What happened
On October 24, 1967, a Mooney Executive 21, registration HB-DEV, departed from Sitterdorf airfield for a VFR flight to Vienna. The aircraft was operating at maximum takeoff weight with three passengers and approximately 20 kg of baggage. Due to the short length of the grass runway and nearly windless conditions, the pilot employed a specific takeoff technique, initially rolling with the flaps retracted before using a hand pump to extend them to a 15-degree takeoff position.
Shortly after liftoff, at an altitude of approximately 2 to 4 meters, the aircraft suddenly lost altitude and struck a wire fence just outside the airfield boundary. The pilot attempted to abort the takeoff, but the aircraft continued to strike several more wire fences, eventually causing the elevator to become blocked. The aircraft finally came to a stop approximately 500 meters past the end of the runway after veering into a marshy area. All four occupants escaped the incident uninjured.
The investigation
Investigators examined the aircraft's hydraulic flap system and the cockpit environment. The investigation focused on why the flaps, which had been extended to 15 degrees, were found in the fully retracted position following the collision. A technical inspection of the flap actuation system performed on the aircraft revealed no mechanical defects or malfunctions in the hydraulic circuitry.
Findings
- The aircraft was operating at its maximum allowable takeoff weight of 1243 kg.
- The primary cause of the altitude loss was the unintentional retraction of the flaps shortly after takeoff.
- Because no mechanical failure was found, investigators considered two possibilities: the pilot may not have fully set the flap lever to the 'down' position during the pre-takeoff check, or a passenger may have inadvertently moved the lever.
- The passenger in the front right seat was noted to be quite tall (1.92 meters), and it was determined that his knee could have potentially bumped the flap release control lever during the climb.