A loose borescope port plug removed as part of a recent inspection may have caused an engine fire that forced an Atlas Air Boeing 747-8 to return to Miami International Airport shortly after departure Jan. 18, a preliminary NTSB report indicates.

A post-incident inspection of the affected GE GEnx-2B67 engine showed damage on the thrust reverser wall directly above an open combustor diffuser nozzle port used to access the engine’s hot section for borescope inspections. The plug that blocks the port when access isn’t needed was “not secured in the case and was found loose in the engine cowling,” the report said.

Maintenance records show the engine underwent a combustor section borescope inspection four days before the incident flight. The work, done by an unnamed vendor at Miami International, required removal of the plug.

“The maintenance work card provided instructions on how to properly reinstall the borescope plugs to ensure the locking feature was properly engaged,” the NTSB report said. “The work card was initialed by the technician performing the work and an inspector, indicating this task had been completed in accordance with the maintenance manual procedure.”

Investigators found no evidence of an uncontained engine failure or combustor damage. Preliminary reviews of the flight data recorder and other operating data “showed no evidence of engine failure,” surge, or stall, the report said.

The incident occurred shortly after takeoff on a scheduled flight, Atlas Air Flight 3885, to San Juan, Puerto Rico. As they were climbing out, the pilots received “overheat” and “engine fire” alerts for engine no. 2. The captain immediately declared a “mayday” and the crew, cleared to return to Miami, worked through the applicable non-normal checklists. The fire was extinguished using onboard systems, and the aircraft landed safely.