Indonesia Asia A320 over Java Sea on Dec 28th 2014, aircraft lost height and impacted waters, loss of rudder travel limiter due to maintenance
Last Update: December 1, 2015 / 10:53:24 GMT/Zulu time
Incident Facts
Date of incident
Dec 28, 2014
Classification
Crash
Aircraft Registration
PK-AXC
Aircraft Type
Airbus A320
ICAO Type Designator
A320
Contributing factors
The cracking of a solder joint of both channel A and B resulted in loss of electrical continuity and led to RTLU (rudder travel limiter unit) failure. The existing maintenance data analysis led to unresolved repetitive faults occurring with shorter intervals. The same fault occurred 4 times during the flight.
The flight crew action to the first 3 faults in accordance with the ECAM messages. Following the fourth fault, the FDR recorded different signatures that were similar to the FAC CB‟s being reset resulting in electrical interruption to the FAC‟s.
The electrical interruption to the FAC caused the autopilot to disengage and the flight control logic to change from Normal Law to Alternate Law, the rudder deflecting 2° to the left resulting the aircraft rolling up to 54° angle of bank.
Subsequent flight crew action leading to inability to control the aircraft in the Alternate Law resulted in the aircraft departing from the normal flight envelope and entering prolonged stall condition that was beyond the capability of the flight crew to recover.
The captain (53, ATPL, 20,537 hours total, 4,687 hours on type) was pilot monitoring, the first officer (46, ATPL, 2,247 hours total, 1,367 hours on type) was pilot flying.
The captain had flown on PK-AXC three days prior to the crash and had experienced a rudder travel limiter unit failure during push back from Surabaya that day, Dec 25th 2014, called maintenance who assured they would have fixed the problem in no time, performed a BITE test, reset both flight augmentation computers (FAC) by pulling the circuit breakers and pushing them back in and the problem apparently went away. The captain asked whether he could repeat the same reset actions, the engineer stated that the captain could repeat that reset action whenever instructed by the ECAM. The aircraft was pushed back again, the problem resurfaced, the captain attemtped the reset procedure but the problem remained. The aircraft was pulled onto the stand again, the engineer advised it might take longer, the passengers were asked to disembark. The engineer replaced FAC2, then suggested to have both engines running, thereafter the problem did not resurface. The passengers were boarded again and the flight departed for a rotation from Surabaya to Kuala Lumpur and back without further incident.
On Dec 28th 2014, the day of the crash, the aircraft departed Surabaya without incident and was enroute at FL320 over the Karimata Strait when the captain instructed to turn anti-ice on and activated the fasten seat belt signs due to weather and possible turbulence ahead. About 3 minutes later the ECAM displayed amber advisory "AUTO FLT RUD TRV LIM 1", the first officer called for the ECAM actions.
One minute later the flight data recorder recorded the failure of both rudder travel limiter units triggering a chime and master caution associated with the ECAM message "AUTO FLT RUD TRV LIM SYS". The captain read the relevant checklists and pressed the pushbutton for FAC1 and FAC2 on the overhead panel to OFF and back to ON one by one. Both FACs successfully reset and both rudder travel limiter units returned to normal function.
Another 3 minutes later the captain requested and was approved a 15nm deviation around weather. The first officer subsequently performed a crew briefing indicating that in case of an emergency the next alternate airport would be Semarang.
9 minutes after the RTLUs failed for the first time the crew received a second indication that the units had failed again, the crew worked the checklists again and successfully reset both RTLUs.
The crew talked to ATC confirming they had deviated left off their track to avoid weather and requested to climb to FL380, ATC verified their position on radar and acknowledged the request by "standby".
A minute later the RTLUs failed again and were again reset using the checklist procedure. However, 110 seconds later the RTLUs failed a fourth time, ATC cleared the flight to climb to FL340 which was not acknowledged anymore.
