Incident Overview
While on approach to the destination airport, the airplane was cleared to descend from 8,000 feet to 5,000 feet. The crew reduced power on both engines to initiate the descent. As the aircraft passed through 6,500 feet, a loud bang was heard, accompanied by a noticeable vibration. Simultaneously, the right engine 'FIRE' warning light illuminated, and the fire bell sounded. The right engine RPM dropped to 80 percent and stabilized. The crew identified the affected engine, discharged the fire bottle, and the 'FIRE' warning light extinguished. The engine was secured, and the airplane continued to land without further incident at IAD.
Examination Findings
Post-flight examination of the airplane revealed that the right engine bullgear had experienced an uncontained separation in flight. A 4 5/8-inch-long section of the bullgear rim penetrated the right side of the fuselage and was subsequently recovered from the left side interior cabin wall above seat 1A's seat cushion. Inspection of the right engine gearbox showed that the bullgear was broken into at least six pieces. The gearbox diaphragm's bullgear bearing bore was measured and found to exceed the engine manual's allowable wear limit.
Maintenance History
Maintenance records indicated that the bullgear had been installed new during the last repair, which occurred 597 hours and 776 cycles prior to the event.
Metallurgical Analysis
Metallurgical examination of the bullgear identified an initial fatigue fracture that originated on the front face of the bullgear, outboard of the hub. Two additional fatigue fractures were found on either side of the initial fracture. The examination also determined that the bullgear's web thickness and material hardness met the manufacturer's design requirements.
Regulatory Response
In response to this investigation, the FAA was considering an airworthiness directive that would mandate recurring oil sampling and analysis for elevated metal levels, which could indicate wear of the bullgear and high-speed pinion gear.