AAIB Bulletin 5-2006

7/28/2019 AAIB Bulletin 5-2006

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AAIB Bulletin: 5/2006 G-BZVG EW/C2004/10/05

ACCIDENT

Aircraft Type and Registration: Eurocopter AS350B3 Ecureul, G-BZVG

No & Type of Engines: Turbomeca Arrel 2B turboshaft engne

Year of Manufacture: 2000

Date & Time (UTC): 8 October 2004 at 300 hrs

Location: Oxford Kdlngton Arport

Type of Flight: Tranng

Persons on Board: Crew - 2 Passengers - None

Injuries: Crew - (Serous) Passengers - N/A

Nature of Damage: Extensve damage to fuselage and man rotors

Commanders Licence: Arlne Transport Plots Lcence wth Instructor Ratng

Commanders Age: 53 years

Commanders Flying Experience: 6,862 hours (of whch 420 were on type)Last 90 days - 44 hoursLast 28 days - 20 hours

Information Source: Arcraft Accdent Report Form submtted by thecommander plus further enqures and examnaton ofthe helcopter and ts control system components

Synopsis

An nstructor and student were carryng out a smulated

hydraulc falure approach and landng. The student

was about to carry out a run-on landng when she

experienced difculty overcoming the control feedback

forces. The nstructor took control and attempted to

clmb the helcopter but t rolled to the left and struck

the ground. No evdence of pre-mpact mechancal

faults was found but the ssue of heavy control forces

in manual ight was well understood by the helicopter

manufacturer. Approprate procedures, advce and

gudance had been ssued, both wthn the helcopters

Flght Manual and through supplementary documents,but the plots nvolved had nether followed the Flght

Manual procedure accurately nor seen all the relevant

supplementary gudance and nformaton. One safety

recommendaton was made about the dstrbuton of

handlng advce and nformaton to plots.

General information

The chef nstructor of the Type Ratng Tranng

Organsaton (TRTO) and the nstructor on the accdent

ight had both own simulated hydraulic failure exercises

n G-BZVG. Both plots had been concerned at what

they consdered to be abnormally hgh hydraulcs OFF

control feedback forces.


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The owner purchased the helcopter n December 2003

and completed hs type ratng on 23 January 2004. On a

day that the owner beleved was 4 Aprl 2004 the chef

nstructor was carryng out a smulated hydraulc falureapproach n G-BZVG wth the owner. Just before touch

down, the owner had difculty controlling the helicopter

whch he attrbuted to hs relatve nexperence. The

chef nstructor took control and he too was unable to

exert enough force on the cyclc control to correct a roll

to the left whch was developng. He dd not want to

re-nstate the hydraulcs at such a low heght n case he

over-controlled causng the man rotor blades to strke

the ground. He rased the collectve lever and was able

to y away from the ground but not before the helicopter

had yawed to the left through 80.

Followng the ncdent, the chef nstructor and the owner

consulted the test plot of the helcopter manufacturers

mport agent. They explaned that they thought the

control feedback forces were abnormally hgh. The owner

asked the test plot to assess the control forces wthout

hydraulic power when he next ew the helicopter. The

test pilot ew G-BZVG on 14 May 2004 and carried out

a full C of A test ight in June 2004; on both occasions

he found the control forces wth hydraulcs OFF to be

normal for the type.

After the owner had experenced heavy control forces

durng a practce manual landng on 4 Aprl, he traned

regularly until he was satised that he had mastered the

technque. Also, between 30 July and October 2004,

the chief instructor conducted ve Licence Skill Tests

usng G-BZVG. A hydraulcs OFF approach to landng

was made durng each test. Although the chef nstructor

dd not handle the controls durng the exercse, none of

the candidates encountered signicant difculties.

On 9 September 2004 the test pilot ew G-BZVG and

agan found the control feedback forces to be normal for

the AS350B3. Ths nformaton was passed to both the

chef nstructor and the owner.

The ight instructor and student involved in the accident

carried out a training ight on 29 September 2004 during

whch a smulated hydraulc falure was attempted.

Both plots consdered the control feedback forces to be

abnormally hgh and the exercse was abandoned. After

the ight, the instructor informed the chief instructor of

the problem. The owner and the chef nstructor went

to see the test plot who re-terated the hgh forces to be

expected.

