MR-SAFETY1_AAIB-Bulletin-promo.jpg AAIB Bulletin

Regulators In UK Critical Of Deferred Maintenance Procedures

UK accident investigators identify maintenance deferral as contributing factor in power loss incident.

Printed headline: Defer It

A recent UK Air Accidents Investigation Branch (AAIB) report highlighted several shortcomings in a cargo operator’s maintenance practices. Investigators specifically pointed to a weakness in the operator’s defect management system that allowed the airline to utilize minimum equipment list (MEL) procedures to further operational objectives—a practice, said the AAIB, unsupported by the “spirit” of European Union Aviation Safety Agency (EASA) regulations.

An October 2018 cargo operation involving a Boeing 737-4Q8 heading to East Midlands Airport from Amsterdam experienced an electrical failure during its final 20 min. of flight. After a successful manual approach and landing, the crew reported that an electrical failure had affected the autopilot, flight instruments, autobrakes, exterior lights and speed and flap indicators.

The electrical failure was ultimately attributed to the incorrect installation of a right-engine generator control unit (GCU), which was not properly secured in a mounting tray and became disconnected in flight.

In the 12 days leading up to the incident, the aircraft operated in accordance with MEL procedures, which allowed for dispatch with one inoperative engine generator as long as the APU generator was operating normally and used throughout the flight. When the acceptable deferred defect (ADD)—later traced to a burnt pin due to a known problem that arises when pins and connectors are not properly connected—was not cleared within the MEL time limits, a one-time Rectification Interval Extension was authorized.

Investigators concluded that while there had been frequent opportunity for the operator to address the cause of the electrical faults, the fault-finding was either stopped or not started due to insufficient time during turnaround to carry out the work. They concluded that the operator “did not appear to use the MEL in the spirit of EASA’s acceptable means of compliance . . . [which] allow the aircraft to return to its main operating base where the faults could be rectified.”

Investigators assert that the operator instead used its MEL procedures to meet operational commitments, evidenced, they said, by the frequent stoppage of fault-finding partway through on a number of occasions. “The burnt pins on the feeder cable was a known fault . . . . An engineer correctly identified that there was a [feeder fault] and inspected the connector between the engine and pylon but ran out of time to check the connector between the pylon and wing where the burnt pin was located,” said the report. Investigators also assert that no evidence was found that the operator had plans to ensure the aircraft was given sufficient downtime to rectify the fault and clear the deferred defect.

While the report did not assert that the ADD in any way contributed to the electrical failures, investigators suggested in their report that the GCU disconnect might not have occurred but for the continuous stops and starts to address the defect. “While there was no record in the aircraft technical log or worksheets for the previous 12 days of [the GCU] having been disturbed, messages on the company’s [Flight Status Reporting system] state that it had been disconnected on three occasions during this period as part of the fault-finding to clear the ADD on the left-engine generator.”

The report also identified several contributing factors, including shortcomings in the operator’s safety management system and lack of communication between line maintenance and the Part 145 repair station that investigators deemed ineffective at highlighting the underlying technical issues on the aircraft. 

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