What happened
On 17 December 2001, a Shorts SD3-60-300, registration D-CFAO, was performing a scheduled cargo flight from Manchester to Paris. Shortly after takeoff, the pilot received a fire warning for the left engine. Following established abnormal procedures, the commander shut down the engine and discharged the first fire extinguisher bottle, which caused the warning light to extinguish. The crew notified air traffic control of their intent to return to Manchester and requested that emergency services be positioned near the runway. The aircraft landed safely with no injuries to the crew.
Two days later, on 19 December 2001, the same aircraft attempted a departure for Paris. While holding at the runway, the No 1 engine fire warning activated a second time. During this second event, the fire warning persisted after the first discharge, necessitating the use of both fire extinguisher bottles. No fire was found upon subsequent inspection.
The investigation
Following the first incident, inspections of the engine showed no physical evidence of fire. However, investigators found that engine oil had been deposited within the engine intake, inside the nacelle, and onto the horizontal stabiliser.
Technical analysis focused on the PT6A-67R engines, noting a known tendency for oil to accumulate in the accessory gear case. Investigators determined that after the initial engine shutdown, the gas generator continued to rotate due to ram air. This rotation likely prevented the scavenge pump from effectively emptying the gear case, causing the oil level to rise until the No 1 bearing was flooded. This flooding allowed oil to vent into the intake and drain toward the stabiliser.
During the maintenance following the first event, engineers identified a minor, unrelated oil leak and found contamination on two fire-wire connectors. While the engine was replenished with oil and tested, the second warning prompted further mechanical changes.
Findings
- The initial fire warning was triggered by oil flooding the accessory gear case, which subsequently leaked through the accessory pad seals.
- The rotation of the gas generator via ram air during the engine shutdown prevented the scavenge pump from maintaining the correct oil level in the gear case.
- The second fire warning was attributed to contamination on the fire-wire connectors, which was resolved by replacing both fire-wire loops and the fire-wire control unit.