Aer Lingus A333 over Iceland on May 11th 2010, broken aileron actuator castings
Last Update: September 20, 2012 / 15:05:06 GMT/Zulu time
1. Fracture of the outer bracket attaching the RH inboard aileronÂ’s active (outside) servo, due to fatigue.
- Manual Â“picklingÂ” process which caused surface pitting greater than that allowed or prescribed in the ManufacturerÂ’s drawings
- Failure to detect cracks during HFEC inspection prescribed in a ManufacturerÂ’s SB.
The AAIU reported the aircraft was enroute at FL330 at position N64.7502 W19.3334 when it encountered turbulence, the crew suspecting it was wake turbulence, resulting in a moderate uncommanded roll of 11 degrees to the left and yaw. The autopilot quickly returned the aircraft to straight and level flight. Soon after the aircraft was cleared to climb to FL380. The crew entered the new altitude into the altitude selector and the autopilot began to climb the aircraft to the newly assigned altitude. The crew observed a lower than expected climb rate at a higher than anticipated fuel consumption rate and began to troubleshoot this loss of performance. During troubleshooting the crew noticed on the flight controls page of the ECAM that the right hand inboard aileron was deflected to about 2/3 of its maximum travel above its normal position, both left hand ailerons were deflected upwards by about 1/3 of their maximum travel. No ECAM warnings or cautions were present.
The captain (52, ATPL, 19,200 hours total, 4,931 hours on type) was assisted by a first officers (35, ATPL, 6,386 hours total, 2,108 hours on type) and a third pilot (49, ATPL, 17,700 hours total, 2,100 hours on type), so that he could leave the cockpit and have a look onto the ailerons himself to verify their physical position corresponded with what was indicated in the cockpit. The flight crew operating manual had no specific procedures for the scenario and suggested the flight could be continued if no abnormal handling was observed, a fuel burn increase of 6 to 16 percent was to be expected. The flight crew contacted dispatch via ACARS, it was considered to reset the primary flight control computer #2 but maintenance advised against this. The flight crew verified the aircraft could be handled normally by selecting a different heading and was satisfied to see the aircraft turned properly. The cabin was secured, the fasten seat belt sign illuminated, cabin crew was seated, then the flight crew disconnected the autopilot and verified they had no control difficulties. Subsequently the autopilot was re-engaged without difficulty.
After consideration of their options the flight crew decided to continue the flight to Chicago. Upon approach to Chicago the crew requested the longest runway 10/28 available advising they had a flight control system problem of undetermined nature, but did not declare emergency. ATC however decided they had to declare emergency in order to get assigned to runway 28. The aircraft landed without any difficulty.
A post flight inspection revealed the brackets attaching the outside active servo-control of the right hand inboard aileron to the right hand wing rear spar were fractured and the lower wing skin underneath the bracket's attachment points was deflected. Further examination showed the brackets on the left hand side were cracked in the same area that the right hand brackets had failed.
The flight data recorder revealed that the autopilot had responded to the initial turbulence event and applied corrective aileron deflections of 14 degrees up and 12 degrees down on the right hand inboard aileron. The aircraft rolled to a right bank angle of 20 degrees, the aircraft rolled again left to 6 degrees then stabilized in wings level attitude. The right hand inboard ailerons stabilized in 15 degrees up position, the outboard right hand aileron stabilized in 7 degrees down position, the left hand ailerons at 7 and 6 degrees up.
Examination by the manufacturer identified the outer bracket as primary point of failure, the cracks originating from a pit of 50 microns depth believed to have existed since manufacturing of the bracket. Following the failure of the outer bracket the inner bracket failed due to overload.
The AAIU analysed that the triggering event for the fracture of the bracket was the turbulence event. The flight crew detected the effects of the failure despite the lack of a specific indication on the ECAM. The flight crew followed a structured approach to troubleshooting and considered all relevant factors before making the balanced decision to continue the flight.
The manufacturer had issued a service bulletin requiring inspection of the brackets using High Frequency Eddy Current inspection. That inspection had been carried out on Dec 22nd 2009 on the incident aircraft, 196 flight cycles before failure, with no cracks detected. The manufacturer stated as result of their examination it was likely however that cracks were already present at that inspection, there was no clear explanation why the inspection did not detect those cracks.
The investigation identified however that the inspection was prone to probe slippage causing false positive readings. The original SB also contained drawing of a wrong bracket pair possibly causing confusion.
The AAIU continued to analyse that the pit was likely the result of the manual pickling procedure, a process which had been automatised in 2003. The automation seems to have addressed the problem.
This article is published under license from Avherald.com. © of text by Avherald.com.
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