On October 2004 the owner and the chef nstructor

carred out one hour of smulated hydraulc falure

training. No signicant problems occurred during the

training and the owner remained condent in his ability

to y the helicopter without hydraulics should the

stuaton arse. The owner also stated that all hs practce

hydraulc falure approaches and landngs had been

carred out wth the hyd test swtch n the depressed

(test) poston.

History of the accident ight

The student was an experenced AS350B plot havng

own approximately 100 hours on that type in the USA

on her FAA lcence. The purpose of the tranng was

to carry out a type converson to have the AS350B3

varant endorsed on her UK PPL. She had accumulated

11.5 hours of ying on the B3 and the accident ight

was the second tranng sorte of that day. The same

nstructor had carred out all her B3 tranng and

was the instructor on the accident ight. During the

earler one-hour dual sorte, varous emergences were

practced ncludng smulated engne governor falure.

Ths exercse necessarly resulted n a low speed run-onlandng nto wnd.


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The nstructor had fully brefed the smulated hydraulc

falure exercse. She had observed the student

satsfactorly demonstrate the safe handlng of ths

exercse on a number of prevous occasons. On thedownwnd leg of a crcut she depressed the hydtest

swtch to smulate hydraulc falure. The student

correctly identied the emergency and reduced airspeed

to 60 kt. When the helcopter was stable the nstructor

swtched the hydraulc cut-off swtch on the collectve

control lever to off. Next the instructor conrmed that

the student was comfortable wth the feel of the controls

due to them being abnormally heavy on a previous ight.

The student consdered them normal and contnued to

y the aircraft around the circuit and made an approach

to the helicopter training area on a nal approach track

of 200. The weather was good wth a surface wnd

of 240/8 kt, vsblty 0 km and the lowest cloud at

3,000 ft. In the last few hundred feet of the approach,

the helcopter was turned nto wnd for the landng.

The approach was smoothly controlled wth speed

reducng gradually, consstent wth the correct approach

prole. As the helicopter neared the ground, still with

forward ground speed, the nose began to rse up and yaw

to the left as the collectve was rased. The nstructor

took control and wth rght tal-rotor-pedal and cyclc

nputs, attempted to lower the nose, correct the yaw

and correct the ncreasng angle of bank to the left. The

lateral cyclc control forces requred were very hgh and

the student asked f she should renstate the hydraulcs

by swtchng on the hydraulc cut-off swtch mounted on

the rght sde collectve control. Gven the large force

the nstructor was exertng and the helcopters close

proxmty to the ground, the nstructor elected to reman

n manual control. Because the nstructors physcal

efforts to correct the yaw and roll had insufcient effect,

she tried to raise the collective lever in an attempt to yaway from the ground. However, the arcraft contnued

to roll left and t struck the grass surface of the helcopter

tranng area. A wtness n another helcopter behnd

G-BZVG, also operatng n the tranng area, saw t

make a steeply banked left turn and strke the ground.The helcopter came to rest uprght on a headng of 020,

almost opposite in direction to its nal approach track

of 240.

ATC activated the crash alarm and the aireld Rescue

and Fre Fghtng Servce promptly attended the scene.

They asssted wth the removal of both plots who had

receved back njures and were subsequently taken to

hosptal. Although there was substantal damage to the

helicopter, there was no re.

Hydraulic system

Purpose and control forces

The helicopter is tted with a single hydraulic system

whch provdes the plot wth hydraulcally boosted

cyclc, collectve and tal rotor controls. Accumulators

n the man rotor servo actuator unts provde a small

energy reserve. The tal rotor servo unt also has an

accumulator and a yaw load compensator; the latter s

mounted n parallel wth the servo actuator to reduce the

control loads n the case of loss of hydraulc power. It

does so by resstng the zero-ptch return moment of the

tal rotor blades (whch s only partly compensated by

boss-type weghts).

In the event of a loss of hydraulc pressure, the man rotor

servo accumulators provde approxmately 30 seconds

of boost to enable the plot ether to land the helcopter f

t s n the hover, or to establsh the recommended safety

speed range (40 to 60 kt), whch mnmses control forces

in forward ight. The tail rotor servo unit accumulator

also powers the load compensator for a perod. The

helicopter can be own without hydraulic assistance but

control forces are hgh. Wthn the safety speed range,


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the lateral cyclc forces requred are as low as 9 lb for left

cyclc movement and lb for forward cyclc movement.

