A foreign object left inside an F-35 engine at Luke Air Force Base, Arizona, in early 2023 caused nearly $4 million in damages, a recent Air Force aircraft accident investigation revealed.

The item — a handheld flashlight — was sucked into the aircraft’s inlet engine during a maintenance ground run on the night of March 15, Air & Space Forces Magazine first reported. The aircraft belonged to the 56th Fighter Wing.

During an idling procedure, a three-man maintainer team started the engine without issue before going through typical checks, including idling for five minutes to monitor for fuel leaks.

It wasn’t until the airmen turned off the engine that any abnormalities arose. During the shutdown, one maintainer noted “abnormal noises.” None of the airmen were injured.

One of the maintainers carried out a tool inventory check after installing a “metering plug into an engine fuel line,” the investigation noted. This process took place before another maintainer used the flashlight to carry out a “Before Operations Servicing” inspection, the report said.

Investigators determined that the maintainers’ lack of adherence to standard cautionary items highlighted in the Joint Technical Data checklist before the run contributed to their leaving the flashlight behind.

Investigators also found that “complacency” related to the F-35′s Autonomic Logistics Information System, or ALIS, is partly to blame.

“The substantial number of checklists and difficulty in accessing the correct ones causes complacency when users reference the required maintenance procedures,” the report notes.

Ultimately, the mishap damaged the aircraft’s second stage rotor, third stage rotor, fifth stage rotor, sixth stage rotor, fuel nozzle, bypass duct, high pressure compressor, high pressure turbine, and fan inlet variable vane, the report found.

In total, estimated damages associated with the accident came in at $3,933,106.

The report concluded that the inspecting maintainer “failed to clear the inlet of foreign objects after completing the required inspections for an engine run after they exited the aircraft inlet. Failure to complete checklist tasks resulted in a flashlight being left inside the inlet.”