What happened
On 16 December 2014, a Bombardier DHC-8-402, registration G-FLBC, was performing a commercial flight from Glasgow to Belfast. During the climb to cruising altitude, the flight crew observed a sudden drop in oil pressure in the left engine. As the crew attempted to follow emergency procedures and reduce power, a brief fire warning occurred, followed by a more persistent engine fire indication.
While the flight deck fire warnings eventually cleared, passengers and crew in the cabin reported seeing blue flames and sparks emanating from the left engine exhaust. The crew declared a MAYDAY and diverted to Belfast Aldergrove Airport. Although the fire warnings had extinguished prior to arrival, the aircraft landed and was met by airport fire services, who identified active flames within the engine nacelle. This prompted an immediate evacuation of the 76 passengers and 4 crew members. One passenger sustained minor injuries during the evacuation.
The investigation
The AAIB investigation focused on the mechanical failure of the left engine's lubrication system. Examination of the engine revealed extensive heat damage, including punctured combustor casing and a damaged fuel pipe. The investigation found that the engine's oil tank had been depleted due to a failure in the scavenge pump assembly.
Detailed analysis of the oil pump assembly showed that a bearing failure had caused the gears in the pump to disengage. This prevented the oil from being returned to the tank, leading to a progressive loss of oil and eventual lubrication starvation. The investigation also noted that the engine' and the manufacturer's subsequent findings were consistent with a specific component failure within the engine's internal structure.
Findings
- The primary cause of the incident was fatigue cracking in an engine bearing key washer.
- This crack allowed a metal fragment to enter the scavenge pump, causing a bearing failure that disengaged the pump gears.
- The resulting failure to return oil to the tank led to oil starvation and the rapid overheating of engine bearings.
- This overheating caused the engine shaft to displace, leading to internal friction and an internal fire that eventually penetrated the engine casing and ignited external fuel.
- The flight crew correctly followed emergency checklists and successfully diverted the aircraft to the nearest suitable airport.