Several seconds later the flight data recorder recorded the failure of the FAC1, 17 seconds later the failure of both FAC1 and FAC2, the FBW reverted to Alternate Law, the aircraft rolled left up to 54 degrees of bank. Nine seconds after both FACs failed the right side stick activated, the bank angle reduced to 9 degrees left and rolled back to 53 degrees left bank angle, the side stick was mainly pulled back, the aircraft pitched up and climbed reaching up to 11,000 feet per minute rate of climb. 34 seconds after both FACs failed a stall warning activated, briefly ceased for 1 second after 4 seconds, then remained active until end of recording.
The standby airspeed indicator recorded a lowest airspeed of 55 KIAS then settled at around 140 KIAS until end of recording. The highest altitude was recorded at 38,500 feet and a left bank angle of 104 degrees, the aircraft subsequently lost height at a rate of descent up to 20,000 fpm.
Descending through about 29,000 feet the wings were level, the airspeed was between 100 and 160 KIAS, the angle of attack was at approximately +40 degrees, the stall warning remained active and the aircraft lost height at about 12,000 fpm until end of recording.
Jakarta Radar lost transponder (ADSB) contact with the aircraft as it descended through 24,000 feet at S3.6143 E109.6974.
The flight data recorder stored the last sample at a radio height of 118 feet, the airspeed was 132 KIAS and the rate of descent 8400 fpm.
The NTSC reported no emergency message was transmitted by the crew.
The NTSC reported that the aircraft documentation recorded 23 events concerning the RTLUs in the year prior to the crash, with increasing frequency, 2 events were recorded in October 2014, 5 events in Nov 2014 and 9 events thereof in December 2014.
The FAC2, that had been removed from the aircraft on Dec 25th 2014 was examined by the manufacturer who found no fault with the unit, the unit returned to service on Jan 26th 2015.
The NTSC wrote: "An evaluation of the maintenance data showed that the maintenance action following the RTLU problems were in accordance with the TSM (TroubleShooting Manual). The actions were mostly resolved by resetting the computer by either pulling the associated CB or resetting the FAC push button and followed by an AFS test. The replacement of FAC2 was the only different action taken by the line maintenance personnel."
The RTLUs recovered from the sea bed were taken to the BEA for examination. The BEA engineering report summarised:
The examination was carried out by performing visual and other inspection of the external part as well as the internal part, including the electronic modules of the RTLU.
The channel A and channel B boards were visually examined under magnification at BEA.
The presence of cracks on solders was confirmed on the surface of both channels (Figure 35).
The summary of the examination found the electronic cards shows the evidence of cracking of soldering of both channel A and channel B. Those cracks could generate loss of electrical continuity and lead to a TLU failure.
The NTSC analysed: "Between 2301 UTC to 2313 UTC the FDR and CVR recordings indicated three Rudder Travel Limiter Unit failures occurred and triggered the chime and master caution, followed by PIC actions to ECAM actions to reset FAC 1 and 2 push-buttons on the overhead panel to OFF then to ON. Thereafter both of Rudder Travel Limiter Units returned to function normally. At 2315:36 UTC, the fourth failure on both Rudder Travel Limiter Units and triggered ECAM message “AUTO FLT RUD TRV LIM SYS” and triggered the chime and master caution light. At 2316:29 UTC, the FDR recorded parameters which indicate that FAC 1 was de-energized leading to the ECAM FAC 1 FAULT message associated with the 5th master caution. 17 seconds later the FDR recorded parameters indicate that FAC 2 was also de-energized leading to the FAC 1+2 FAULT message associated with the 6th master caution. The FAC 1+2 FAULT was followed by rudder deflected 2° to the left, the aircraft flight control status reverted from Normal Law to Alternate Law and the Auto Pilot (A/P) and the Auto thrust (A/THR) disengaged. As consequence, the pilot should fly the aircraft manually. The fault on FACs was associated with electrical interruption due to loss of 26VAC and 28VDC. Refer to the information provided by Airbus, when the loss of 26VAC was detected by the FAC, the FAC logic associated to the computation time and rudder movement inertia created a Rudder movement of about 2°. As both FAC were disengaged this rudder movement was not automatically compensated. The FDR recorded that when FAC 1 was de-energized, the rudder deflected of about 0.6° at this time the FAC 2 took over the function of FAC 1 and the auto-pilot was still engaged. The FDR also showed the deflection of aileron to compensate the aerodynamic roll caused by rudder deflection hence the FDR did not record any heading change. The FDR did not record re-engagement of the FAC 1. Seventeen seconds after the FAC 1 being de-energized, the FDR recorded that the FAC 2 was also de-energized leading to the FAC 1+2 FAULT message. As a consequence the A/P and A/THR disengaged, flight control law reverted from Normal Law to Alternate Law, and the rudder deflected 2° to the left causing the aircraft rolled to the left with rate of 6°/second. After the auto pilot disengaged the pilot had to fly the aircraft manually. However when the aircraft rolled, neither pilots input the side stick to counter the aircraft roll until nine seconds later thereby the aircraft rolled left up to 54°. The investigation concluded that the un-commanded roll was caused by the rudder deflection, the autopilot disengaged and no pilot input for nine seconds."
