ACAS II Bulletin 12 - Focus on Pilot Training

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February 2011 | N12

WELCOME Event 1: overreaction to RA causes injuries Welcome to this new look ACAS IIBulletin. Although we have a new layout,the aim of the publication remains thesame, to alert operators and pilots toimportant safety issues, bothoperational and technical, relating toTCAS.

TCAS is the only commercially availableimplementation of the ICAO standardfor ACAS II. Whilst the terms are virtuallyinterchangeable, we tend to refer to ACAS when talking about the concept,and TCAS when talking aboutequipment, incidents, or encounters.

A clear understanding of the definitions,system operations and operating procedures is vital if ACAS is to achievesafety benefits. ACAS II has beenmandated for several years, yetsituations with inappropriate pilotresponses are still being reported. Theseinappropriate responses have led tosevere loss of separation or, in extremecases, even accidents.

This ACAS Bulletin focuses on pilottraining, featuring a number of events

based on real-life incidents. Pilot actionsin the featured events serve to underlinethe importance of both initial andrecurrent training so that flight crewsrespond correctly to RAs. This Bulletinalso presents summary findings of somerecent research into how pilots react toRAs.

Stanisaw Drozdowski Air Traffic Service UnitEmail: [email protected]

A B777 at FL340 is flying a heading ofapproximately 220 degrees. A B757flying in the opposite direction iscleared, erroneously, to descend fromFL390 to FL310 at a time when theaircraft are flying head-on,approximately 70 NM apart.

Some four minutes later, when the B757is descending through FL343 and is 13NM in front of the B777, the controllerdetects the conflict and instructs theB757 (using non-standard phraseology)

to stop descending. There is noresponse from the B757 pilot but theaircrafts vertical rate starts to decrease.Five seconds later the controllerinstructs the B777 to turn right onto aheading of 270 which is acknowledgedby the crew. Coordinated RAs aretriggered the B777 receives a ClimbRA while the B757 receives a DescendRA.

The B777 crew responds to the ClimbRA establishing a climb rate of 1550

ft/min within 9 seconds; however, theydo not report the RA to ATC.

When the Descend RA is issued theB757 is at FL338. The pilot pushes hardon the controls and the descent rateincreases rapidly. Simultaneously, thepilot wrongly notifies ATC that he isresponding to a Climb RA. Within 8seconds of the RA the B757 vertical rateincreases to almost 12100 ft/min. Then,the pilot pulls back to arrest theexcessive descent rate, reducing it to

approximately 1350 ft/min in the next11 seconds. The RA onboard the B757weakens to Adjust vertical speed andsoon after a Clear of Conflict messageis posted.

The excessive reaction to the DescendRA caused the B757 to descend over2000 ft in just 17 seconds, its airspeedincreased by 30 kts and its pitch anglechanged from 4.4 degrees nose up to17.8 degrees nose down in just 7seconds. When the pilot pulled back to

01-2 Event 1: overreaction to RA causes injuries03-4 Events 2 & 3: contrasting responses to altitude crossing RAs03 Research confirms need for pilot training04 ICAO guidelines for recurrent training04 Sources of training materials04 Conclusions
mailto:[email protected]:[email protected]


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Event 1continued

arrest the excessive descent rate theacceleration changed from 1.06g to+2.48g (in 4 seconds) The resulting G-

forces caused injuries to 21 occupants (4seriously) and damage to the interior ofthe aircraft. The aircraft had to divert to a

nearby airport to seek medicalassistance for the injured.

Learning points: Pilots should avoid excessive responses to RAs: RAs never require vertical rates as excessive as described above.Responses to RAs must be followed as indicated on the flight deck instruments.

