BOMBARDIER DHC-8-402Horizon Air

The flight was climbing through flight level 200 when the flight crew received a No. 2 engine audible fire warning, which was accompanied by a bang sound. The crew subsequently shut the No. 2 engine down and discharged both of the engine's fire extinguishing bottles. The cabin crew reported no visible fire, but the flight crew continued to receive a fire indication. The captain then declared an emergency, began a descent to the departure airport, and landed about 9 minutes after the event. After the airplane was stopped on the runway and the No. 1 (left) engine was shut down, an emergency evacuation was conducted using the main cabin door. An initial assessment by company personnel indicated that the fire damage to the airplane was limited to what was visible on the aircraft's exterior. A visual postincident examination of the engine revealed that sooting was present on external case surfaces, which were concentrated on the aft half of the No. 2 engine. A disassembly of the engine confirmed that about half of the SED outer dome within the combustion section had separated and was subsequently ingested into the gas path. To determine the reason for the separation, three locations on the outer dome were sectioned and examined. All three locations exhibited insufficient weld penetration so that the welds remained totally contained within the SED outer dome and did not penetrate through to the inner duct as required by the manufacturing specifications.