What happened
On 23 March 2024, a Cessna 208B, registration G-CPSS, was performing a ferry flight to its main base following maintenance. While cruising at 3,300 ft near Cranfield Airport, the pilot observed a low oil pressure warning light accompanied by smoke emitting from the engine exhaust. The pilot immediately diverted to Cranfield Airport, performing a rapid descent.
Upon landing on Runway 21, the smoke remained visible, particularly as the engine power was reduced during the landing flare. After taxiing off the runway, the pilot shut down the engine and discovered what appeared to be flames within the right engine exhaust. Airport fire services attended the scene and used CO2 extinguishers to manage the situation. There were no injuries to the pilot.
The investigation
The investigation focused on the engine's condition and the maintenance history of the aircraft. The engine was sent to the manufacturer in Canada for a detailed teardown. Investigators found a significant amount of oil and water trapped within the exhaust duct, which could not drain because the drain hole was blocked by carbon buildup.
Further inspection revealed that the reduction gearbox (RGB) rear housing had developed two holes due to significant corrosion. This corrosion allowed engine oil to leak from the gearbox into the exhaust system. The investigation also uncovered that the operator had been performing weekly engine compressor washes internally. These washes were conducted without removing the drain plug and were not documented in the aircraft logbook, nor was a Certificate of عدم compliance to service issued.
Findings
- The primary cause of the oil loss was significant corrosion in the reduction gearbox housing, which created holes that allowed oil to escape into the exhaust.
- The corrosion was caused by numerous engine compressor washes performed without removing the drain plug, which prevented water from draining away.
- The operator had implemented a weekly washing routine to combat blade corrosion but did not follow the manufacturer's Approved Maintenance Manual (AMM).
- The maintenance procedure used by the operator did not include the necessary steps to ensure the drain plug was removed or that fluid could drain freely.
- The lack of formal documentation and the failure to use approved maintenance personnel meant the risks associated with the improper washing technique were not identified.