BOEING 747-87UFATLAS AIR INC

During the airplane’s initial climb after departure, the flight crew received engine overheat and fire warning indications for the No. 2 engine. The engine was subsequently shut down, one fire bottle was discharged, and the engine fire light extinguished. After declaring a mayday, the flight returned to the airport and made an uneventful landing. A review of the airplane’s maintenance records revealed that, 4 days before the flight, the airplane had undergone a borescope inspection of the No. 2 engine performed by a third-party vendor. The inspection required the removal of the port M borescope plug. The maintenance work card provided instructions for properly reinstalling the borescope plug to ensure that the locking feature was properly engaged. By initialing the work card, the technician who performed the work and the inspector who reviewed the work indicated that this task had been completed in accordance with the maintenance manual procedure. A postincident inspection of the No. 2 engine revealed that the engine cases were intact with no evidence of an uncontained engine failure. The inspection also revealed burn through of the thrust reverser fan duct fixed inner wall located directly over the combustor diffuser nozzle (CDN) port M borescope plug. The CDN case port M borescope plug was not secured in the engine case and was found loose in the engine cowling. The open CDN port allowed hot gases to escape from the engine, which caused thermal damage to the thrust reverser. A postincident inspection of the port M borescope plug revealed no anomalies to the plug’s material composition, dimensions, threads or locking components. The plug was threaded into an exemplar engine without binding or stiffness and locked as designed. Although the work card had been initialed by a technician and an inspector, it is likely that the port M borescope plug was not properly secured and inspected according to maintenance manual procedures and that the improperly secured plug resulted in the engine fire.