 5.49

Twin-turbine 4.67 4.83 4.37

Bell Model 206 single-turbine 8.70 14.07 4.28

Table 5. U.S.-Registered General AviationAccident Rates (NTSB/FAA Data 1984-1988)

Accident Rateper 100,000

Type of Aircraft Flight-hours

Single-piston helicopter 17.83

Single-piston airplane 8.55

Single-turbine helicopter (all) 5.49

Twin-piston airplane 5.12

Twin-turbine helicopter 4.37

Bell Model 206 single-turbine helicopter 4.28


Engine material failures represent just one as-pect of material failures that cause accidents(also called airworthiness failures), as shownin Table 7 and Figure 5. Non-engine materialfailures that cause accidents are also repre-sented. The single-piston accident rate per 100,000flight hours for non-engine material failureaccidents is the highest rate, followed by twinturbines, all single turbines, and with the low-est rate, the Model 206 single-turbine. Table 7shows the combined engine and non-enginematerial failures (all-airworthiness failures),and indicates that the accident rate for all-airworthiness failures in twin turbines is muchlower than for single pistons and slightly lowerthan for all single turbines. However, the twin-turbine all-airworthiness-failure accident rateis 51.4 percent higher than the single-turbineModel 206 rate. From an overall airworthinessstandpoint, these figures could be used to in-dicate that there is no justification to requiretwin-turbine engines on all helicopters for allmission applications.

Comparisons of the all-airworthiness-failure accident rates of three ICAO states(United States, United Kingdom andCanada) are presented in Table 8 andFigure 6, which show the variabilitythat is a function of the mix of aircraftmodels within a type and reflects vary-ing helicopter utilization in the differ-ent ICAO states. The rates of twin tur-bines and Model 206s appear to be quiteconsistent. The single-turbine Model 206has the lowest airworthiness accidentrate in two of the three states and thesecond lowest in the remaining one.

Fatal Accidents Per FlightHours Are Questioned

Since safety is a condition of freedom fromrisk of harm, injury or loss, measurement ofthose accidents involving fatal injuries is rel-evant to the relationship of safety to humansuffering. A fatal accident is an accident inwhich at least one person is fatally injured. Afatal accident rate is the number of fatal acci-dents per 100,000 flight hours. Figure 73 showsthe fatal accident rates for various families ofU.S. general aviation aircraft, plus three indi-vidual models for 1975 through 1979. Notethat most aircraft types have approximatelythe same fatal accident rate. This method ofmeasuring safety is still inaccurate because itdoes not account for the number of people onboard that had the chance of being fatally in-jured. For example, regardless of a helicopter’sseating capacity, there is five times the chanceof someone being killed with 10 people aboardas there is with two people aboard. This is due

Figure 5. Seriously injured occupants by accident cause(NTSB)


Table 7. U.S.-registered Helicopter Accident Rates (Source NTSB/FAA for 1984-1988) (accidents per 100,000 flight hours)

Engine Only Non-engine AllType of Aircraft Airworthiness Airworthiness All Causes

ALL HELICOPTERS 1.22 1.08 2.30 8.54

Single-piston 1.99 2.09 4.09 17.83

Twin-turbine 0.35 1.25 1.59 4.37

Single-turbine (all) 1.08 0.61 1.69 5.49

Bell Model 206 single-turbine 0.88 0.17 1.05 4.28


to the occurrence of 10 people impacting theground in one airframe versus two people inthe other airframe. Obviously, then, the num-ber of helicopter seats is not important; thenumber of people aboard is important. There-fore, fatal accident rates can be misleadingbecause they are related to aircraft airframeaccidents, not to the occupants.

Risk Around HeliportsAddressed by Neighbors

Some neighbors around heliports have voicedconcern about the safety of helicopters approach-ing or leaving a heliport. Accident statisticsindicate that these concerns are unfounded.NTSB data on helicopters for 1970 through1975 show that approximately five percent ofthe accidents occur in the traffic pattern orwithin a 1/2-mile radius of an airport or heli-port. The actual risk to the surrounding neigh-borhood from helicopters was analyzed7 to de-termine the likelihood of a helicopter accidentwithin a 1/2-mile radius of the heliport orairport. This analysis was based on the Model

Table 8. 1980s U.S., U.K., and Canadian All-airworthiness-failure Accident Rates (accidents per 100,000 flight hours)

Canada United Kingdom United StatesType (1982-87) (1980-87) (1984-88)

Single-piston 8.91 18.45 4.09

Single-turbine (all) 2.12 4.17 1.69

Twin-turbine 1.27 1.93 1.59

Bell Model 206 single-turbine 1.43 1.17 1.05

206 rate of 4.33 accidentsper 100,000 flight hours inthe United States for theperiod of 1975 through 1978.One can then calculate thelikelihood of an accidentwithin the 1/2-mile radius,which becomes a functionof how many takeoffs andlandings are made.

A conservative three-minutetime period was assumed for time the helicop-ter spent flying over the 1/2-mile radius dur-ing an approach or a landing. The term “cycle”is used for the combination of a takeoff andlanding (six minutes over the 1/2-mile zone).Using the average number of cycles per dayfor a year, the average number of years be-tween accidents can be determined using Fig-ure 8. For example, a busy heliport conduct-ing five cycles per day (182.6 hours per yearover the 1/2-mile radius), the expected aver-age interval between accidents should be oncein 128 years.

The likelihood of a helicopter striking a resi-dence or building within a 1/2-mile radius ofa heliport can be estimated using Figure 9.

The accident frequency used was for all heli-copter accidents (single-piston, single-turbine,and twin-turbine) involved in striking a resi-dence or building. For the five-cycle-per-daycase, a helicopter striking a building or resi-dence is estimated, on average, once every4,000 years. Figure 10 shows the likelihood of



Figure 6. Airworthiness failure accident ratesfor U.K., U.S., and Canada in the 1980s

Figure 7. U.S. general aviation fatal accidentrates (1975-1979)


an on-the-ground person (not a crewman orpassenger) being injured within this 1/2-mileradius. For the five-cycle-per-day case, the av-erage interval between injuries is estimated tobe about 5,000 years.

The heliport operating at five cycles per dayover the period of a year is extremely busy.For a private heliport or limited-use facilitythat averages fewer than one cycle per dayduring the year, the risk is significantly lower.Using one cycle per day as an average, thelikelihood of an accident in the 1/2-mile area,the likelihood of a helicopter striking a resi-dence/building, or the likelihood of an on-the-ground person being injured are once in635 years, 22,400 years, and 25,000 years, re-spectively. These average-year values in them-selves are not important, but their magnitudesindicate the extremely remote threat from he-licopters operating over a congested area.

If only airworthiness-failure-caused accidents


Figure 8. Helicopter accidents within 1/2 mileradius of heliport

are considered using the Model 206 and twin-turbine helicopter rates of Table 7, a compari-son of the likelihood of an airworthiness-causedaccident over a heliport’s neighborhood canbe done. For a constant five cycles per dayusage, the expected accident frequency withinthe 1/2 mile radius of the heliport is an acci-dent once in 34.4 and 52.2 years for a twin-turbine helicopter and Model 206 single-tur-bine helicopter, respectively. The likelihood ofan accident involving either of these helicop-ter types is extremely remote, although oneshould expect the occurrence of a Model 206accident significantly less often than a twin-turbine helicopter accident. According to thesefigures, there is no more justification to pro-hibit a Model 206 than there is to prohibit

twin-turbine helicopters from flying over con-gested (populated) areas.

Study Looks for Causes of Acci-dents Resulting in Fatalities

A worldwide study of Bell civil and militaryturbine-powered helicopter accidents was con-ducted by the author to determine the acci-dent causes that resulted in fatalities. The in-volved period was from January 1970 throughMarch 1987. The size of the Bell turbine fleetdelivered at the time was approximately 19,700single-turbine aircraft and 1,800 twin-turbineaircraft. An engine failure was the initiatingcause that resulted in six percent of all fatali-ties in single-turbine helicopter accidents andthree percent of all fatalities in twin-turbine


Figure 9. Helicopter strikes of residence orbuilding (within 1/2 mile radius of heliport)


Figure 10. On-the-ground personal injury (within1/2 mile radius of heliport)


helicopter accidents as shown in Figure 11.However, the percentage of fatalities due toremaining airworthiness failures (non-enginematerial failures) was 12 percent and 22 per-cent for single-turbine and twin-turbine heli-copters, respectively. Therefore, the total per-centage of fatalities for all-airworthiness fail-ures was 18 percent for single-turbine heli-copters and 25 percent for twin-turbine heli-copters. More complex twin-turbine helicop-ters, with more moving parts, will have a highertotal number of material failures (engine andnon-engine) with a corresponding higher to-tal number of fatal injuries than a simpler single-turbine helicopter.

