Failure of the inboard programming roller cartridge was due to undetected fatigue cracking that occurred in an area that was not included in the detailed flap actuation system inspection.
Both Boeing and the ATSB agree that failure of the aft flap programming roller will result in a reduction in safety margins. Boeing's analysis shows this to be minor and the ATSB accepts that failures such as this will have only a limited effect on controllability. However, in accordance with the ATSB's risk management standards, given the nature of the operation and the predicted frequency of occurrence, a minor reduction in safety margins still presents a safety issue. The ATSB still believes that the opportunity exists to enhance the current inspection regime to limit the likelihood of recurrence, despite Boeing's position on the adequacy of the current inspection program. However, given the agreed position on the minor reduction in safety margins and limited effect on controllability, the ATSB has closed the safety issue.
Response by The Boeing Company
On 15 November 2022, The Boeing Company provided the following response:
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.
ATSB comment in response
The ATSB acknowledges that Boeing’s risk management program does not classify this as a safety issue, but based on the reduction in safety margins, size of the operation and the frequency of occurrence—particularly in the past 5 years—the ATSB’s risk analysis procedures identify these programming roller failures as an ongoing safety issue. 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. While the cartridges are inspected in the general visual inspection of the flaps, inclusion of the cartridges in the detailed inspection would provide the greatest opportunity for cracks to be identified prior to failure.
The Australian Transport Safety Bureau 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.
Boeing provides recommended inspection intervals for the outboard aft flap, including the affected roller cartridge, in maintenance task cards that suggest inspection intervals of 6600 flight cycles or 36 months, whichever occurs first. The final inspection interval for these components is defined by each operator individually, and approved by the regulatory agency for that operator.
After learning of this report from the ATSB, Boeing performed a review of the outboard aft flap roller cartridge to determine the history of similar failures, the accumulated time on the affected airframes, and the effect (if any) the failure had on the successful completion of the flight. The ATSB final report references 10 similar occurrences that were identified in that review, some of which included failures of the roller itself (which is covered by a different inspection). In each of the reported events, the flight crew noticed and corrected for slight changes in airplane roll control, but otherwise completed their scheduled flights to their destination without further issue. These outcomes align with Boeing’s prior safety analysis for this failure, which identified a very low risk of impact to the continued safety of flight. When evaluating the rate of these occurrences across the total accumulated hours of the 737 fleet, Boeing’s review showed that the failure rate for this component remains below the maximum allowed by the FAA for airplane certification.
In light of these results, Boeing’s review of the available data showed that the current design of the roller cartridge ensures the flap system remains damage tolerant and ensures the safety of flight, and that no changes are required to the current design or maintenance program for the component.
The ATSB accepts Boeing's conclusion that failures such as this will have only a limited effect on controllability. Both Boeing and the ATSB agree that failure of the aft flap programming roller will result in a reduction in safety margins. Boeing's own analysis shows this to be minor, which the ATSB accepts. However, in accordance with the ATSB's risk management standards, given the nature of the operation and the predicted frequency of occurrence, a minor reduction in safety margins still presents a safety issue.
