What happened
On May 17, 2011, a Cabri G2 helicopter departed from the Frankfurt-Egelsbach airfield for a sightseeing flight accompanied by a passenger. While flying at approximately 1,500 ft AMSL, the pilot performed a "push over" maneuver involving reduced load factors. During this maneuver, the engine suddenly failed.
The pilot immediately initiated an autorotation and, after several attempts, successfully restarted the engine at approximately 50/500 ft GND. Following the restart, the pilot elected to return to Egelsbach. No further engine malfunctions or technical issues occurred during the remainder of the flight or the landing process. There were no fatalities and no damage to the aircraft or third parties.
The investigation
The BFU investigation focused on the cause of the engine failure and the subsequent series of engine failures observed during troubleshooting test flights. The investigation included an examination of the aircraft's Lycoming O-360-J2A engine and its Avstar MA-4SPA carburetor.
Following the initial incident, the operator performed extensive troubleshooting, which included replacing the carburetor multiple times and inspecting the electrical fuel pump and gascolator. The manufacturer also conducted test flights using different carburetors. These tests revealed that engine power loss and failures continued to occur specifically during low-G maneuvers (load factors below approximately 0.2 g), often accompanied by visible smoke.
Findings
Technical analysis of the carburetors revealed that the engine failure was caused by fuel overflow within the carburetor during maneuvers with low load factors.
Specifically, investigation of the original carburetor (S/N AV1127488) identified defects in the needle, needle seat, and float holder, which caused increased friction and impeded the movement of the float. A second carburetor (S/N AV11274416) was also found to suffer from leaks due to defects in the needle and needle seat area. These manufacturing or component defects allowed fuel to overflow at higher pressures/load changes, leading to the loss of engine power.