Relative Risk of Serious InjuryConsiders the Individual

Accident rates compare the frequency of air-craft that must be reported as an accident be-cause there is significant damage or there areserious personal injuries. In the majority ofaccidents, there is no serious injury, so theaccident reporting is basically an aircraft damagemishap frequency. This information is usefulin forecasting the number of aircraft expectedto be damaged, repaired, replaced or otherconsiderations based on aircraft damage. Itdoes not address the safety of the occupant.Risk must be assigned to an individual occu-pant to be meaningful. Occupant safety mustbe determined for each individual occupantbased on individual exposure. This is donewith a formula that gives the relative risk ofserious injury (RSI). RSI is the probability of

an accident occurring multiplied by the prob-

ability of serious (major or fatal) injury. RSI =probability of an accident X probability of se-rious injury. It can be calculated by:

The RSI, or an individual occupant’s risk ofserious injury for every 100,000 occupant hoursof exposure, is shown in Figure 12 for all-airworthiness failure causes. This is the truemeasure of occupant safety related to the air-craft design.

Therefore, an occupant’s risk of serious injurydue to accidents caused by all-airworthinessfailures is the same in the generic single-tur-bine and twin-turbine helicopters. The occupant’srisk in a Model 206 single-turbine helicopteris nearly half that of a generic single-turbineor a twin-turbine helicopter. The reasons thatrisks are generally higher in twins than singlesare:

• More parts and increased complexityyield more non-engine material failures.

• There are more free-standing passen-ger seats and resulting seat failures intwins.

• There are more passenger seats with-out a shoulder harness.

Figure 11. Percentage of fatalities by accidentinitiator


Figure 12. RSI from airworthiness failures


Number of accidentsFlight hours flown

XNumber of people witn major or fatal injury

Total number of people on board in accidentsRSI =

with


• More fuel cells lead to increased likeli-hood of post-crash fires.

The introduction of passenger shoulder har-nesses, energy attenuating seats for all occu-pants, and crash-resistant fuel systems maylower the RSI values. FAA Amendments 27-25and 29-29 of November 13, 1989 requires shoulderharnesses and dynamically tested, energy at-tenuating seats for all occupants in future he-licopter designs. FAA Notice of Proposed RuleMaking (NPRM) 90-24 in progress is address-ing a requirement to include crash-resistantfuel systems in large and small helicopters tominimize thermal injuries due to post-crashfires. The result could be that occupants offuture helicopter designs may have even lowerrisk of serious injury regardless of what causesthe accidents. [This subject has been addressedby Flight Safety Foundation since the early 1960s.A successful program to develop a crash-resistantfuel tank for helicopters was conducted by FSFthrough its Aviation Safety Engineering and Re-search division in 1965, which led to a U.S. Armyretrofit for approximately 5,000 helicopters. S.Harry Robertson received the FSF Admiral LuisDeFlorez Award for directing development of thesystem that was estimated to be capable of achiev-ing a 70 percent reduction in loss of life due tocrash fires. Helicopter crashworthiness was thefocus of a 1984 FSF Regional Helicopter SafetyWorkshop in Rio De Janeiro, Brazil.— Ed.]

A study of U.S. Army helicopter accidents andinjuries8 found similar results in civil helicop-ters. Table 9 shows the RSI for the four Armyhelicopters in the study. The UH-60 is the twin-turbine helicopter and the remainder are single-turbine powered. The risk of injury was foundto be lower in single-turbine helicopters thanin twin-turbine ones. There are several rea-

sons for this, two of which are the greatercomplexity of the UH-60 and its higher speedsat impact. Again, one must be careful to evaluateall aspects of an aviation system, because im-provements in one area can have detrimentaleffects in another. One of the goals in safety isto strive for the best mix to produce the lowestrisk.

Safety Is Risk Management

To manage risk, one must first understand thetotal risk. Prudent risk management will re-duce both probabilities in the RSI formula (prob-ability of an accident and probability of seri-ous injury) and achieve the lowest possiblerisk. Accident prevention programs attemptto reduce the probability of an accident. Train-ing, standardization, equipment, maintenanceand positive management attitude toward safetyare key factors in reducing the probability ofan accident. Pre-accident planning, flight fol-lowing, aircraft/occupant survival gear andtraining, and aircraft crashworthiness addressthe reduction of the probability of serious in-jury. Totally effective accident prevention is aworthwhile goal, but an expectation of abso-lute elimination of accidents is unrealistic.

Australian CAA Studies Single vs.Twin Helicopter Transfer of

Marine Pilots

The Australian Civil Aviation Authority (CAA)conducted a study9 in September 1989 to re-spond to a recommendation from within theCAA and elsewhere to mandate that twin-en-gine helicopters be utilized for transferringmarine pilots between ship and shore ratherthan the single-engine helicopters that had beenused. The study reviewed worldwide accidentdata. Applicable paragraphs from the studyfindings and conclusions are quoted below:

“The CAA believes that greater weight shouldbe given to actual accident performance fig-ures (where these are available) than to theo-retical assumptions about fatal accident ratesderived from, say, engine shutdown. For ex-ample, it would fail to account for the trade-

Table 9. Relative Risk of Serious Injury inU.S. Army Helicopters

Type of Helicopter RSI/100,000 Occupant Hours

UH-60 (twin-turbine) 5.11

AH-1 (single-turbine) 4.13

OH-58 (single-turbine) 2.91

UH-1 (single-turbine) 1.36


off between the extra reliability from having asecond engine and the lower reliability of themore complex helicopter system… .

“Informal advice from the industry suggestthat it would approximately double the costof transferring marine pilots by helicopter iftwin-engine helicopters were made compul-sory… .

“This report does not pursue costing furtherbecause of the lack of conclusive evidence oftwin-engined helicopters leading to lower fa-tal accident rates… .

“Marine Authorities have indicated that in somecases the higher cost of twin-engined helicop-ters could lead to them reverting to launchesto transfer pilots, which these authorities havestated is less safe than transfer by helicop-ter… .

“Conclusion — The CAA believes the proposalto regulate to make it compulsory to use twin-engined helicopters for the transfer of marinepilots to and from ships should be shelved atthis time. The CAA concludes that the pro-posal should be shelved because the presentvery low engine-failure accident rate is ac-ceptable, and because there is no conclusiveevidence that using twins would result in alower fatal accident rate.”

Helicopter Accidents at ElevatedStructures Are Considered

The accident histories of turbine-powered he-licopters at elevated structural platforms werecompared because of the ICAO Annex 6 pro-hibition of single-engine helicopter operationfrom elevated structures. Accident data fromthe NTSB for 1984 through 1988 were used.There were no distinctions made between typeof operations being conducted, such as air trans-port versus aerial work. ICAO defines airtransport as commercial air transport opera-tion — an aircraft operation involving the trans-port of passengers, cargo or mail for remu-neration or hire. Aerial work is defined as anaircraft operation in which an aircraft is usedfor specialized services such as agriculture,

construction, photography, surveying, obser-vation and patrol, search and rescue, aerialadvertising, etc. Since the vast majority ofhelicopter uses are for hire or remuneration, itis not practical to use the ICAO definitionsbecause of some overlap and the fact that thedefinitions as adopted vary among ICAO states.

Many helicopter operations in the United Statesdo not fit perfectly into a particular ICAO defi-nition; also the helicopters in use can changecategories of work several times in a day. Forexample, a helicopter used for EMS purposescan be included in the operational categoriesof business, unscheduled air taxi and otherwork. If the helicopter owner is a government/municipal entity or if the civil operator con-tracts with a government agency for helicop-ter services, the same helicopter can also beconsidered an exclusive lease aircraft. The ac-cident data should be considered in its en-tirety to be consistent with flight hours.

Each NTSB helicopter accident narrative forthe latest available data (1984 through 1988)was used to determine all accidents that oc-curred on an elevated landing site or approach-ing/departing the elevated structure. A keyword search was used for the following wordsin the NTSB accident narratives. These keywords were

Elevated Helipad Net

Structure Helideck Rail

Platform Heliport Pad

Rig Hospital Raised

Roof Building Deck

The resulting accidents were then separatedinto movable landing structures and station-ary landing structures. Accidents at the mov-able landing structures that included landingdollies, trailers, trucks, boats, barges and por-table landing structures were eliminated asnot being applicable to the safety history ofhelicopters operating on an elevated structure.The stationary elevated structure accidents arethose located at rooftops or offshore platforms.


