What happened
On 31 May 2009, a Rockwell Commander 114 Commander, registration G-BDYD, was conducting a private flight from Oban to Abbeyshrule. While cruising at 2,000 ft near Cookstown, Northern Ireland, the pilot experienced a sudden vibration, a loud rattling sound, and a rapid increase in propeller speed. This sequence lasted approximately 15 seconds, during which engine oil pressure dropped to zero before the engine failed entirely.
Following the engine failure, the pilot transmitted a MAYDAY call and searched for a suitable landing site. Due to the small and uneven nature of available fields, the pilot opted to land downhill in a cultivated field. The pilot lowered the landing gear and shut off the fuel and battery master switches. The aircraft struck a ditch and a hedge at the far end of the field, causing the aircraft to be destroyed. Both the pilot and the passenger sustained serious injuries, though they were able to exit the aircraft via the left door.
The investigation
The AAIB examined the wreckage and the engine components. While no holes were found in the engine casing, the engine contained significant metallic debris, and less than half a lubricative pint of oil remained. Rotating the propeller by hand revealed severe internal damage, including a broken connecting rod. The investigation focused on the oil filter converter plate, which was found to have a deteriorated seal.
Maintenance records were also reviewed. The investigation found that the maintenance organisation had incorrectly believed that an Airworthiness Directive (AD) regarding gasket replacement was no longer applicable. Consequently, the gasket had not been replaced for approximately 100 engine hours beyond the required interval.
Findings
- The engine failure was caused by a complete loss of engine oil following the failure of a gasket on the oil filter converter plate.
- The gasket was likely an older type made of ethylene propylene rubber, which is unsuitable for use with petroleum-based oils.
- The failure of the gasket led to the extrusion of the seal, allowing oil to escape.
- The loss of oil pressure caused the propeller speed to increase uncommanded as the hub piston moved toward a fine pitch position.
- The maintenance organisation had mistakenly concluded that the relevant Airworthiness Directive was no longer applicable, leading to the gasket exceeding its service life.