What happened
On July 9, 1967, a private flight departed from Zurich Airport bound for Grenchen. The aircraft, a Meta-Sokol L-40 with registration HB-TBE, was performing a standard takeoff from Runway 28. After reaching an altitude of approximately 30 to 40 meters, the pilot retracted the landing gear and flaps. Immediately following these actions, the engine suffered a total loss of power.
Recognizing the emergency, the pilot maintained control and attempted an emergency landing on a grassy area adjacent to the runway. While the pilot had initially targeted the field, a roadside access road with earth and stone embankments was discovered too late to avoid. The aircraft, with its landing gear retracted, struck the embankment approximately 35 meters before the obstacle. The impact resulted in two light injuries to the occupants and caused heavy damage to the aircraft.
The investigation
Investigators examined the engine components to determine the cause of the power loss. The inspection revealed that the engine had no compression and that fuel was no longer reaching the engine from the injection pump. The investigation focused on the drive mechanism connecting the crankshaft to the camshaft and the fuel injection pump.
Technicians discovered that three teeth had broken off a bevel gear within the drive train. Additionally, the vertical drive shaft (king shaft) was found to be bent by approximately 1.5 mm. A small, foreign metal piece made of chrome-nickel steel was found wedged within the gear teeth. While the exact origin of this metal fragment could not be definitively confirmed, its presence was linked to the mechanical failure of the gear assembly.
Findings
- The primary cause of the engine failure was a mechanical blockage in the gear train caused by a foreign metal object.
- This blockage caused the bevel gear to fail, breaking three teeth and interrupting the drive to both the camshaft and the fuel injection pump.
- The resulting loss of valve actuation and fuel delivery led to the immediate cessation of engine power.
- The pilot's decision to attempt a landing on the adjacent grass was appropriate given the low altitude and sudden nature of the failure.
- The collision with the obstacle was a direct consequence of the aircraft's flight path during the emergency maneuver.