Vertical rates for initial RAs: For initial RAs requiring a change in vertical speed, initiation of a response in the properdirection must be made within 5 seconds of the RA being displayed. For example, the Descend and Climb RAs requirevertical speeds of 1500 ft/min. The change in vertical speed is accomplished by an acceleration of approximately 1/4 g to obtainthe required vertical speed. Vertical rates for subsequent RAs: The response time is reduced to 2.5 seconds for subsequent RAs. The accelerationwithin which the change in vertical speed is accomplished is 1/3 g for increase rate and reversal RAs and 1/4 g for weakeningand strengthening RAs. Practical advice how to achieve the required acceleration is provided in JAA-TGL 11: An acceleration of approximately1/4 g will be achieved if the change in pitch attitude corresponding to a change in vertical speed of 1500 ft/min is accomplished inapproximately 5 seconds, and of 1/3 g if the change is accomplished in approximately 3 seconds. The change in pitch attitude requiredto establish a rate of climb or descent of 1500 ft/min from level flight will be approximately 6 degrees when the True Air Speed is 150 kts,4 degrees at 250 kts, and 2 degrees at 500 kts. (These angles are derived from the formula: 1000 divided by TAS.)

Events 2 & 3: contrasting responses to crossing RAsRAs requiring the pilot to cross throughthe altitude of an intruder aircraft arerare. They account for less than 2% of allRAs. ACAS is designed to select non-altitude crossing RAs if these provide thedesired vertical separation at the ClosestPoint of Approach. Only when thatcannot be achieved will an RA withaltitude crossing be posted. They areless intuitive than other RAs as they maygive the pilot an impression that theyare being wrongly directed towards theother aircraft. The following two eventsillustrate contrasting flight crewresponses to crossing RAs, withcorrespondingly different outcomes.

Event 2An RJ85 is cleared after departure toclimb to FL150 on a heading of 330degrees. An A330 is flying on a headingof 300 degrees descending towards itsdestination. The predicted trajectories ofboth aircraft are expected to cross with

a horizontal separation of less than 1mile. The air traffic controller planned toclear the A330 to FL160 (1000 ft abovethe RJ85). However, the controller clearsthe A330 to FL150 by mistake.

Some time later, the controller instructsthe RJ85 to turn right onto a heading of345 degrees. When the aircraft are 2.5NM and 2100 ft apart TAs are issued forboth aircraft. A few seconds later, Short Term Conflict Alert warns the controllerof the impending conflict. The controllerissues avoiding action instructions toboth aircraft:A330 turn right heading 360 degreesRJ85 turn left heading 270 degrees.He subsequently gives the A330 afurther instruction to turn onto aheading of 035 degrees and providestraffic information to the RJ85 pilot.

When the aircraft are 1.6 NM and 850 ftapart, a first RA is issued for the A330

Maintain vertical speed, crossingmaintain. The A330 at this point isdescending at almost 2500 ft/min andthis RA tells the pilot to continue thisvertical speed crossing through the levelof the threat aircraft. Two seconds later,the RJ85 which is climbing at 1500ft/min, also receives a Maintain verticalspeed, crossing maintain RA. Both pilotsfollow their RAs and make reports toATC.

When the aircraft pass each other theA330 is over 600 ft below the RJ85, bothRAs weaken to Adjust vertical speed,adjust which is soon followed by aClear of conflict announcement.

Learning points: RAs provide successfulmitigation against the risk of a mid-air collision. Follow RAs: Correct andprompt responses to RAs arenecessary to reduce the risk ofserious incidents or accidents. Crossing RAs: Although theyare rare, training programmesshould cover them.

Event 3A Cessna Citation is on the groundrequesting start up and departureclearance. The Citation is not equippedwith TCAS II. Its maximum take-off mass

is below 5700 kg, so the aircraft isoutside the current equipage mandate. The tower controller clears the Citationto climb to 3000 ft after take-off. Thepilot reads back 4000 ft and thisincorrect read-back is not detected by

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the controller. Consequently, onceairborne the aircraft is climbing to 4000 ft.At the time when the Citation getsairborne, a B777 is being vectored for anILS approach to its destination. The B777is cleared to descend to 4000 ft on aheading of 185 degrees. The B777 andthe Citation are on radio frequencies ofdifferent ATC sectors.