There were no single-piston helicopter acci-dents related to stationary elevated platformstructures, but some occurred on movable land-ing structures.

There were 15 single-turbine helicopter acci-dents at stationary elevated platform struc-tures. Twelve were at offshore platforms andthree at a rooftop sites. Of the 15 accidents,four power losses were reported. There wereno material failures found during the investi-gation of two of these power losses. The re-maining 11 clearly resulted from human causesas follows:

• Takeoff with aircraft tied down

• Landing gear caught on safety net

• Landing gear caught on deck obstruc-tion

• Main rotor blade strike

• Blown off platform during engine startby wind

• Elevator cover not removed prior to flight.

There were 13 twin-turbine helicopter acci-dents at elevated platform structures. Nine wereat offshore platforms and four were at roof-tops. Of the nine offshore platform accidents,two were due to material failures of tail rotordrive shafts and one pylon mounting failurethat allowed ground resonance. The remain-ing seven offshore platform accidents werehuman factors related as follows:

• Tail or tail rotor strike

• Main rotor strike

• Flight controls restricted (maintenanceerror)

• Takeoff with wheel in safety net

• Flight control loss.

Of the four rooftop accidents, two were powerlosses due to fuel exhaustion. A tail rotor strike

accident and a flight controls restricted acci-dent (loose object in cockpit) made up the tworemaining accident causes. Two of these twin-turbine helicopter accidents on stationary el-evated structures were deleted prior to theaccident rate calculation because flight-hourswere not available for the year of the acci-dents. These accidents involved two twin-tur-bine Aerospatiale SA-330J helicopters whichwere included above to show the types of ac-cidents (13 accidents) but are deleted in Table10 when accident rates are used (11 accidents).All single-turbine accidents (which were Model206s) on stationary elevated structures wereusable accidents.

Table 10 shows the U.S. elevated structure he-licopter accident history for 1984 through 1988.This table also identifies the stationary elevatedstructure accidents that were related to powerlosses. For all accidents at elevated structures,the accident rates for the single-turbine andtwin-turbine helicopters were 0.21 and 0.76per 100,000 flight hours, respectively. There-fore, the single-turbine rate was 72.4 percentlower than the rate for twin-turbine helicop-ters. Considering only those related to powerlosses, the single-turbine and twin-turbine he-licopter accident rates were 0.071 and 0.139per 100,000 flight hours, respectively. The single-turbine rate for power loss accidents was 48.9percent lower than the twin-turbine rate.

The second part of Table 10 is similar to thefirst, except that the fleet flight hours usedwere for only the models that were involvedin elevated structure accidents. In this analy-sis, the single-turbine and twin-turbine acci-dent rates for all causes were 0.29 and 1.18 per100,000 flight hours, respectively. The single-turbine rate was 75.4 percent lower than thetwin-turbine rate. Considering the power-lossaccidents, the single-turbine and twin-turbineaccident rates were 0.096 and 0.214 per 100,000flight hours, respectively. The single-turbinerate for power-loss accidents was 55.1 percentlower than the twin-turbine rate. Thus, theactual helicopter accident experience relatedto helicopter operations at a stationary elevatedstructure does not justify the prohibition ofsingle-engine helicopters in such operations.


Evaluating Offshore HelicopterOperator Experience

Petroleum Helicopters Inc. (PHI) is the largestcommercial helicopter operator in the world.Most of the company’s flying is for offshoreoil support and is an excellent example of safehelicopter operations in a difficult environ-ment. The latest PHI-furnished flight-hour in-formation and NTSB accident data on PHI he-licopters from 1984 through 1988 indicate thatsingle-turbine helicopters are being operatedsafely over water and onto elevated platforms.PHI flight hours in Table 11 show that 66.1percent of PHI’s flying was in single-turbinehelicopters. Table 12 compares the PHI acci-dent rates for all causes with the U.S. civilhelicopter fleet rates for all causes. The PHI

accident rate for single-turbine helicopters was65.8 percent and 62.2 percent lower than thegeneral U.S. single-turbine and twin-turbinehelicopter rates, respectively. This illustratesthat a safe operation can be conducted usingsingle-turbine helicopters without operationalrestrictions as proposed by the recent ICAOAnnex 6, Amendment 1 change.

Time of Accident, Day vs. Night,Considered as Factors

Since the actual hours flown at specific timesof the 24-hour day are not known, it is diffi-cult to determine relative safety of night flightversus daylight flight. However, it is possibleto approximate the distribution of flying at

night by considering the random na-ture of material failures. For the pe-riod of 1982 through 1988, the U.S.distribution of accidents (all causes)by the time of day from NTSB dataare shown in Figure 13. The break-points between light and dark wereassumed at 0600 and 1959 hours. Thisdistribution of accidents is consid-ered conservative, because most fly-ing is done during the summer months

Table 10. U.S. Elevated Structure Turbine Helicopter Accident History (1984-1988)

Fleet Power-lossFlight All All Causes Power-loss Accident

Type of Aircraft Hours Accidents Rate* Accidents Rate*

Single 7,035,846 15 0.21 5 0.071

Twin 1,442,116 11 0.76 2 0.139

Using hours of aircraft models involved in accidents:

Single

Bell Model 206 5,215,001 15 0.29 5 0.096

Twin

222 932,438 11 1.18 2 0.214

AS355

B0105

S58T

S76

* Accidents per 100,000 flight hours

Table 11. Petroleum Helicopter Inc (PHI) Flight Hours(1984-1988)

Type of Aircraft Flight Hours Percentage of Total

Single-turbine 1,064,439 66.1

Twin-turbine 545,670 33.9

Total 1,610,117 100

Bell Model 206 only 982,611 61.0


when the length of daylight is highest. Thisindicates that 91.8 percent and 82.8 percent ofall single-turbine and twin-turbine helicopteraccidents, respectively, are occurring in day-light hours.

Figure 14 shows the time of accident distribu-tion of airworthiness-failure accidents (allmaterial failures including the engine). Forall-airworthiness-failure accidents, 98.2 per-cent and 94.1 percent of single-turbine andtwin-turbine helicopter accidents, respectively,are occurring in daylight hours. The two fig-ures have similar distribution; therefore, acci-dents due to material failures do not appear tobe adversely affected by lighting, and there-fore, there is insufficient justification to pro-hibit single-engine helicopters from flying atnight.

The major difference in helicopter and fixed-wing aircraft emergency landings is that thefixed-wing aircraft requirement for a long cleared

Table 12. Petroleum Helicopter Inc. (PHI) vs. U.S.Helicopter Accident Rates* (accidents from

NTSB, hours from FAA and PHI, 1984-88)

Type of Aircraft U.S. (NTSB/FAA) PHI (NTSB/PHI)

Single-turbine 5.49 1.88

Twin-turbine 4.37 1.65

Bell Model 206 only 4.28 1.73

* Accidents per 100,000 flight hours

landing site increases the likelihood of in-jury during the final phase of the emer-gency. Conversely, a helicopter (regard-less of the number of engines) can use alanding site that is quite small in com-parison to the needs of the fixed-wing air-craft. Likewise, visibility at night is not ascritical in a helicopter as in a fixed-wingaircraft due to the helicopter’s lower speedand greater maneuverability during auto-rotation.

Likelihood of a Material Fail-ure Accident at Night Examined

Assuming a Model 206 and a twin-turbine he-licopter flew during 10 hours of darkness ev-

ery night throughout one full year, each heli-copter would fly 3,652.5 hours during that time.Using the NTSB/FAA accident data for 1984through 1988 (Table 7), the likelihood of anaccident due to a material failure (which in-cludes engine) for the twin-turbine helicopteris estimated at once in 17.2 years, whereas thelikelihood in a Model 206 is estimated at oncein 26.1 years. Thus, the likelihood of any ma-terial-failure-caused accident is 51.4 percenthigher in a twin-turbine helicopter than in thesingle-turbine Model 206. There is insufficientjustification to support the prohibition of nightflights using single-engine helicopters withrespect to material failures.

