What happened
On June 5, 2018, a Cessna F150 M, registration OY-BJF, was conducting a private cross-country flight from Holbæk to Roskilde under visual flight rules. While cruising at approximately 1,200 feet with the engine running at 2,300 RPM, the pilot experienced sudden engine vibrations accompanied by a slight drop in engine speed.
In response to the instability, the pilot implemented emergency procedures, reducing engine RPM to mitigate the vibrations to a manageable level. However, the aircraft was unable to maintain its altitude, prompting the pilot to prepare for an emergency landing in a nearby field. After notifying air traffic services and extending the flaps to the full 40-degree position, the aircraft landed in a field approximately 5 kilometers south of Frederikssund. The aircraft came to a stop without immediate visible damage, and both occupants on board remained uninjured.
The investigation
The investigation, which included interviews and a technical examination of the engine, focused on the mechanical failure of the Continental O-200-A engine. Investigators discovered that the exhaust valve for cylinder number one had broken. This failure caused significant internal damage to both the cylinder and the piston of the first cylinder. No other engine components were found to be damaged.
Records indicated that the engine had been installed in the aircraft in 1995 and had accumulated a total of 3,012 flight hours at the time of the incident. While the manufacturer's recommended time between overhaul (TBO) for this engine type is 1,800 hours, the engine had not undergone an overhaul since its installation in 1995.