The collectve lever has a neutral force poston at about

40% torque and any movement up or down from thatposton requres ncreasng amounts of force.

If the plot attempts to hover the helcopter wthout

hydraulc assstance, the control forces change n both

drecton and ntensty as the plot attempts to mantan a

steady poston. The plot has to exert longtudnal and

lateral forces of up to 2 lb whch can change quckly

n drecton. Ths results n excessve plot workload

and controllablty problems. Durng a run-on landng

at about 0 kt, the plot may have to exert a forward

longtudnal force of up to 37 lb for less than 30 seconds

wth low lateral forces. The maxmum forces whch

may be encountered are at the extremes of the speed

envelope. These may be as hgh as 33 lb left or rght

lateral cyclc and 37 lb forward longtudnal cyclc. A

force of up to 30 lb may be requred to rase or lower

the collectve control to ts maxmum up or down travel.

The tal rotor control pedals also exhbt hgh feedback

forces, partcularly the rght pedal when the collectve

lever s rased. These forces are descrbed as very hgh

f the yaw load compensator s nactve.

System control

The hydraulc system s controlled usng the hydraulc

cut-off swtch located on the rght seat collectve lever

and the hydraulc test pushbutton on the centre console.

Hydraulc cut-off swtch

The cut-off swtch s a two poston guarded swtch (on/

off), normally remanng n the on poston. It allows

the man and tal rotor servos to be powered when the

hydraulc system s operatng normally. When selectedto off, the system s depressursed and the accumulators

on the man rotor servo safety unts are depressursed

smultaneously; ths prevents asymmetrc exhauston

of the accumulators. Asymmetrc exhauston could

cause control difculties; consequently, selecting thisswtch to off s a requred acton for ether a real or a

smulated hydraulc falure. However, the tal rotor

servo accumulator s not depressursed by the cut-off

swtch; the tal rotor servo and compensator retan ther

accumulator assstance. If system hydraulc pressure s

avalable, selectng the swtch toonmmedately renstates

hydraulc pressure to the servos and re-pressurses the

accumulators.

Hydraulc test pushbutton

The hydtest pushbutton, mounted on the centre console

between the two plots seats, has two postons. The

test poston (button pushed n) ntates the test functon

and the button out poston restores normal operaton.

The prmary functon of the hydtest pushbutton s to

enable the plot to check the functonng of the servo

accumulators before ight but it is also used to simulate

the onset of hydraulc falure durng tranng. Selectng

the test poston results n the solenod valve openng on

the regulator unt, whch mmedately depressurses the

hydraulc system. It wll also open the tal rotor servo

solenod, depressursng the tal rotor accumulator, and

wth t the tal rotor load compensator, but t allows the

man rotor servos to be powered by ther accumulatorsuntl the energy stored n them s exhausted.

Hydraulic system failure training

Hydraulc system falure s smulated by carryng

out a specic sequence of switch selections and

correspondng actons whch are documented n

the arcraft Flght Manual wthn Supplement 7.

Practce hydraulcs OFF approaches are conductedin two phases: rstly, a transition to recommended


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safety speed range from steady ight conditions and

secondly, a transton to landng.

The nstructor depresses the hyd test pushbuttonto the test poston and the student reduces arspeed

to between 40 and 60 kt. The man rotor controls

are pressursed through ther accumulators but no

hydraulc assstance s provded for the tal rotor servo

and load compensator. Once the student has stablsed

the helicopter at the safety speed, the rst phase of the

exercse s complete.

When in a steady ight condition, the instructor resets

the hydtestpushbutton to theonposton whch restores

system pressure and recharges the man and tal rotor

accumulators. Next the student selects the collectve

hydraulc cut-off swtch to the offposton whch, wthn

two seconds, ntroduces the man rotor manual control

loads. The tal rotor accumulator contnues to assst

the tal rotor servo and load compensator. Ths swtch

conguration ensures that if hydraulic power is required,

selectng the collectve hydraulc cut-off swtch to on

wll mmedately renstate the powered controls.

The recommended procedure for landng s to select a

clear at area and make a shallow nal approach which

mnmses operaton of the collectve lever. The plot

should perform a no hover, slow run-on landng, at about

10 kt, with the helicopters nose into wind. Specically,the helcopter should not be hovered or taxed wthout

hydraulc pressure assstance.