Figure 13. Time of accidents due to all causes


Figure 14. Time of accidents due to airworthi-ness failures



Human Error Accidents Related toWeather Considered

Analysis of human error accidents involvingweather show a changing trend in the UnitedStates. NTSB accident data and FAA flight hoursfor 1984 through 1988 were divided into anearly period of 1984 through 1986 and com-pared to the later period of 1987 and 1988. Theresults are represented in Table 13. The year1987 was when Bell Helicopter Textron Inc.began concentrated safety training programsto reduce human error accidents. The range ofhuman error accident rate reductions due topoor weather decisions in the most recent timeperiod have been significantly reduced by be-tween 45 and 72 percent. Bell believes thatthis reduction is due to safety training, not tomandatory regulations.

Bell Institutes SafetyTraining Approach

Accident data analyses can be used to deter-mine if safety programs or other factors aremaking a change in the accident frequencies.Two out of three accidents are not caused byairworthiness failure but are basically due tohuman error. Accidents caused by human er-ror (otherwise called pilot error) present an

Table 13. U.S. Human Errors Accidents Involving Weather

Bell ModelSingle- 206 Single-

Single-piston turbine Twin-turbine turbine

Flight Hours

1984-86 1,899,081 4,167,156 821,679 2,997,911

1987-88 1,062,171 2,868,690 620,437 2,217,090

HE WX Accidents

1984-86 26 40 7 25

1987-88 8 8 2 5

HE WX Accidents per

100,000 Flight Hours

1984-86 1.37 0.96 0.85 0.83

1987-88 0.75 0.28 0.32 0.23

HE WX Rate Reduction -45.3% -70.8% -62.4% -72.3%

HE = Human Error

WX = Weather

extremely complex problem with a large num-ber of root causes and an even larger numberof potential solutions. Engineers and regula-tory agencies are comfortable working on me-chanical problems because their performanceand failure modes are fairly predictable. Thus,aviation safety efforts in the past have madesignificant gains in minimizing airworthinessfailures.

More attention is now being made toward theunderstanding, and eventual reduction, of humanerror accidents. A worldwide engineering studyin 1985 and 1986 into human error accidentsof Bell civil helicopter models found that poorjudgment was the common factor in all of theseaccidents2. Two directions of concentrated ef-fort at Bell were launched in 1987 to aggres-sively attack the complex human error prob-lem, with the emphasis on judgment training.

The company’s Human Factors Engineeringstaff took the approach of developing an arti-ficial intelligence-based software program thatwould allow a pilot to use a personal com-puter (PC) as a judgment (decision-making)simulator. This program, called the CockpitEmergency Procedures Expert Trainer (CEPET),also includes emergency procedures training.A CEPET was developed for the Bell JetRanger(206BIII) and LongRanger (206L-3), with one


completed late in 1990 for the Model 212/412.A CEPET package is provided with each newBell aircraft delivery; pilots can also purchasea separate CEPET package.

The other direction focused on concentratedsafety education. Bell’s System Safety Engi-neering personnel developed a three-hour safetybriefing for immediate use with groups of pi-lots and managers. This safety briefing includeshow to measure one’s risk, what happens in acrash, how one can improve the chances ofsurvival, causes of accidents, root causes ofhuman error, and judgment training. Judgmenttraining emphasizes the use of all resourcesavailable to the pilot

The emphasis of judgment training is on situ-ational awareness and internal pilot monitor-ing. Portions of the FAA study, DOT/FAA/PM-86/45, “Aeronautical Decision Making forHelicopter Pilots10,” are used in this safety brief-ing and the FAA report is given to the studentfor further self study. The safety briefing isgiven at operator meetings and regional safetyseminars, and is included in Bell’s weekly Model206 pilot’s ground school as part of theHELIPROPS (Helicopter Professional PilotsSafety) program.

The company’s chief training pilot also con-ducts customer HELIPROPS safety briefingson safety awareness, professionalism andmanagement’s role in safety. These safety brief-ings are held at the factory, customer sites and

regional safety seminars. The Customer Sup-port and Service Department (CSSD) initiatedthe HELIPROPS program to add continuityand coordination of these safety education ef-forts in 1988.

The worldwide effects of this four-year safetyeducation effort on the human error accidentrate since the Model 206 effort was initiated in1987 is shown in Table 14. There have beenmore than 5,000 Model 206 series helicoptersproduced, or 70 percent of Bell’s entire civilturbine helicopter model fleet. Bell also con-ducts flight training in Model 206s. Based onthese two factors, the concentrated safety edu-cation effort has been directed at Model 206pilots. For comparison, the same worldwidedata for Bell’s medium civil helicopters mod-els (204B, 205A1, 214B, 212, 214ST, 222, and412) are also shown in Table 14. These me-dium helicopter data indicate some reductionsin human error causes, but were offset by non-human-error causes; thus the accident rate forall causes was basically the same during thetwo four-year periods.

Conversely, accident rates due to human errorin a Model 206 for the four-year period beforethe initiation of this safety effort (1983-1986)compared with the four-year period since then(1987-1990) show a 36.2 percent reduction inhuman error accidents. This is a significantsafety improvement and only a small portionof the world’s Model 206 pilots have been reachedso far. The overall (all causes) Model 206 acci-

Table 14. Worldwide Bell Turbine Accident Rates (per 100,000 flight hours)

Causes of AccidentsAircraft and Non-human

Period Flight Hours Human Error and Unknown All Causes

Bell Model 206

1983-1986 7,903,072 3.90 2.05 5.95

1987-1990 9,341,573 2.49 1.89 4.38

Percent change -36.25% -7.8% -26.3%

Bell Medium Helicopters

1983-1986 2,438,515 2.62 2.01 4.63

1987-1990 2,472,091 2.31 2.39 4.69

Percent change -11.8% +18.9% +1.3%


dent rate was reduced by 26.3 percent. Be-cause many pilots fly other helicopters in ad-dition to the Bell Model 206, some spillover ofthe beneficial effects of judgement training canbe expected that should affect the overall heli-copter industry accident rate.

Judgment training is also called pilot decisionmaking (PDM) and aeronautical decision making(ADM). The Canadian government is integratingPDM into its pilot training requirements as ofApril 1991. PHI introduced judgment trainingas an integral part of its internal training aboutwhen Bell introduced its judgment trainingprogram in 1987, and has subsequently cut itsaccident rate in half. The company expectsfurther accident rate reductions as the pro-gram continues. Judgment training has moresafety improvement potential than the totalelimination of all-airworthiness failure causes.

A reflection of the success of the safety educa-tion efforts by several manufacturers of heli-copters in the United States is found in Table15. This shows a significant reduction in hu-man error accident rates in the turbine heli-copter fleet, although more work is needed inthe single-piston fleet. Since safety educationis an ongoing effort, it will take several yearsto benefit all helicopter pilots.

The Helicopter’s Uniqueness MustBe Taken into Account

Helicopters behave differently than fixed-wingaircraft after an engine failure. The helicopter ’s

ability to autorotate allows the selection ofsuitable landing sites and a low-speed emer-gency landing from an engine failure.

Safety decisions on any one aspect of helicop-ters should not be made without consideringall the other safety aspects, as well as the hu-man causes. The safety measurement methodto consider is strictly determined by the sub-ject of primary concern. The denominator ofthe frequency rate will include this primaryconcern. If aircraft damage frequency is theprimary concern, then an accident per aircraftflight hour method is appropriate. If the mis-sion is the primary concern, then accidentsper mission (takeoff, departure, takeoff, flight,trip, passenger mile or patient transport) isappropriate. If the primary concern is the riskof an accident in a neighborhood without re-gard to the aircraft occupants, then years-be-tween-accidents measurement for that specificneighborhood exposure is appropriate. Withthe safety of the aircraft occupant as the pri-mary concern, the relative risk of serious in-jury per occupant flight hour is the best method.♦

References

1. Amendment No. 1 to International Stan-dards and Recommended Practices, Opera-tion of Aircraft, Annex 6 to the Conventionon International Civil Aviation, Part III,International Operations — Helicopters, In-ternational Civil Aviation Organization(ICAO), March 1990.

Table 15. Safety Education Effects on Human Error Accident RatesNTSB/FAA (U.S.-registered)

Rate* Before Rate* SinceType of Helicopter (1984-86) (1987-88) Percent Changes

Single-piston 11.16 10.92 -2.2%

Other Than Bell Model 206 single-turbine 4.11 3.07 -25.3%

Twin-turbine 2.56 1.61 -37.1%

Bell Model 206 single-turbine** 3.40 1.76 -48.2%

* Human error accidents per 100,000 hours

** Concentrated HELIPROPS safety education


2. Fox, R. G., “Helicopter Accident Trends,”American Helicopter Society/Helicopter As-sociation International/FAA Seminar, Ver-tical Flight Training Needs and Solutions,September 1987.

