Incident Description
Following a flight, the aircraft's thrust reversers did not operate during the landing roll. An engineering investigation revealed that MEL lockout pins were installed in the hydraulic control units (HCU), deactivating the thrust reverser system. The pins were installed as part of required maintenance and not removed before the flight.
Maintenance Findings
The required maintenance included replacement of the horizontal stabilizer actuator. The operator's task card, a supplemental procedure to the AMM, required a functional check of the thrust reversers after reactivation. The task card specifically noted that post-maintenance checks should be performed as stand-alone tasks. However, contrary to the written procedure, the engineers did not follow the task card sequentially. They signed off the operational check based on testing completed earlier in the day, rather than performing it in the required sequence. If the functional check had been completed in sequence, the deactivated thrust reverser would have been discovered.
Procedural Factors
Thrust reverser deactivation requires a lockout pin with a red flag to provide visual indication. In this case, an MEL lockout pin was used instead of a maintenance-specific pin. The MEL pin was designed for in-service use and was less visually obvious. Additionally, the AMM procedure for thrust reverser deactivation required specific warning labels in the cockpit stating 'thrust reverser HCU is de-activated.' However, it was reportedly common practice to use only a generic maintenance warning notice. This combination of actions removed opportunities to identify the status of the thrust reverser system during final inspection.
Contributing Factors
The maintenance schedule was affected by the replacement of the horizontal stabilizer actuator and a change to the revenue flight departure time. The engineering team felt pressure to expedite maintenance, working through meal breaks. Engineers stated that returning all tools to the tooling crib so the certifying engineer could complete paperwork was an influential factor in their decision to use the MEL lockout pin. Although the required operational check of the thrust reverser would have prevented the incident, other maintenance actions hindered detection of the HCU lockout.