As the aircraft converge, the Citation isturning onto a northerly heading,passing through 3000 feet climbing3300 ft/min. At this point the B777 crewgets a TA against the Citation. Althoughnot required, the B777 pilot reports the TA to ATC, We have a traffic alert.During this radio transmission aCrossing descent RA is generated.

The B777 controller sees the conflictdeveloping and suspects that an RAcould already have been issued (but not

yet reported). He is aware that he shouldnot issue any instructions to aircraftexperiencing RAs, so he asks the B777crew if they can climb back to 5000 ft,rather than instructing them to climb.Soon after that the RA on board theB777 strengthens to Increase descent.

The B777 crew does not respond to theRAs and the B777 levels off at 4000 ft.Later, the Captain stated that seeing the

other aircraft on the TCAS display hewas concerned that a descent wouldput his aircraft closer to the Citation.

The aircraft pass each other on areciprocal heading just 0.5 NM aparthorizontally and 100-200 ft vertically.Just seconds before they pass eachother, the B777 receives a Climb RA. The B777 crew responds to this RA butby then the Citation is already behind.

Learning points:

B777 crew: Increase descent or Increase climb (i.e. strengthening RAs). RAs mustbe followed as a matter of priority. An initial RA will strengthen if insufficientresponse to the initial RA is detected, or if either aircraft accelerates toward theother aircraft. Pilots must be prepared to respond to strengthening RAs within 2.5seconds. Climb NOW or Descend NOW (i.e. reversal RAs) will be posted when theintruder aircraft manoeuvres vertically in the direction of the other aircraft and thecurrent RA is deemed to be not effective any more. RA reversals are rare (less than1% of all RAs). Pilots must be prepared to respond to reversal RAs within 2.5seconds. TCAS traffic display: it must not be used to try to determine the degree ofcollision risk. Reporting to ATC: RAs that require a departure from the current ATCclearance or instruction must be reported to ATC. TAs are not required to bereported to ATC.

Citation crew: Vertical speed: 1000 feet before level off the vertical speed should not exceed1500 ft/min if the flight crew is made aware of another aircraft at or approachingan adjacent altitude or flight level.

ATC: In the absence of an RA report , ATC horizontal avoiding instructions will notadversely affect TCAS II RAs.

Do pilots always react to RAs and arethey always flown correctly?

Recent research by E UROCONTROL hasfound that climb/descend RAs are

correctly complied with in 65% of casesin en-route airspace and only 55% ofcases in TMAs. The graph adjacentprovides a breakdown of RAs that arenot correctly complied with in TMAsand airspace respectively.

ACAS training is crucial to ensure thatpilots correctly interpret and react toRAs and the ACAS pilot training isvery important. Prompt and correctreaction to RAs is fundamental. Thepotential consequences of not reacting

to an RA are clear, and responses thatare too weak or too aggressive can havea negative impact on the effectivenessof TCAS.

Research shows the importance of pilot training

Reaction to RAs in TMAs ( FL135)

Study results for climb/descend RAs

In 19% of the cases in TMAs and 21% in En-Route airspace, climb/descend RAs arefollowed, but with an inappropriate vertical speed, either too high or too low. In26% of the cases in TMAs and 14% in En-Route airspace, there is either no reactionfrom the crew or an opposite response.

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Sources of training material

Replay Interface for TCAS AlertsRITA2 (Replay Interface for TCAS Alerts)is a tool designed for the ACAS IItraining of air traffic controllers and alsoas an additional source of information to

reinforce the ACAS training of flightcrews.

RITA2 provides the synchronous replayof reported and analysed ACAS II (TCASII Version 7) events, which have beenselected for their relevance to trainingissues. It shows on the same screen,both what the pilots and the controllercould see and the radio exchanges.

