What happened
During the landing roll of a flight, the thrust reversers failed to operate. Subsequent engineering inspections determined that the thrust reverser system had been deactivated because MEL lockout pins remained installed in the hydraulic control units (HCU). These pins had been placed in the units as part of a necessary maintenance procedure but were not removed before the aircraft's departure.
The investigation
The investigation focused on why the deactivation was not detected during the post-maintenance inspection. While the Aircraft Maintenance Manual (AMM) did not mandate a functional check of the thrust reversers following reactivation, the operator's specific task card required it. The investigation found that the engineers did not follow the task card in the correct sequence. Instead, they signed off on the operational check based on testing performed earlier in the day, likely in an attempt to accelerate the aircraft's return to service.
Several factors hindered the detection of the lockout pins. The specific MEL lockout pin used was less visually prominent than the standard maintenance pin, making it harder to identify. Additionally, the engineers did not use the required specific cockpit warning labels, opting instead for a generic maintenance notice. The investigation also noted that recent changes to the maintenance schedule and the need to meet a revenue flight departure time created significant pressure on the engineering team.
Findings
- The engineers failed to follow the sequential steps outlined in the operator's task card.
- The failure to perform the required functional check prevented the discovery of the deactivated thrust reverser system.
- Pressure to expedite maintenance and return tools to the crib influenced the decision to deviate from established procedures.
- The use of a less visible MEL lockout pin and generic warning labels reduced the opportunities for error detection during final inspections.