- Pilot of Boeing 737 noticed aircraft tended to roll to right after take-off from Sydney;
- Post-flight inspection found several components in left outboard aft flap actuation system had failed;
- ATSB recommends improving inspection specifications to capture potential fatigue cracking in key parts of flap mechanism.
An Australian Transport Safety Bureau investigation found multiple occurrences involving fatigue cracks and failures on 737 wing flaps in a location not included in the detailed flap actuation system inspection.
The investigation stemmed from an incident involving a passenger flight from Queensland's Gold Coast Airport to Sydney, NSW, operated on 27 April 2022 by a Virgin Australia 737-800, registered VH-YFZ.
"Immediately after take-off the pilot noticed the aircraft tended to roll to the right, and so trimmed the rudder to keep wings level," ATSB Chief Commissioner Angus Mitchell said.
The aircraft no longer required trim when the flaps were retracted for cruise, but the issue returned when the flaps were extended for landing into Sydney.
"A walk-around inspection after the flight found the outboard aft flap on the left wing had not completely retracted, and a subsequent inspection found several components in the aft flap actuation system had failed," Mr Mitchell said.
The ATSB determined that a pre-existing fatigue crack progressed through the aft flap's inboard programming roller cartridge, resulting in component failure.
"The last general visual inspection had been carried out on VH-YFZ's left outboard flap, according to Boeing's specifications, in October 2020, and no defects were found," Mr Mitchell said.
"While it could not be determined whether the fatigue crack was present at that inspection, 10 other instances of cracking and/or failure of the programming roller were reported to Boeing between 2017 and 2022, and at least six of these were old enough to have been inspected several times prior to failure.
"Significantly, the area in which the fatigue cracks developed was not included in the detailed inspection that Boeing specified for the flap actuation system."
Boeing has advised the ATSB that it does not agree that this issue warrants safety action - noting that a review of prior failures showed that aeroplane-level effects were correctly mitigated by flight crews, and the affected aircraft landed without further incident.
"While the ATSB acknowledges that Boeing's risk management program does not classify this as a safety issue, the ATSB believes the reduction in safety margins involving a passenger-carrying aeroplane, and the frequency of occurrence - particularly in the past five years - warrants safety improvement in the detection of fatigue cracking prior to failure," Mr Mitchell said.
"A detailed inspection of the flap actuation system already exists, and while it includes the aft flap rollers, it does not include the cartridges that house them. Inclusion of the cartridges in the detailed inspection would provide the greatest opportunity for fatigue cracks to be identified prior to failure."
You can find here the report: AO-2020-029: Flight control systems occurrence involving Boeing 737-800, VH-YFZ Gold Coast Airport, Queensland, on 27 April 2022
