What happened
On 12 July 2003, a Falcon XP microlight, registered G-BUYF, was conducting a private flight from Manchester, Barton Airfield, to Kemble Airfield to participate in a rally. While joining the circuit for Runway 26, the pilot found himself positioned between two other aircraft on the downwind leg. To allow the following aircraft to overtake, the pilot adjusted his heading to the left and reduced power.
Upon attempting to re-apply power once clear of the other aircraft, the engine failed to respond. At an estimated altitude of 600 to 800 feet, the pilot determined that reaching the airfield was not possible and began searching for an emergency landing site. After evaluating two potential fields, the pilot selected one for a forced landing. During the landing sequence, the aircraft lost directional control, spinning 180 degrees and sliding backwards. This motion caused the main undercarriage to collapse and the nosewheel to bend. The rear-mounted propeller struck the ground, resulting in a broken propeller and damage to the underside of the fuselage. There were no injuries to the pilot or the passenger, both of whom were wearing four-point harnesses.
The investigation
The investigation focused on the sudden loss of engine power. Following the accident, the pilot noted that fuel was present in both carburettor bowls, leading to an initial suspicion of an electrical malfunction. A subsequent teardown of the Rotax 503 UL engine revealed that the rear crankshaft oil seal had failed. This failure allowed the fuel-oil mixture to escape and subsequently contaminate the ignition points for one of the two cylinders. Because the engine relied on a single ignition system per cylinder, the contamination rendered the engine unable to operate on the remaining cylinder, leading to the loss of power.