Flight Manual supplements

At the tme of the accdent, Flght Manual Supplement 7

Revson 0 (zero) was current (see Appendx A). Whlst

t requred the same tranng procedure for conductng

the smulaton of a hydraulc falure, t contaned lesscomprehensve addtonal nformaton than Revson ,

whch superseded Revson 0, partcularly regardng the

magntude of expected control forces.

Revson was rased by Eurocopter n the 25th week

of 2003. DGAC approval for the revson was granted

on 4 May 2004 wth EASA approval ganed on

2 June 2004. By that tme EASA approval was vald

for all European operators and so Eurocopter ssued

Revson to all European countres on 30 June 2004.

However, when the UK CAA receved Revson a few

days later, t was deemed not acceptable because the CAA

required Eurocopter to take account of modications

whch the CAA had requred before grantng type

approval to AS350B3 helcopters regstered wthn the

UK. At the begnnng of October 2004, when Eurocopter

dscovered that UK operators had not receved Revson ,

they prepared a new master for the UK and ssued t

wthout CAA approval (because t dd not need t snce

t had already been approved by EASA). Ths master

(revson) was released on 2 October 2004; t reached

the UK agent for the arcraft type on 29 October 2004,

days after the accdent2.

Between the rasng of Revson and ts crculaton,

Eurocopter TELEX INFORMATION, T.F.S.

No 0000053 dated 9 December 2003 was crculated

regardng hydraulc power. The TELEX was ssued as

a CAUTION and drectly applcable to the AS350B3.

Wth regard to hydraulc system falure tranng, the

followng advce was ncluded:

Footnotes

Untl September 2003, Flght Manuals ntended for European

operators were approved and ssued n accordance wth four dfferent

layouts according to the country of certication (DGAC for France,

LBA for Germany, ENAC for Italy and CAA for UK). Snce

September 2003 the EASA approved Flght Manual verson was

applcable n all member States of the European Communty.

2 At the end of December 2005, Eurocopter Servce Letter 73-00-05

was ssued to explan to operators that they wll gradually receve

normal revsons wth code letter A (EASA approved) when no

denition specicity applies, or with a code letter E when including

denition specicity.


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Over a clear and at landing area, apply the

landing procedure in accordance with the Flight

Manual: Make a at approach, nose into wind,

and perform a no-hover slight running landing at

low speed (10 kt are sufcient).

Within Revision 1 were several notes which amplied

the recommended tranng procedure. One of these

notes reterated the advce above contaned n the

TELEX message. Other notes and cautons explaned

the mportance of not attemptng to hover the helcopter

and of returnng the hydtest pushbutton to the offposton, thereby restorng system hydraulc pressure to

all the actuators and accumulators before swtchng the

hydraulc cut-off swtch to off.

The TRTO had not receved a copy of the TELEX

and neither the chief instructor nor the accident ight

nstructor had seen a copy of the TELEX. The UK agent

for the helcopter manufacturer had receved the TELEX

but t was unable to provde a record of when the TELEX

was receved or a dstrbuton lst of where and when t

was re-dstrbuted wthn the UK.

Previous incidents

On 6 July 2004, some three months before ths accdent,

the helcopter manufacturer ssued a cautonary TELEX

message (TFS No 0000088) relevant to a number of

helcopter types ncludng the AS350B and B3 versons.

The cauton on page stated the information and

instructionscontained inthistelex informationare

intended for flight crews. The message descrbed

a prevous occurrence of hydraulc problems whch

resulted n a hard landng and attrbuted some of the

difculties experienced to inadvertent operation of

the hydtest pushbutton. The stated purposes of ths

message were: to remind ight crews of the functionof the (yaw) load compensator; to remind ight crews

of the proper use of the hydraulc test functon; and

to nform plots of the consequences of unntentonal

actuaton of the hydtest pushbutton.

Airworthiness Directive

Soon after ths accdent, on 0 November 2004,

Arworthness Drectve No F-2004-74 was ssued

by the French DGAC on behalf of EASA. It requred

ncorporaton of Revson to Supplement 7 of the

Flght Manual wthn one month (t also appled to other

varants of the AS350 helcopter). The reason stated

was:

This AD is issued after having noted that some

crews do not understand how to comply with the

emergency procedures in the event of a hydraulic

power system failure or during emergency

procedure training (hydraulic failure training

procedures). The Flight Manuals have been

revised to prevent misunderstanding.

