What happened
On July 9, 1972, a Dornier DO-27, registration HB-HAE, was conducting a commercial VFR flight near Beromünster when the engine failed. The aircraft had previously dropped four skydivers from an altitude of 1,500 meters. During the landing approach, the pilot reduced manifold pressure and set the flaps to 35 degrees. While on the base leg, the pilot heard abnormal engine noises, followed shortly by a complete engine shutdown.
In response to the failure, the pilot reduced the flaps to 15 degrees and turned toward runway 34 at Beromünster. Despite verifying that the fuel pump was active and attempting to restart the engine using the starter, the engine failed to relight. The aircraft approached the runway at a low speed with a windmilling propeller and impacted a cornfield approximately 100 meters before the runway threshold. After sliding for about 35 to 40 meters, the aircraft performed a nose-over and came to rest on its back.
The investigation
The investigation focused on the mechanical condition of the Lycoming GO-480-B1A6 engine and the pilot's actions. Physical examination of the engine revealed extensive internal damage, including broken counterweight brackets on the crankshaft and foreign metal debris found in all four main crankshaft bearings. The damage was so severe that the connecting rods, pistons, camshaft, and several cylinders, along with the crankcase, were rendered irreparable.
Findings
- The engine failure was caused by the failure of the valve train mechanism.
- The primary mechanical trigger was the breakage of a counterweight bracket on the crankshaft, which subsequently caused the camshaft gear mounting to fail.
- At least one fastening bolt from the camshaft gear mounting entered the gear train, leading to the destruction of the intermediate gear carrier eye.
- The aircraft was within weight and balance limits at the time of the accident.
- The pilot was found to be in good health with no medical impairments noted at the time of the event.
Safety action
- The investigation concluded that the accident was the result of an engine failure due to valve train malfunction, followed by an aircraft capsize caused by an insufficient distance for the forced landing.