Event Description
After landing, the number four engine thrust reverser remained deployed. A tailpipe fire occurred. No cockpit indications of either the fire or the extended reverser were reported. Ground personnel observed the fire and notified the crew, who subsequently shut down the engine.
Investigation Findings
A teardown inspection revealed that the thrust reverser pneumatic drive unit (PDU) had been incorrectly assembled during its previous overhaul. This overhaul had taken place 113 hours and 26 cycles before the failure. The required stroke of the PDU interlock mechanism is between 1.318 and 1.326 inches. Upon disassembly, the measured stroke was 1.230 inches. This stroke is necessary to allow the gear change mechanism to fully shift from the extend position to the retract position. The fracture surfaces of the PDU retract pawl were indicative of overstress.
Environmental Conditions
The lowest ambient surface temperature the airplane had been exposed to at various airports during the 26 landing cycles was -13 degrees Fahrenheit. The surface temperature at the time of the PDU failure was -27 degrees Fahrenheit.