Engineering examination

A detaled examnaton of the wreckage was undertaken

after t was recovered to the helcopters mantenance

organsatons hangar at Oxford Arport. The tal rotor

blade ptch control system was found to be connected but

sezed. Examnaton found that the sezure was caused

by severe mpact damage between the tal rotor blade

balance weghts and the ptch shaft outer sleeve casng;

ths resulted n the casng beng deformed onto the shaft.

There was no evdence of a pre-mpact restrcton or

dsconnecton wthn the man rotor control systems.

All the components of the helcopters hydraulc system

were taken to the helcopter manufacturers test faclty

n France where full functonal tests on each component

were carred out. All but two of these componentsfunctioned within the manufacturers specications.


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Two of the three man rotor hydraulc servo actuators

faled to functon correctly. These two actuators were

dsmantled and t was found that they had faled the

functonal test because of damage caused durng thempact sequence.

Examnaton of the mantenance records showed that

approximately two ying hours before the accident the

tal rotor ptch control hydraulc servo actuator had been

replaced. It was replaced wth the helcopters orgnal

servo actuator that had prevously been returned to the

manufacturer for modication. This hydraulic servo

was one of the tems that, when tested, was found to

function within the manufacturers specications.

Analysis

During the accident ight the instructor had correctly

ntated the exercse by depressng the hyd test

pushbutton and the student had reduced the arspeed

to the recommended safety speed. The exercse then

devated from that requred n the Flght Manual n that

the hydraulc cut-off swtch was selected to offbefore

the hyd testpushbutton was selected out to restore

hydraulc power. The pushbutton was not moved and t

remaned n the depressed testposton for the remander

of the ight. This omission had two unwanted effects.

Frstly t depressursed the tal rotor load compensator

and thereby ncreased the rght pedal force subsequently

requred to control yaw at low arspeed. Secondly,

although the nstructor dd not accept the students offer

to select the cut-off swtch to on, even f the collectve

mounted swtch had been selected on, no hydraulc

power would have been avalable due to the system

beng n the testmode.

The circuit and initial approach had been own correctly

wth the arcraft reducng speed n the descent consstentwith the required prole. The rst indication of

difculty was the uncorrected yaw to the left. Although

the angular dsplacement was not large, the reducton

n speed caused the helcopters nose to ptch up. The

effect of the crosswnd from the rght due to the yawof the helcopter probably caused the man rotor dsc

to apback to the left to some degree. The effect of

yaw to the left would also have caused the helcopter

to roll to the left. Havng taken control, the nstructor

was surprsed by the magntude of force she needed to

exert on the cyclc control n order to try and correct the

stuaton. She consdered these forces were greater than

normal when practsng a hydraulcs OFF landng.

The physcal demands of the combned feedback forces

and the rate of change n atttude led the nstructor to

beleve that rasng the collectve was the best opton n

order to clmb away from the ground.

Conclusion

The accdent occurred durng a tranng exercse when

the helcopter was at a low heght wth hydraulc power

selected off. The approach was own with the helicopters

nose nto wnd but the nstructor had not followed the

correct sequence of hydraulc swtch selectons. Havng

taken control, the instructor was unable to exert sufcient

force on the controls to counteract the movement of the

helcopter and so control was lost.

When he ew G-BZVG on several occasions, the import

agents test plot found the control forces normal for the

type, perhaps because he was usng the correct hydraulc

falure smulaton technque. However, the TRTOs chef

nstructor and the accdent nstructor were not complyng

wth the tranng procedure stated n the Flght Manual

at Supplement 7 Revision 0. Specically, they were not

resettng the hyd test swtch before commencng an

approach to land. Ths may explan why they felt thecontrol forces were too hgh.


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Appendix A

Extract from G-BZVGs Flght Manual

AAIB WARNING NOTE: - THIS SUPPLEMENT IS OUT OF DATE


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Appendix A (Cont)

Extract from G-BZVGs Flght Manual

AAIB WARNING NOTE: - THIS SUPPLEMENT IS OUT OF DATE


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Appendix B

Example E-mal alert generated by the T.I.P.I System

Documents

AAIB Bulletin 5-2006