The Australian Transport Safety Bureau has recommended Boeing “take safety action to increase the detection of fatigue cracks in the roller cartridges of 737‑800 prior to failure.” The move comes after an investigation into an incident with a Virgin Australia 737-800, registration VH-YFZ.
The incident flight was the first flight of the day for both the aircraft and crew. Immediately after take off from the Gold Coast Airport on 27 April 2022, the pilot noticed the aircraft tended to roll to the right. The crew trimmed the rudder to counteract the roll.
As the aircraft was going through 10 000 feet, the cabin manager told the flight crew that a “muffled rumble” could be heard on the left side of the aircraft, while the captain believed they heard more of a “whine”. The crew initially put the sound down to slight damage to one of the fan blades that had already been noted and logged for maintenance.
Upon retraction of the flaps trim was no longer required, but the rolling issue returned when the flaps were extended for landing. There were no warnings in the cockpit regarding the flap symmetry.
After landing at Sydney, a walk around inspection found the left outbound aft flap hadn’t fully retracted. the ATSB investigated the incident and determined “a pre-existing fatigue crack progressed through the aft flap’s inboard programming roller cartridge resulting in component failure. This cartridge held a roller that guided the aft flap during extension and retraction. The failed cartridge affected aft flap performance when flaps were deployed and resulted in the aircraft’s tendency to roll to the right.” It’s believed the failure occurred between touch down of the previous flight but before take off of the incident flight.
Boeing specifies “a general visual inspection of the left outboard flap” when an aircraft has reached 6000 flight cycles or 36 months. The incident aircraft had flown 6377 flight cycles since it had been manufactured, with the required inspection carried out in October 2020 with no defects detected. A more thorough examination, including the programming rollers is recommended by Boeing every 12000 cycles. The ATSB notes the incident aircraft hadn’t reached that many cycles.
The ATSB notes that Boeing has received 10 reports of similar instances between 2017 and 2022. The ATSB also noted, “Boeing performed a safety analysis of this condition to determine whether the event presented a potential safety issue to the affected 737 fleet. …. Boeing found that the aircraft remained well within the bounds of controllability by pilots, and the issue was evaluated to result in a slight increase in workload for the pilot in the worst possible case. Based on prior occurrences, Boeing determined that fleet experience aligned with that severity study, and that the rate of occurrence was less than that required by the United States Federal Aviation Administration for the level of hazard presented.”
As a result of the investigation the ATSB issued a safety bulletin stating the ATSB “recommends that The Boeing Company takes safety action to increase the detection of fatigue cracks in the roller cartridges of 737‑800 aircraft prior to failure.”
In the Safety Issue bulletin the ATSB included Boeing’s response dated 15 November 2022: “Boeing does not concur that this event represents a safety issue, when analyzed within our FAA‑approved risk management program. A review of prior failures shows that airplane-level effects were correctly mitigated by flight crews and the affected aircraft landed without further incident. Boeing’s review indicates that the event does not represent a significant reduction in airplane safety margins, and that the current inspection program is adequate.”
The ATSB comment in response to Boeing: “The ATSB acknowledges that, based on consideration of the consequence of the component failure, Boeing’s risk management program does not classify this as a safety issue. However, the ATSB also believes that the reduction in safety margins involving a passenger‑carrying aeroplane and the frequency of occurrence—particularly in the past 5 years— warrants safety improvement in the detection of fatigue cracking prior to failure.
“The ATSB also notes that 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.”
The full ATSB report can be found here.
The full ATSB Safety Issue bulletin can be found here.