What happened
On June 10, 2018, a Robin DR 400-140 B, registration HB-KFD, departed from Lausanne-La Blécherette (LSGL) for a local flight. The aircraft was carrying a pilot and one passenger. After climbing to an altitude of 5,500 feet, the pilot began performing standard cruise flight checks.
During the manipulation of the mixture control, the engine RPM dropped significantly. Despite the pilot's attempts to move the mixture lever back to the "rich" position, the engine power continued to decrease, making it impossible to maintain altitude. Recognizing the impending engine failure, the pilot identified a nearby grassy field and executed an emergency approach. The aircraft landed in a downward-sloping field but was unable to stop before entering a rye crop approximately one meter in height. The occupants sustained one slight injury, and the aircraft remained intact with no visible damage to the propeller or airframe.
The investigation
Investigators examined the engine compartment and the mixture control system to determine why the cockpit controls were unresponsive. While the cockpit lever was set to "rich," the investigation revealed that the mixture lever at the carburetor was actually in the "lean" position.
Technical inspection of the control mechanism showed that the twisted cable sheath had become detached from its stop. The rigid steel control cable was found to be worn at its end, and the sheath was sliding freely within the stop without resistance. Additionally, investigators noted tool marks from pliers on the cable stop, indicating a prior intervention. The engine had been operating slightly beyond its standard 2,000-hour overhaul interval, though it was within the permitted 10% extension allowed by regulatory authorities.
Findings
- The primary cause of the engine power loss was the disconnection of the mixture control sheath from its stop.
- Excessive play in the cable stop allowed the sheath to gradually pull out of position during flight.
- Because the carburetor mixture lever lacked a return spring, the internal mechanism remained stuck in the "lean" position despite the pilot's efforts to adjust the cockpit lever to "rich."