What happened
While on arrival to the destination airport, the crew was cleared to descend from 8,000 feet to 5,000 feet. The descent was initiated by reducing power on both engines. As the aircraft descended through 6,500 feet, the crew experienced a loud bang and vibration, followed by the illumination of the right engine 'FIRE' warning light and the sounding of the fire bell.
The right engine RPM dropped to 80 percent and stabilized at that level. The crew identified the right engine as the source of the issue and discharged the fire bottle, which caused the 'FIRE' warning light to extinguish. After securing the engine, the airplane landed uneventfully at IAD.
The investigation
Post-flight examination of the aircraft revealed that the right engine bullgear had undergone an uncontained separation in flight. A section of the bullgear rim, measuring 4 5/8 inches in length, penetrated the right side of the fuselage and was found inside the cabin on the left side interior wall above the seat cushion for seat 1A.
An inspection of the right engine gearbox showed that the bullgear had broken into at least six pieces. Investigators measured the bull gear bearing bore of the gearbox diaphragm and determined it exceeded the allowable wear limit specified in the engine manual. Maintenance records indicated the bullgear had been installed new during the previous repair, 597 hours and 776 cycles prior to this event.
A metallurgical examination of the bullgear identified an initial fatigue fracture originating on the front face outboard of the hub, along with two additional fatigue fractures on either side of the primary fracture. The investigation confirmed that the web thickness and material hardness of the bullgear met all manufacturer design requirements.