3. Fox, R. G., “Relative Risk, the True Mea-sure of Safety,” Flight Safety Foundation,Proceedings of the 28th Corporate AviationSafety Seminar, April 1983.

4. Commuter Airline Safety, Special Study,National Transportation Safety Board, Re-port NTSB-AAS-80-1.

5. Collett, H. F., “Accident Trends for Air Medi-cal Helicopters,” Hospital Aviation, Febru-ary 1989.

6. Fox, R. G., “Helicopter Crashworthiness,”Flight Safety Foundation, Proceedings of the34th Corporate Aviation Safety Seminar, April1989.

7. Fox, R. G., “Helicopters Are Safe Neigh-bors,” Helicopter Association International,1990 Helicopter Annual, January 1990.

8. Shanahan, D. F. and M. O., “Injury in U.S.

Army Helicopter Crashes, October 1979 toSeptember 1985,” Journal of Trauma, April1989.

9. Pardy, B. T., “Preliminary Safety ImpactStatement Twin-Engined Helicopters for Ma-rine Pilot Transfers,” Australian Civil AviationAuthority, Report A SR-2, September 1989.

10. Adams, R., and Thompson, J., “Aeronauti-cal Decision Making for Helicopter Pilots,”Federal Aviation Administration, DOT/FAA/PM-86/45, February 1987.

About the Author

Roy G. Fox directs the System Safety EngineeringGroup as Chief Safety Engineer for Bell Helicopter,Textron. He joined Bell in 1966 immediately aftergraduation with a Bachelor of Science Degree inMechanical Engineering from New MexicoUniversity. In addition to his work in safetyengineering, Fox has been deeply involved in crashsurvival of military and civil helicopters. In thisfield, he has directed the Crashworthiness ProjectGroup for the helicopter industry.

Fox is a member of the General Aviation SafetyPanel for seat restraints and post-crash fire protection,and participates in the SAE committees on seatand restraint requirements. He is a lecturer oncrash survival and human performance at the BellTraining School and the U.S. Federal AviationAdministration Helicopter Safety and AccidentInvestigation course.


Aviation Statistics

The U.S. Federal Aviation Administration (FAA)has reported that near midair collision inci-dents, air traffic controller operational errorsand pilot deviations continued to show a de-cline in 1990 as compared with those recordedin previous years, but runway incursions showedan increase two years in a row. This informa-tion was provided in the final report, 1990Aviation Safety Statistics, prepared by the FAA’soffice of the assistant administrator for avia-tion safety.

A near midair collision refers to an incidentassociated with the operation of an aircraftduring which a possibility of collision occursas a result of a proximity of less than 500 feet

Near Midair Collisions, OperationalErrors and Pilot Deviations

byShung C. Huang

Statistical Consultant

from another aircraft, or an official report isreceived from an air crew member stating thata collision hazard existed between two or moreaircraft. The 452 near midair collisions, re-ported during last year, were 606 (57) percentless than the 1,058 recorded in 1987 and wasthe lowest level since 1984. The 1990 figurerepresents the third consecutive year that nearmidair collision reports dropped since the FAAupgraded its reporting system in 1985 to en-sure more complete data collection on theseincidents.

Figure 1 shows the downtrend of near midaircollision incidents for the past six years, andFigure 2 illustrates how midair collisions hap-pened more often in the summertime than anyother time of the year. In the months of July,August and September, air traffic for both civiland military were the heaviest during eachyear.

An operational error refers to an occurrenceattributable to an element of the air trafficcontrol system which results in less than theapplicable separation minima between two ormore aircraft, or between an aircraft and ter-rain or obstacles and obstructions as requiredby FAA Handbook 7110.65 and supplementalinstructions. Obstructions include vehicles,equipment, personnel or runways. Beginningin 1984, the FAA began installing computersoftware in all domestic air route traffic con-trol centers that automatically records viola-tions of the agency’s aircraft separation stan-dards. The 881 operational errors made byFigure 1. Near Midair Collisions 1985-1990



controllers in 1990 were the lowest since 1983.Figure 3 shows the trend of air traffic control-ler operational errors for the past six years.Air traffic controller operational errors droppedfrom a high of 1,406 in 1985 to 881 in 1990,down by 524 errors, or 37 percent. Figure 4shows the occurrence of operational errors by

month for the past six years. Note that themonthly frequency of operational errors ap-pears to have an almost identical pattern eachyear. Air traffic controllers usually commit-ted fewer operational errors in January andFebruary. The frequencies of operational er-rors increased in March and April, dropped inMay and continued to rise into July; they wentup again in August and gradually declinedfrom September to the end of the year.

Pilot deviation refers to those actions of a pi-lot that result in the violation of U.S. FederalAviation Regulations or airspace violation ofa North American Air Defense (NORAD) Com-mand Air Defense Identification Zone (ADIZ)tolerance. Figure 5 shows the trend of pilotdeviations over the past six years. Pilot de-viations increased from 1,800 in 1985 to 3,625in 1987, the highest in recent years, then de-creased to 2,460 in 1990. Although the 2,460pilot deviations in 1990 were only slightly lowerthan those in 1989, the figure is well below the1987 level. Figure 6 shows the annual fre-quency of pilot deviations by month whichalso indicates that pilots committed more er-rors during the summer of each year.

Runway incursion refers to an occurrence atan airport involving an aircraft, vehicle, per-

Figure 2. Near Midair Collision Reports byMonth 1985-1990

Figure 3. Air Traffic Controller Operational Er-rors 1985-1990

Figure 4. Air Traffic Controller Operational Er-rors by Month 1985-1990





son or object on the ground that created acollision hazard or resulted in loss of properseparation with an aircraft taking off, intend-ing to take off, landing or intending to land.Against the downtrends of other safety indi-cators, runway incursions steadily increasedfrom 179 in 1988, to 233 in 1989 and to 267 in1990. Figures 7 and 8 show the trend andannual frequency distribution by month of run-way incursions. Almost every year in the pastfour years, runway incursions were relativelyhigher in March, June, August, October and

December and often relatively lower in Febru-ary, April, July, September and November.During the most recent 15 months, there werefive fatal accidents at U.S. airports, involvingU.S. air carrier aircraft, accounting for 33 fa-talities. That was the highest number of fatalaccidents in any 15-month period in U.S. aircarrier safety records. Three of the fatal acci-dents were ground collisions between aircraft.One pedestrian was killed on a runway by anaircraft during takeoff and one mechanic wasfatally injured by a tug during towing. ♦

Figure 5. Pilot Deviations 1985-1990 Figure 7. Runway Incursions 1987-1990

Figure 6. Pilot Deviations by Month 1985-1990





Figure 8. Runway Incursions by Month 1987-1990


Reports Received at FSFJerry Lederer Aviation Safety Library

Reference

Federal Aviation Regulations, Part 135-Air TaxiOperators and Commercial Operators; Change 38.—Washington, D.C. : United States. Federal Avia-tion Administration [1991].

Summary: This change incorporates SpecialFederal Aviation Regulation (SFAR) 38-6, Cer-tification and Operating Requirements, effec-tive June 5, 1990, and Amendment No. 135-39,Minimum Equipment Lists (MEL), effective June20, 1991, in Federal Aviation Regulation Part135.

Reports

Aircraft Accident Report: Avianca, the Airline ofColombia, Boeing 707-321B, HK 2016, Fuel Ex-haustion, Cove Neck, New York, January 25, 1990/United States. National Transportation SafetyBoard. — Washington, D.C. : U.S. NationalTransportation Safety Board; Springfield, Vir-ginia, U.S. : Available through NTIS*, AdoptedApril 30, 1991, Notation: 5255B. Report NTSB/AAR-91/04; NTIS PB 91-910404. vi, 6 p. : ill.

Key Words1. Aeronautics — Accidents — 1990.2. Aeronautics — Accidents — Fuel

Exhaustion.3. Aeronautics — Accidents — Pilot Lan-

guage Proficiency.4. Aeronautics — Accidents — Air Traffic

Control Procedures.5. Aeronautics — Accidents — Pilot Fatigue.6. Aeronautics — Accidents — Windshear.7. Aeronautics — Accidents — Flight

Planning.8. Aeronautics — Accidents — Takeoff/

Landing.9. Avianca Airlines — Accidents — 1990.