The NTSC continues analysis: "At 2316:39 UTC, the FDR recorded that the FAC 1 was re-energized indicated by stopping of parameter alternation. However because the FAC1 pushbutton on overhead panel was not reset by put to OFF then ON, the FAC1 functions remained unavailable and all equipment controlled by FAC 1 did not operating.
At 2316:46 UTC, the FDR parameters indicated that FAC 2 was also de-energized leading to the FAC 1+2 FAULT message associated with the 6th master caution ... At 2316:54 UTC the FAC 2 was re-energized indicated by stopping of parameter alternation. ... Returning FAC CB back in during flight does not automatically make the FAC functions to be re-engaged and recover the function of the FAC, it requires resetting the FAC push button on the overhead panel as mentioned on ECAM Procedures. Without resetting the FAC pushbutton the FAC and all related systems remain not engaged even though the FDR shows some FAC FDR parameters are re-computed and recorded."
The NTSC analysed with respect to resetting the FACs: "The Airbus developed the statement to open the possibility for the operator in some circumstances allowed to reset another computer CB when “fully understand the consequences”. One way of doing this is by consulting to Airbus. The PIC had seen the engineer resetting the FAC CB on the ground. Having experience of witnessing and performing FAC CB reset, the PIC might consider that he “fully understand the consequences”. Resetting the FAC CB on the ground and in flight has different consequences. The FAC CBs were not included in the list of the CB allowed in OEB and TDUs to be reset in flight. The consequences of resetting FAC CBs in flight are not described in Airbus documents. It requires good understanding of the aircraft system to be aware of the consequences."
With respect to the left roll the NTSC analysed: "After electrical interruption the autopilot disengaged and the ruder deflected at 2° then the aircraft rolled to the left without pilot input with a rate of 6° per second. This rate of roll was two times faster than normal roll rate operation. The SIC who acted as Pilot Flying responded 9 seconds after the autopilot off when the roll angle had reached 54°. Normally a pilot will respond immediately to level the wings when an aircraft is rolling without input by the pilot or normal system. During the autopilot disengages and the ECAM message changed which triggered the master caution and chime this might attracted the crew attention. The delayed response of SIC as PF was likely due to his attention not being on the PFD, however the investigation could not determine to what the SIC‟s attention was directed at that time."
With respect to the response to the stall warning the NTSC analysed: "The operator manual (FCOM and QRH) stated that at this condition, the flight crew must apply the stall recovery procedure by lowering the nose to reduce AOA as soon as they recognized any stall indication either the stall warning or aircraft buffet. Stall recovery procedures have been trained for both pilots. During the stall warning activated, the right side stick was at neutral then moved forward for two seconds. It caused the AOA decreased below 8°, and the aural stall warning stopped. The pitch up input of the right side stick has made the aircraft AOA increase and activated the stall warning which ceased after pitch down action was performed."
