What happened
Following the landing of the aircraft, the number four engine thrust reverser remained in the deployed position. During this time, a tailpipe fire developed. The crew received no indications within the cockpit regarding either the extended state of the reverser or the occurrence of the fire. Ground personnel observed the situation and notified the flight crew of the fire, after which the engine was shut down.
The investigation
A teardown inspection of the engine components was conducted to determine the cause of the failure. Investigators found that the thrust reverser pneumatic drive unit (PDU) had been incorrectly assembled during its previous overhaul, which had occurred 113 hours and 26 cycles prior to the incident.
Technical measurements revealed that the required stroke for the PDU interlock mechanism is between 1.318 and 1.326 inches; however, upon disassembly, the measured stroke was only 1.230 inches. This insufficient stroke prevented the gear change mechanism from fully shifting from the extend position to the retract position. Additionally, examination of the PDU retract pawl fracture surfaces showed evidence of overstress.
Temperature data indicated that while the lowest ambient surface temperature the aircraft had encountered during the preceding 26 landing cycles was -13 degrees Fahrenheit, the surface temperature at the time of the PDU failure was -27 degrees Fahrenheit.