Summary: On January 25, 1990, at approxi-

mately 2134 eastern standard time, AviancaAirlines flight 052, a Boeing 707-321B withColombian registration HK 2016, crashed in awooded residential area in Cove Neck, LongIsland, New York. AVA052 was a scheduledinternational passenger flight from Bogota,Colombia, to John F. Kennedy InternationalAirport, New York, with an intermediate stopat Jose Maria Cordova Airport, near Medillin,Colombia. Of the 158 persons aboard, 73 werefatally injured, including all three flight crewand five of six cabin crew members. Becauseof poor weather conditions in the northeast-ern part of the United States, the flight crewwas placed in holding three times by air traf-fic control for a total of about 1 hour and 17minutes. During the third period of holding,the flight crew reported that the airplane couldnot hold longer than five minutes, that it wasrunning out of fuel, and that it could not reachits alternate airport, Boston-Logan International.Subsequently, the flight crew executed a missedapproach to John F. Kennedy International Air-port. While trying to return to the airport, theairplane experienced a loss of power from allfour engines and crashed approximately 16miles from the airport.

The NTSB determined that the probable causeof this accident was the failure of the flightcrew to adequately manage the aircraft’s fuelload, and their failure to communicate an emer-gency fuel situation to air traffic control be-fore fuel exhaustion occurred. Contributingto the accident was the flight crew’s failure touse an airline operational control dispatch systemto assist them during the international flightinto a high-density airport in poor weather.Also contributing to the accident was inad-equate traffic flow management by the Fed-eral Aviation Administration and the lack ofstandardized understandable terminology forpilots and controllers for minimum and emer-gency fuel states.


Recommendations A-91-33 through A-91-38 andA-90-9 through A-90-11 were issued as a re-sult of this accident. [executive summary]

Aircraft Maintenance: Additional FAA OversightNeeded of Aging Aircraft Repairs. Vol. I and II.Report to the Chairman, Subcommittee on Avia-tion, Committee on Public Works and Trans-portation, House of Representatives/UnitedStates. General Accounting Office (GAO). —Washington, D.C., U.S. : U.S. General AccountingOffice*, May 24, 1991.Report GAO/RCED-91-91A and GAO/RCED-91-91B, B-242727.71 p. ;57 p. ; ill.

Key Words1. Air Planes — Maintenance and Repair —

United States.2. Jet Transports — Maintenance and Repair

— United States.3. Aeronautics, Commercial — Safety

Measures — United States.

Summary: Volume I describes that portion ofthe U.S. aircraft repair industry that performsheavy airframe maintenance on large trans-port aircraft. Specifically, it examines increasesin demand for heavy airframe maintenance;constraints on supply, including parts, skilledmechanics, and hangar space; and air carriers’efforts to comply with new requirements foraging aircraft and the Federal AviationAdministration’s (FAA) oversight of air carri-ers as they attempt to comply with the newrules. Volume II provides the questionnaireresponse of the 48 air carriers and 35 indepen-dent repair stations participating in the re-view on the issues examined in Volume 1. [In-troductory letter]

To improve FAA’s oversight of aging aircraftAD compliance, GAO recommends that theSecretary of Transportation direct the Admin-istrator, FAA, to (1) require domestic air carri-ers to submit periodic reports on their imple-mentation of FAA’s new rules for aging air-craft, (2) submit to the chairmen of the avia-tion authorization subcommittees in the U.S.House and Senate a semiannual report on theindustry’s progress in complying with FAA’saging aircraft mandates, and (3) explore op-tions for extending compliance deadlines or

granting alternative means of compliance whenwarranted by resource shortages and ensuredairworthiness of each aircraft. [recommenda-tions]

Annual Review of Aircraft Accident Data. U.S.Air Carrier Operations, Calendar Year 1988/UnitedStates. National Transportation Safety Board.— Washington, D.C. : U.S. National Transpor-tation Safety Board; Springfield, Virginia, U.S.: Available through NTIS*, April 18, 1991.Re-port NTSB/ARC-91/01, NTIS PB91-176040. 76p.; charts, graphs.

Key Words1. Aeronautics — Accidents — 1988.2. Aeronautics — Accidents — Statistics —

1988.3. Aeronautics — Accidents — United States—

1988.4. Aeronautics, Commercial — Accidents—

United States.

Contents: Introduction — 14 CFR 121, 125,127 Operations — Scheduled 14 CFR 135 Op-erations — Nonscheduled 14 CFR 135 Opera-tions — Midair Collision Accidents — Explana-tory Notes — Cause/Factor Table-14 CFR 121,125, 127 —Cause/Factor Table-Scheduled 14CFR 135 — Cause/Factor Table-Nonscheduled14 CFR 135 — NTSB Form 6120.4.

Summary: Presents the record of aviation ac-cidents involving revenue operations of U.S.air carriers including commuter air carriersand on-demand air taxis for calendar year 1988.[author abstract]

Aviation Safety: Changes Needed in FAA’s ServiceDifficulty Reporting Program. Report to the Chair-man, Subcommittee on Aviation, Committeeon Commerce, Science, and Transportation, U.S.Senate/United States. General Accounting Office.— Washington, D.C., U.S.: U.S. General Ac-counting Office*, March, 1991. Report GAO/RCED-91-24; B238393. 16 p.

Key Words1. Jet Transports — Airworthiness.2. Jet Transports — Inspection — United States.3. Aeronautics — Accidents — United States.


Summary: GAO evaluated the effectivenesso f t h e p a r t o f t h e F e d e r a l Av i a t i o nAdministration’s (FAA) Service Difficulty Re-port (SDR) program related to large, airline-operated aircraft. GAO found that several factorsstemming primarily from FAA’s managementinattention limit the program’s usefulness.Information that one airline considers report-able may go unreported by another airline;useful information does not reach subscribersfor more than six weeks because of delays inmanual data processing through a paper-basedsystem; FAA does not analyze the data, as re-quired by FAA policy, to detect malfunctiontrends in specific aircraft models or focus theefforts of FAA’s inspection work force becauseof insufficient staff and unreliable data. Alter-natives do exist, such as major equipment manu-facturers managing the program. Several policyissues regarding cost, liability and the manu-facturers’ roles in regulating air safety need tobe addressed before an alternative is chosen.[Results in brief]

Donning Times and Flotation Characteristics ofInfant Life Preservers: Four Representative Types.Final Report/Gordon E. Funkhouser and Gre-gory W. Fairlie (Civil Aeromedical Institute).— Washington, D.C. : United States. FederalAviation Administration Office of AviationMedicine; Springfield, Virginia, U.S.: Availablethrough NTIS*, April, 1991. Report DOT/FAA/AM-91/6. 12 p. : ill.

Key Words1. Aeronautics, Commercial — Safety

Measures.2. Survival (After Airplane Accidents,

Shipwrecks, etc.).3. Life-preservers.4. Drowning — Prevention — Equipment and

Supplies.5. Infants.

Summary: Four currently available represen-tative types of infant life preservers were testedto assess the donning times and flotation char-acteristics for infant subjects (six months totwo years old). Donning times were recordedfrom the time the unwrapped device was handedto the parent until the last connection or ad-justment was made. The device that was most

quickly donned was an inflatable type with avest attached to the top of the upper chamber(median donning time was 28.8 seconds). Thisinfant life preserver also exhibited good bodysupport with the head well above the water.The two fixed-foam devices were designed tohave approximately one-third of the buoyancyof the two inflatable types and relied on assis-tance from an adult to maintain the infant in asafe flotation attitude. It appears that the fixed-foam infant life preservers would provide morethermal protection than the inflatable life pre-servers. [author abstract]

Electronic Checklists: Evaluation of Two Levels ofAutomation/Everett Palmer (U.S. National Aero-nautics and Space Administration (NASA)-AmesResearch Center) and Asaf Degani (San JoseState Foundation). — Moffett Field, Califor-nia, U.S.: NASA Ames Research Center, 1991.6 p. ; ill.

Key Words1. Airplanes — Piloting — Automation.2. Airplanes — Piloting — Checklists.3. Airplanes — Cockpits — Automation.

Notes

Paper presented at the Sixth International Sym-posium on Aviation Psychology, April 29-May2, 1991, Columbus, Ohio, U.S..

