F-22A APU Overheat Mishap at Nellis Air Force Base (06-4109)

Incident Overview On 30 October 2020, at approximately 0930 local time, an F-22A with tail number 06-4109 experienced an overheat condition in the Auxiliary Power Unit (APU) exhaust bay at Nellis Air Force Base, Nevada. The aircraft was assigned to the 422d Test and Evaluation Squadron, 53d Wing, headquartered at Eglin Air Force Base, Florida, and was maintained by the 757th Aircraft Maintenance Squadron, 57th Wing, at Nellis AFB. The estimated cost to replace damaged parts and repair the aircraft was $2,690,000. ## Maintenance Background The aircraft began an extensive modification on 26 June 2020 to prepare for operational test missions. On 28 October 2020, the APU Mixing Exhaust Duct (AMED) was removed to facilitate troubleshooting of the modification. During that work, applicable circuit breakers were not pulled and collared, and warnings were not applied to the aircraft structure or digital forms in accordance with Technical Order guidance. These errors were not corrected by the on-scene 7-Level supervisor who verified the work. ## Sequence of Events On 30 October 2020, the aircraft required defueling and reconfiguration of doors via Aerospace Ground Equipment, but a decision was made to use the APU instead. The APU Emergency-Off Switch was incorrectly set to “Normal” by an unknown person. During pre-procedural checks, a maintenance member failed to recognize that AMED installation was required before APU operations. After the APU was started, smoke began emanating from the APU exhaust bay and into the left main landing gear wheel well. The maintenance member delayed emergency APU shutdown to review digital forms for fault reporting codes. A nearby maintenance member then set the APU Emergency-Off Switch to “Emergency Off,” shutting down the APU manually. ## Findings The Accident Investigation Board President found, by a preponderance of the evidence, that the cause of the mishap was improper maintenance procedures resulting in the start of the APU while the AMED was removed. Four additional factors were found to have substantially contributed: (1) the culture of the mishap unit, including limited use of circuit breaker collars and inconsistent use of warnings; (2) the design of test instrumentation on the aircraft, which obscured access to applicable circuit breakers; (3) the extensive nature of the aircraft’s modification; and (4) distractions caused by several non-standard events scheduled on the day of the mishap.