What happened
On September 22, 2006, an experimental BX-2 Cherry, registration PH-OOY, was performing an overland flight from Teuge to Stadlohn, Germany. Shortly after takeoff, while at an altitude of approximately 1,600 feet, the pilot reduced power, which caused the engine RPM to drop unexpectedly. Attempts to restore power were unsuccessful, and the engine began to run at idle.
As the pilot communicated with Teuge airport to declare the situation, a significant loss of altitude occurred. Realizing the airport was no longer reachable, the pilot attempted an emergency landing in a nearby grassland, but ultimately had to touchdown in a beet field. During the descent, the pilot retracted the landing gear to perform a belly landing. Upon impact, the right wingtip struck the ground, causing the aircraft to rotate 9/0 degrees before coming to a halt. The aircraft sustained severe damage, including a broken exhaust and propeller damage, though both occupants escaped without injury.
The investigation
The investigation focused on the engine, a BMW R1100S motorcycle engine modified for aviation use. While the pilot had checked the oil level before departure and found it sufficient, investigators discovered an oil trail on the engine cowling and the right side of the airframe.
Upon inspection of the engine, investigators found that the oil sight glass, used to monitor levels, had been displaced from the crankcase. This created a hole through which oil escaped. Evidence suggested that increased crankcase pressure had likely forced the glass out of its mounting. The investigation also looked into the crankcase ventilation system, which had been custom-developed by amateur builders rather than using the standard BMW design.
Findings
- The primary cause of the engine failure was the loss of oil due to the displacement of the oil sight glass.
- Increased crankcase pressure is the suspected mechanism that forced the sight glass out of the engine block.
- Such pressure spikes can occur if the crankcase ventilation system becomes obstructed, which can happen if plastic lines collapse due to heat or if debris/insects block the vent.
- The pilot did not monitor the oil pressure or oil temperature gauges during the period of power loss.