What happened
On 30 October 2020, at approximately 0930 local time, an F-22A, tail number 06-4109, experienced an overheat condition within its Auxiliary Power Unit (APU) exhaust bay while stationed at Nellis Air Force Base, Nevada. The aircraft, assigned to the 422d Test and Evaluation Squadron under the 53d Wing, was undergoing extensive modifications for operational testing.
Prior to the incident, during maintenance performed on 28 October 2020, the APU Mixing Exhaust Duct (AMED) had been removed. During this process, required circuit breakers were not pulled or collared, and necessary warnings were not applied to the aircraft's structure or digital forms. On the morning of the mishap, a decision was made to use the APU for defueling and door reconfiguration rather than using Aerospace Ground Equipment.
During the procedure, the APU Emergency-Off Switch (AES) had been incorrectly set to the "Normal" position by an unidentified individual. Upon starting the APU, smoke began to emerge from the exhaust bay and into the left main landing gear wheel well. A maintenance member eventually manually set the AES to "Emergency Off" to stop the unit. The damage to the aircraft is estimated at $2,690,000.
The investigation
The Accident Investigation Board (AIB) examined the maintenance procedures and the organizational environment of the 757th Aircraft Maintenance Squadron. Investigators looked into the handling of the AMED removal, the accuracy of digital maintenance forms, and the physical accessibility of circuit breakers. The board also reviewed the impact of ongoing aircraft modifications and the specific workload present on the day of the incident.
Findings
- Improper maintenance procedures resulting in the APU being started while the AMED was still removed.
- A systemic unit culture characterized by the infrequent use of circuit breaker collars and inconsistent application of digital warnings.
- The design of test instrumentation on the aircraft, which physically obstructed access to certain circuit breakers, making the use of collars difficult.
- The complexity of the ongoing aircraft modifications, which created a massive volume of maintenance data that obscured critical information.
- Significant distractions caused by several non-standard events scheduled for that day, including a flight-line visit from a Distinguished Visitor and a morale event.