The most appropriate use of RITA2 isthrough briefing sessions to small

groups. The replay of relevant eventswith RITA2 may also help introductorypresentations on ACAS II (performances,operational impact, regulation, etc.).

The package is available to qualifiedstakeholders as a CD-ROM for PC. Theuser guide includes descriptions,commentaries and lessons-learnt abouteach incident.

ICAO guidelinesfor recurrent trainingICAO guidelines for ACAS stipulate bothpractical manoeuvre training usingaircraft simulators (or suitable CBT) andtheory training for pilots. ACAS shouldbe included in recurrent trainingsessions and flight crews must be testedto ensure they are fully conversant withACAS procedures, capabilities andlimitations, and know how to respondcorrectly to RAs.

The ICAO guidelines for pilot training onACAS / TCAS are contained in Doc. 9863and in the attachments to PANS-OPSDoc. 8168. The relevant sections of thesedocuments as well as training materialscan all be found on the E UROCONTROL website: www.eurocontrol.int/acas .

Presentation for pilotsWhilst many aircraft operators havedeveloped their own training materialsand company bulletins, other sources ofmaterial exist, including an updatedEUROCONTROL Overview of ACAS II presentation for pilots covering:

The EUROCONTROL presentation can bedownloaded from:www.eurocontrol.int/acas

ACAS II BulletinsACAS II Bulletins provide practical event-related information and learning points. They can be downloaded free of chargefrom Bulletins and Safety Messagespage of the E UROCONTROL website:www.eurocontrol.int/acas

ACAS trainingACAS training material is also availableon the E UROCONTROL IANS E-learningsystem. Whilst developed originally forair traffic controllers, it also containsinformation relevant to pilots including:

The full module can be accessedwithout prior registration at:trainingzone.eurocontrol.int under theDirect Access Modules, Air TrafficControl folders.

TCAS videos There are TCAS training videos clipsproduced by NATS and Flight SafetyFoundation available on SKYbrary:www.skybrary.aero

How TCAS works How pilots should respond to TAsand RAs Examples of common problemsand learning points

Conclusions Whilst RAs are rare events, whenthey happen the situation may becritical, and correct action must betaken. Recurrent training will improveflight crew understanding of how

TCAS works, how they shouldrespond to RAs, as well as thelimitations of TCAS. Monitoring programmes haveidentified a number of situationswhere pilot responses areinappropriate aircraft operators andtraining facilities should considermaking these the focus of recurrenttraining sessions. Updated training material forpilots is available on the E UROCONTROL

ACAS training pages:www.eurocontrol.int/acas

All events described in this ACAS IIBulletin are based on real-life incidents.They have been de-identified here tosupport training and promote safetyculture. Descriptions of the events havebeen simplified and/ or abbreviated forclarity and to facilitate the training process. The sole objective of the ACAS IIBulletin is the prevention of futureaccidents and incidents rather thanassigning blame.

Overview of the ACAS concept Description of the responsibilitiesof both pilots and air traffic controllersduring a TCAS RA encounter Description of the TCAS II logic andits operation.

Whilst this issue focuses on training forlight crew, training is equally important

for air traffic controllers. This will be thefocus of the next ACAS II Bulletin.

f

Contact

Email:[email protected] February 2011 - European Organisation for the Safety of Air Navigation (Eurocontrol) This document is published by Eurocontrol for information purposes. It may be copied inwhole or in part provided that Eurocontrol is mentioned as a source and to the extent justified by the non-commercial use (not for sale). The information in this document may notbe modified without prior written permission from Eurocontrol. The use of this document isat users sole risk and responsibility. Eurocontrol expressly disclaims any and all warrantieswith respect to the document, expressed or implied. Additionally, the disclaimer availableunder www.eurocontrol.int/acas applies to the information contained in this bulletin.
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ACAS II Bulletin 12 - Focus on Pilot Training