Summary: Two versions of an electronic check-list, differing in degree of pilot involvementin conducting the checklists, and a paper checklist(as a control condition) were evaluated in line-oriented simulation. Two aircrews from onemajor air carrier flew a routine, four leg, short-haul trip. This paper presents and discussesthe portion of the experiment that was con-cerned with measuring the effect of the degreeof automation on the crews’ performance. Itdiscusses and presents evidence for a poten-tial downside of implementing an electronicchecklist that is designed to provide fully re-dundant monitoring of human procedure ex-ecution and monitoring. [modified author ab-stract]

Philosophy, Policies, and Procedures: The ThreeP’s of Flight-deck Operations /Asaf Degani (San


Jose State University Foundation) and Earl L.Wiener (University of Miami). — Moffett Field,California, U.S.: NASA Ames Research Cen-ter, 1991. 8 p.

Key Words1. Aeronautics — Accidents — Human

Factors.2. Aeronautics — Accidents.3. Airplanes — Operational Procedures.3. Airplanes — Piloting — Human Factors.

Notes

Paper presented at the Sixth International Sym-posium on Aviation Psychology, April 29-May2, 1991, Columbus, Ohio, U.S..

Research was supported by NASA Ames Re-search Center Grants NCC2-327 to the San JoseState University Foundation and Grant NCA2-441 to the University of Miami.

Includes references.

Summary: Standard operating procedures (SOP)are drafted and provided to flight crews todictate the manner in which tasks are carriedout. Failure to conform to SOP is frequentlylisted as the cause of violations, incidents andaccidents. However, procedures are often de-signed piecemeal, rather than being based ona sound philosophy of operations and policiesthat follow from such a philosophy. A frame-work of philosophy, policies and proceduresis proposed. [author abstract]

The Use and Design of flight crew Checklists andManuals. Final Report/John W. Turner (EF&GDynatrend) and M. Stephen Huntley Jr. (U.S.Department of Transportation Research andSpecial Programs Administration). — Wash-ington, D.C. : United States. Federal AviationAdministration Office of Aviation Medicine;Springfield, Virginia, U.S. : Available through

NTIS*, April, 1991. Report DOT/FAA/AM-91/7. 75 p. : ill.

Key Words1. Airplanes — Piloting — Checklists.2. Airplanes — Piloting — Handbooks,

Manuals, etc.3. Airlines — Operational Procedures.4. Air Pilots — Handbooks, Manuals, etc.5. Aeronautics, Commercial — Safety

Measures.

Summary: A survey of aircraft checklists andflight manuals was conducted to identify im-pediments to their use and to determine ifstandards or guidelines for their design wereneeded. Information for this purpose was col-lected through the review of checklists andmanuals from six Part 121 and nine Part 135carriers, review of NTSB and Aviation SafetyReporting System (ASRS) reports, analysis ofan Air Line Pilots Association (ALPA) surveyof air carrier pilots, and by direct observationin air carrier cockpits. The survey revealedthat some checklists and manuals were diffi-cult to locate and were poorly designed foruse in the cockpit environment, the use of check-lists by flight crews was not always well de-fined, the use of checklists interfered with otherflight operations and flight operations oftenmade it difficult to use checklists effectively.The report contains recommendations for theformat and content of checklists and manuals,their use by flight crews, and areas of researchrelevant to checklist design. [author abstract]

*U.S. Department of CommerceNational Technical Information Service (NTIS)Springfield, VA 22161 U.S.Telephone: (703) 487-4780

*U.S. General Accounting Office (GAO)Post Office Box 6012Gaithersburg, MD 20877 U.S.Telephone: (202) 275-6241


This information is intended to provide an aware-ness of problem areas through which such occur-rences may be prevented in the future. Accident/incident briefs are based upon preliminary infor-mation from government agencies, aviation orga-nizations, press information and other sources.This information may not be accurate.

Accident/Incident Briefs

Air CarrierAir Carrier

Who Left theDoor Open?

Boeing 747: Minor damage. No injuries.

The widebody aircraft was preparing to de-part the terminal for a regularly scheduledflight. The flight was being operated duringthe daytime, early in the afternoon.

After all pre-taxi checks had been completed,the aircraft was being pushed back from itsparking position at the terminal gate. A leftfront passenger door, that had inadvertentlybeen left open, collided with the jetway andwas damaged as the aircraft rolled back. Theaircraft had to be taken out of service for re-pairs and the passengers transferred to otherflights.

Overshot Altitude WhileLooking for Traffic

Boeing 737: No damage. No injuries.

The air carrier had just departed the airport

and was climbing to its initially assigned alti-tude of 14,000 feet. The captain was provid-ing initial operating experience for a new firstofficer who was flying the aircraft.

As the aircraft approached the 13,000-foot level,air traffic control (ATC) issued a traffic advi-sory for a target at 11 o’clock at 15,000 feet. Atfirst, neither pilot was able to see the reportedtraffic and both were distracted looking for it.As a result, the aircraft continued climbingthrough its assigned clearance level of 14,000feet. The pilots received an altitude alert, andan ATC altitude deviation advisory at 14,300feet and were able to reverse the climb by14,500 feet. Despite a quick return to the clearedaltitude, legal vertical separation of 500 feethad been lost between the 737 and the otheraircraft.

Among lessons learned are ensuring that some-one is flying the aircraft at all times, that therate of climb is closely monitored during thelast 1,000 feet of climb, and that procedures beestablished for receiving clearances and set-ting the altitude alert system.

Strange Noise at NightMakes Flight Interesting

Fokker F.27 Friendship: No damage. No injuries.

The aircraft was departing on a scheduled flightduring the night. The aircraft and systemshad checked OK during preflight checks.

Air Taxi/CommuterAir TaxiCommuter


However, after rotation the noise level in thecockpit increased substantially. The soundbecame worse as the aircraft increased speedduring the climbout. The flight crew investi-gated the sound and the noise appeared to becoming from the first officer ’s side window.The crew reduced the airspeed and it was de-termined that the first officer ’s window wasajar, even though the locking handle was inthe down and locked position. When it hadbeen visually inspected during the preflightchecks, the window had appeared to be closed;but the locking bolts had not been engagedproperly.

The first officer opened the window and closedit properly and the noise stopped. The flightwas continued without further incident.

Too Busy TalkingTo Passenger

Piper PA-31 Navajo Chieftain: Substantial dam-age. No injuries.

The aircraft was inbound on an approach. Therewere a pilot and two passengers aboard. Thepilot had total of 13,000 flying hours, 1,500 intype and 200 during the previous three months.

The aircraft was cleared for a straight-in ap-proach to land. The pilot was explaining thepre-landing checks to one of the passengers.During the conversation, the pilot noticed thatthe nose gear landing lights were not illumi-nated. Because he thought the landing gearhad been placed down, he assumed that thelanding gear indicator bulbs had failed.

The pilot continued the approach — and theaircraft landed with the landing gear retracted.There was no fire, but the aircraft sustainedsubstantial damage to the propellers, flaps andunderside of the fuselage. There were no inju-ries to the occupants.

The pilot reported that he forgot to lower thelanding gear, and that the power setting wastoo high for the warning horn to actuate.

Man-made WindshearSnaps Gear Leg

Cessna 303 Crusader: Substantial damage. Noinjuries.

The aircraft was approaching to land after amorning flight. There were a pilot and threepassengers aboard the twin-engine aircraft.

The pilot had obtained destination airportweather information en route that reportedwind from 280 to 300 degrees magnetic at 20knots, gusting to 34 knots. He would be usingrunway 25. Approaching the airport, he at-tempted to contact the Unicom to determineweather conditions, but there was no operatoravailable.

During final approach, the aircraft descendedrapidly from a height of approximately 100feet and struck the ground hard enough thatthe right main gear assembly broke away. Thepilot elected to abort the landing and continueto his home base airport where better emer-gency and repair facilities were available. Ashort time after the aborted landing, the windat the airport was reported to be gusting from20 to 36 knots from the northwest.

After a flyby of the control tower at the homeairport it was confirmed that the right gearwas missing. The runway was foamed andthe pilot accomplished a gear-up landing withno further difficulties. The aircraft sustainedfurther damage due to the landing but thepassengers deplaned with no injuries.

The pilot was cited for failing to adjust theairplane’s approach speed for the gusty windconditions at the airport where the gear hadbeen damaged. He was familiar with the air-

Corporate ExecutiveCorporateExecutive


port and the accident report stated that heshould have been aware of a published cau-tionary warning that there was a possibility ofwindshear when landing on runway 25 withnorthwest winds in excess of 10 knots.

The windshear pilots are cautioned about iscaused by buildings and farm silos near thethreshold of the runway.

Severe Icing Is aSevere Threat

Beech 65 Queen Air: Aircraft destroyed. Fatalinjuries to 1.

