The advanced fifth-generation fighter was not destroyed and no one was injured in the incident, according to an accident investigation board report released Friday. But it was a Class A mishap, the most serious classification. F-22s cost an estimated $250 million each. Lockheed Martin stopped manufacturing the aircraft in 2012.
The Raptor was assigned to the 422nd Test and Evaluation Squadron at Nellis, according to the Air Combat Command report; it was maintained by the 757th Aircraft Maintenance Squadron.
According to the report, maintainers started troubleshooting problems with extensive hardware and software modifications made to the aircraft last summer. On Oct. 28, a maintainer removed the auxiliary power unit's mixing exhaust duct, which steers extremely hot gases safely outside of the aircraft, to try to figure out what had gone wrong.
However, the maintainer failed to place the required "collar" clips, with bright orange "remove before flight" streamers attached, to the circuit breakers involved, which would have warned others not to reset them and start the aircraft, and did not put the proper digital or physical warnings in place, the report states. Clip-on collars are designed to stop circuit breakers from being reset inadvertently. Maintainers told investigators that installing the collars is difficult and often not done; one maintainer said he didn't know what they are.
The maintainer also did not replace that exhaust duct, the report adds.
Two days later, the F-22's auxiliary unit was improperly started up with its exhaust duct uninstalled and its emergency cutoff switch incorrectly set to normal. Scorching hot exhaust gas went straight into the exhaust bay and the wheel well of the left main landing gear, when a properly installed duct would have safely diverted it out of the plane.
As smoke spewed out of the exhaust bay, the maintainer mistakenly tried to run diagnostics and review the warning codes. It wasn't until another maintainer manually shut off the power unit that the overheating stopped. By then, the damage was done.
Temperatures in the exhaust bay had reached 600 to 700 degrees Fahrenheit. The overheating damaged the F-22's airframe, systems, wiring, hydraulics and surrounding structure, and parts of the auxiliary power unit's exhaust bay.
The report found that the maintenance unit's culture, in which circuit breaker collars and warnings were used rarely or inconsistently, contributed to the mishap.
In an email Tuesday evening, after this article's original publication, Nellis said it has taken steps to make sure these problems don't happen again. Lt. Col. Bryon McGarry, head of Nellis' public affairs department, said the leaders of the 757th squadron and the 57th Maintenance Group reviewed the technical orders involved and revamped maintainers' training to make sure they follow the safety rules.
He added that the maintainers involved in the incident were held accountable, decertified and retrained on the processes involved with the mishap.
According to the investigators, the incident could have been avoided if the collars had been in place.
The report also placed some blame on the extensive nature of the earlier F-22 modifications, stating that the "sheer amount of data" being processed by one maintainer made a mistake more likely.
Editor's note: This story has been updated to add a response from the Air Force.