The NTSC analysed that there initially there had been responses to the upset on the right hand side stick only, later followed by responses on the left side stick and wrote: "The first left side stick input was at 2317:03 UTC for 2 seconds, then 15 seconds later another input for 2 seconds, and at 2317:29 continued in dual input until the end of the recording. The sidestick priority logic, when one pilot operates the sidestick, it will send the control signals to the computers. When both pilots move both sidesticks simultaneously in the same or opposite direction and neither takes priority, the system adds the signals algebraically. When this occurred, the two green Side Stick Priority lights are ON and followed by “DUAL INPUT” voice message activation. If this occurred, the PF or depending on the PIC instruction, should stop provides input on the sidestick or a pilot should stop the „dual input‟ by pressing the priority pushbutton for 40 seconds or more to latch the priority condition. The FDR did not record neither pilots pressed such button for more than 40 seconds. The CVR did not record “DUAL INPUT” voice message as it was supressed by “STALL” voice warning. The FDR recorded at 2317:15 UTC the aircraft pitch reached 24° up. The PIC commanded „pull down...pull down‟ and at 2317:17 UTC the FDR recorded second Stall Warning. Following the command „pull down...pull down‟ the FDR recorded the SIC side stick backward input increased. The aircraft pitch and AOA were increasing. The average of the side stick inputs recorded on the FDR since the A/P and A/THR disengaged until the aircraft encountered the second stall warning indicated that the SIC was pulling almost full back input while the PIC was slightly pushing nose-down. The sum of both side stick inputs commanded nose up pitch. The pitch up input resulted in the AOA reaching a maximum of 48° which was beyond the flight director envelope and the flight director would have been disappeared from the PFD. The pilot would no longer have guidance from the flight director. The pilot training for stall was intended to introduce the indications of approach to stall condition and recover it. While the aircraft system designed to prevent the stall by providing early warning. The pilot training and the aircraft system were intended to avoid stall. The condition of AOA 40° as recorded on the FDR was beyond any airline pilot training competency as they never been trained or experienced. The degraded SIC performance and ambiguous command of the PIC may have decreased the SIC‟s situational awareness. Consequently, the SIC did not react appropriately in this complex emergency situation. This resulted in an aircraft upset from which recovery was beyond the procedures and philosophy of training that was provided to flight crew and the increasing difficulty of aircraft handling as the result of the rudder deflection which provided roll tendency."
With respect to maintenance activities the NTSC analysed: "Based on PK-AXC 1 Year report, 23 occurrences related with the RTLU problem were recorded since January 2014. The line maintenance personnel performed similar action by resetting the FAC and doing the AFS Operational test which resulted satisfactory and the problem was considered close. Any repeating defect was treated as a new defect. Refer to the CMM chapter 5.3 Defect & Repetitive Defect stated : A defect is deemed to be repetitive when it has been reported more than once in 7 flight sectors or 3 days where 3 rectification attempts have not positively cleared the defects. Evaluation of MR1 data December 2014 found 10 pilot reports related to RTLU occurred on 1, 12, 14, 19, 21, 24, 25 (two cases), 26 and 27 December 2014. On 19 December 2014, the repetitive RTLU problem was inserted to MR2. Repetitions of the problem were not classified as repetitive problem as the rectification by AFS test were resulted satisfactory and the problems were considered solved. Actually the rectification by AFS test did not completely solve the problem. ... The company did not clearly state the policy of recording defect handling captured by the CFDS system or printed PFR and mainly based on MR1. It resulted in the line maintenance personnel did not aware of similar problem and repeat similar maintenance action, and also the problem was not recorded as a repetitive problem. None of the issues reported was identified as meeting the repetitive defect definition which would have triggered maintenance actions under the CMM requirements."
Incident Facts
Date of incident
Dec 28, 2014
Classification
Crash
Aircraft Registration
PK-AXC
Aircraft Type
Airbus A320
ICAO Type Designator
A320
This article is published under license from Avherald.com. © of text by Avherald.com.
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