By Mike Mangano
ATSB Recommends Improving Boeing 737 Inspections
The Australian Transportation Safety Bureau (ATSB) has recommended that inspections for Boeing 737 wing flaps be improved, following an investigation of an incident earlier this year in Australia. The said incident — involving a Virgin Australia 737-800 — occurred on a flight from Queensland, Australia’s Gold Coast Airport to Sydney, New South Wales on April 27, 2022.
According to ATSB Chief Commissioner Angus Mitchell, an issue occurred on the Boeing aircraft, registered as VH-YFZ, “immediately after take-off.” The pilot observed the aircraft began a right-rolling tendency. Although the roll stopped after the retracting flaps, the roll occurred once again when the flaps were extended for landing in Sydney.
Mitchell said, ” 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.”
Following the investigation, the ATSB concluded the cause was due to a pre-existing fatigue crack in the flap’s inboard programming roller cartridge, which houses and secures the flap’s roller. During the inspection, part of this cartridge was found to have caught on the underside of the main flap from which the outboard flap extends, damaging the flap and preventing a smooth retraction. The remaining sections of the roller and cartridge were not found, with the ATSB noting the parts had detached from the aircraft.
Commenting on the last inspection, Mitchell said, “The last general visual inspection had been carried out on (the aircraft’s) left outboard flap, according to Boeing’s specifications, in October 2020, and no defects were found,” adding “the ATSB believes the reduction in safety margins… warrants safety improvement in the detection of fatigue cracking prior to failure.”
Boeing, according to the ATSB, does not agree that the fatigue cracking issue requires further safety action. According to Boeing’s guidance on the aircraft’s programming roller cartridge, a general visual inspection is scheduled for either every 6000 flight cycles or every 36 months.
The general inspection, described by Boeing, is to involve “a visual inspection of an interior or exterior area, installation or assembly to detect obvious damage, failure or irregularity,” adding that the inspection is to be made from “within touching distance” of the parts mentioned. According to the ATSB report, Boeing only recommends a detailed visual inspection of inboard and outboard programming rollers every 12,000 flight cycles.
Being manufactured in 2017, the Virgin Australia aircraft had only flown a total of 6,377 cycles, meaning the parts were not due for a more scrupulous inspection. The ATSB went on to say that Boeing noted a “review of prior failures showed that airplane-level effects were correctly mitigated by the flight crew,” and that the aircraft affected had landed without any incident.
Mitchell concluded by stating, “Inclusion of the cartridges in the detailed inspection would provide the greatest opportunity for fatigue cracks to be identified prior to failure.”