The aircraft was carrying cargo across the south-east corner of Australia some time after 0300hours in July, winter season down under. Thearea forecast included a freezing level at 4,500feet east of an approaching front, moderate tosevere icing in cumulus clouds, visibility lessthan four miles in heavy rain showers and lessthan 1,000 feet in snow showers.

After a normal departure, the pilot flew theaircraft to a cruising level of 8,000 feet. Hereported passing his first checkpoint and wasgiven further clearance. This was the last con-tact air traffic control personnel had with thepilot.

Persons sleeping in the path of the plannedflight were awakened by very loud engine noisesfrom an aircraft that was obviously flying at avery low altitude in an approximately south-ern direction. They saw a flash of light fol-lowed almost immediately by the sound of athud.

The weather in the vicinity of the accidentincluded snow and fog.

Several hours later, searchers found the wreckageof the aircraft; it had been destroyed and thepilot was fatally injured. The aircraft hadstruck power lines at a height of 100 feet andcrashed. Weather in the area included snowand fog. Other pilots flying in the vicinity hadreported encountering rime icing that some-times was severe.

Forced Landing PracticeTaught Another Lesson

Cessna 206: No damage. No injuries.

The aircraft was being used for dual instruc-tion. An instructor and a student were theonly persons aboard the single-engine aircraft.

The maneuver was forced landing practice and,although the terrain was not suitable for anactual emergency landing, the instructor re-duced power by pulling back the throttle anddeclared a simulated power failure. The stu-dent pilot accomplished the proper proceduresand established a landing approach to a stretchof road. When the aircraft had descended towithin a couple of hundred feet of trees, theinstructor ordered the student pilot to addpower and end the simulated forced landingapproach. The student applied full power.

The engine did not respond.

The instructor checked the position of the fuelselector and called for the fuel boost pump tobe activated, while watching an automobilemoving in the section of road intended for the“simulated” forced landing. The engine startedbefore an actual landing was necessary andthe aircraft climbed back to altitude withoutfurther mishap. The flight was completed andneither pilot thought further about checkingout the reason for the engine’s failure to re-spond during the forced landing practice.

The aircraft sat for a week without being flown.During preflight engine runup prior to a longflight, the pilot advanced the throttle rapidlyand the engine died. Remembering the prac-tice forced landing episode, the pilot recalled

Other GeneralAviation

OtherGeneralAviation


from the station towards which he was headed.

The pilot reported continuous light rime icingand was cleared lower to 7,000 feet, the mini-mum radar vectoring level. Upon levellingoff, however, the pilot requested a still-lowerlevel but that was not available and ATC of-fered a vector to the south off the airway andtoward lower terrain. The pilot declined theoffer, and the controller questioned if the air-craft was picking up ice and whether the pilotcould maintain his altitude. The pilot reportedthat the icing was light and that the flight wasOK.

Within the minute, the pilot requested a loweraltitude and was observed to have descendedto 6,000 before the clearance was given twominutes later. The controller indicated therewas an alternate airport eight miles away andasked if he could maintain that altitude, towhich the pilot answered “yes.”

Almost immediately, the pilot announced thathe had an engine problem and could not maintainaltitude. He requested a vector toward thealternate airport and advised that one enginewas shut down. The aircraft disappeared fromradar coverage and that was the last radiocommunication from the aircraft.

Subsequently, the aircraft entered heavy rainand showers of ice pellets as it descended be-neath the clouds and, unable to maintain alti-tude, the pilot made a successful forced land-ing in rugged terrain. One passenger was in-jured seriously, and the other occupants sus-tained minor injuries, but they all were able toexit the aircraft before post-impact fire destroyedit.

At no point during the incident did the pilotdeclare an emergency to ATC.

The basic cause of the accident was that thepilot continued flight into severe icing condi-tions in an aircraft that was not properlyequipped. Contributing factors were that hewas advised of the severe icing by a previousflight along his route and that by not declar-ing an emergency, he precluded timely assis-tance.

previous trouble with the engine fuel pump.During an engine change 200 hours previously,the fuel pump had not responded well to pressureadjustment, and maintenance personnel hadbeen having trouble adjusting the idle mix-ture recently. Further checking by the over-haul shop disclosed that the fuel pump’s low-pressure adjustment screw was loose and itwas not possible to maintain pressure adjust-ments; an excessive amount of adjusting hadcaused the screw to become loose.

Lessons learned included paying attention tosigns that the engine was not functioning prop-erly; that practice forced landings should bemade where good approaches to actual land-ing sites are available in case the engine failsto respond; and that the engine should be clearedproperly during idle descent to ensure its avail-ability for the go-around.

One Engine Out in IceBut no Emergency

Piper PA-23-250 Aztec: Aircraft destroyed. Seri-ous injuries to one, minor injuries to three.

The pilot and three passengers were returninghome from a skiing vacation in the light twin-engine aircraft.

After receiving a telephone weather briefing,the pilot filed an instrument flight rules (IFR)flight plan for 12,000 feet and, after ATC re-ported severe turbulence and heavy icing inthe general area he requested a higher alti-tude. Cleared to 14,000 feet, the pilot remainedin instrument meteorological conditions (IMC).

The pilot then requested a descent and wascleared to 10,000 feet even though severe tur-bulence and moderate to heavy icing was re-ported at that altitude. He reported that hewas only experiencing light rime icing at thetime.

During the descent, the VOR navigation re-ception from the station behind the aircraftwas interrupted and the pilot was given radarvectors to help him stay on the airway untilreliable VOR reception could be established


Dead TreeSnags Helicopter

Bell 206B Jetranger III: Substantial damage. Noinjuries.

The pilot was on the way from his home baseto pick up a passenger at a small airstrip in awooded area. Arriving at the destination, heoverflew the landing site at a height of ap-proximately 500 feet and began a wide, de-scending approach to a downwind leg.

While turning to base leg, the pilot heard athump sound and noticed that both lower vi-sion bubbles were broken. He assumed thehelicopter had struck a bird and checked theengine instruments and caution lights but noth-ing was amiss, so he continued the landing.

After touching down, the pilot left the enginerunning while he inspected for damage. Therewas damage to the lower vertical tail fin but,with the tail rotor still turning, he did notinspect the area closely. He cancelled the pas-senger flight, and cleaned the cockpit area aroundthe broken plexiglass, throwing out brokenpieces of the canopy and some pieces of woodhe assumed had blown in during the landing.Lifting off into a hover, the pilot checked thatthere were no unusual vibrations and flew backto home base.

Inspection by maintenance personnel after theaircraft had been shut down at the home facil-ity revealed widespread damage to the mainand tail rotor blades, dents and scrapes else-where on windshield and metal skin of therotorcraft and a damaged antenna. There wereno feathers or blood indicative of a bird strike;however, bits of wood and bark pointed to a

tree strike.

A visit to the landing site revealed that thehelicopter had collided with the top of a 150-foot-high dead tree approximately 250 feet awayfrom the center of the landing strip. Althoughmuch taller than surrounding trees, the deadtree had no leaves to draw attention to it.

There were numerous contributing factors in-volved. The company was short of pilots andthe workload was high; the pilot had workeda demanding 12-hour schedule the previousday. He had just returned to work from aholiday that involved visitors in the home anda sick wife, resulting in his having to assistwith the children’s schooling needs. The dayof the tree strike, the pilot had begun work at0545 and flew five attention-demanding flights;the accident flight was expected to be an easyone and he was relaxed. He made a wide lefttraffic pattern to give himself plenty of roomfor the approach, and in a helicopter that hasa blind area on the lower left, failed to see thetree.

Third AutorotationWas Not a Charm

Bell 206B: Substantial damage. No injuries.

The aircraft was being used for autorotationtraining. A flight instructor and a studentpilot were the only occupants.

The student had successfully completed twoautorotations to touchdown and was nearingthe ground on a third simulated power-off ap-proach. At a height of three feet, the studentbegan applying cushioning collective pitch andleveled the rotorcraft. Then, when the aircraftwas one foot above the ground, the studentapplied aft cyclic control, causing the rear portionof the landing skids to hit the ground. Boththe instructor and the student applied forwardcyclic control and the aircraft rocked back andforth through several cycles. By the time itcame to rest, the helicopter had sustained sub-stantial damage. The instructor and the stu-dent pilot were not injured. ♦

RotorcraftRotorcraft

Documents

Measuring Safety in Single- and Twin-engine Helicopters · 2 FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • AUGUST 1991 egories and recommends certain operational limitations