What happened
On 10 April 2010, a YAK-50, registration G-YAKK, was conducting a private flight from Carlisle Airport when the engine failed. During the flight, the engine experienced a sudden loss of oil pressure. This failure caused oil to spray onto the aircraft, resulting in the windscreen becoming partially obscured by an oil film. The pilot managed to perform a forced landing in a field near Bothel, Cumbria. The aircraft sustained substantial damage during the impact. The pilot, who sustained minor injuries, was able to exit the aircraft without assistance before later seeking hospital treatment as his injuries became more evident.
The investigation
The investigation focused on the engine's oil system following the loss of pressure. The aircraft had recently undergone significant maintenance, which included an overhaul of the oil scavenge pump assembly. During subsequent flights after this maintenance, a small oil leak had been detected near the pump. At that time, the leak was addressed by an engineer tightening the vertical bolts near the pressure adjustment valve.
Upon inspection of the engine following the accident, it was discovered that the oil pressure adjustment valve had fallen out of the oil scavende pump housing. The lower cowling was found to be missing the wire locking that should have secured the component in place.