What happened
On 21 June 2018, a Yakovlev YAK C11, registration G-OYAK, was conducting a private flight near Little Gransden Airfield in Cambridgeshire. The flight was intended to perform flight test schedules and aerobatics following annual maintenance. The aircraft was carrying a pilot and one passenger.
During the return to the airfield, the pilot began an overhead join for Runway 28. As the aircraft entered the downwind leg, the engine began to misfire. In an attempt to manage the situation, the pilot flew a tighter circuit. However, during the final approach, the engine failed to respond to throttle increases, resulting in a total loss of power. Unable to reach the runway threshold, the pilot performed a wheels-up landing approximately 150 m short of the runway. During the landing, the propeller struck the ground, causing the aircraft to slew. The passenger was uninjured, but the pilot sustained one minor injury after striking his head on the gun sight.
The investigation
The investigation focused on the cause of the engine power loss. The magnetos used on the engine were BCM 7MJ models dating from the 1950s. Following the incident, both magnetos were removed for inspection and testing.
During testing, both units experienced a sudden failure, or "dead cut," as they reached higher temperatures. The right magneto failed after two hours of operation when its temperature reached 62°C, though a bent rotor arm was noted. The left magneto failed after one hour when it reached 53°C, at which point the coil was noted to feel soft. The aircraft sustained damage to the engine mounts, propeller, flaps, and the lower fuselage.