LONDON – An Australian Transport Safety Bureau (ATSB) investigation found multiple occurrences involving fatigue cracks and failures on Boeing 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.

The incident

On the morning of 27 April 2022, a Boeing 737-800 registered VH-YFZ and operated by Virgin Australia Airlines departed from Gold Coast Airport, Queensland.

Immediately after take-off, the pilot noticed the aircraft tended to roll to the right, and trimmed the rudder to keep wings level.

The aircraft no longer required trim when the flaps were retracted for cruise, but the issue returned when the flaps were extended for landing.

There were no warnings of flap skew or asymmetry provided to the flight crew. A subsequent engineering inspection found several components in the aft flap actuation system had failed.

ATSB finding

“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.”

ATSB rectifying action

While it was identified that failure of this component would not significantly affect the controllability of the aircraft, the ATSB issued a safety recommendation to The Boeing Company.

The recommendation urged Boeing to take safety action to increase the detection of fatigue cracks in the roller cartridges of 737‑800 prior to failure.

Boeing disagrees

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.”

A full copy of the ATSB Report can be read here.