What happened
On July 25, 1979, at 17:07, a flight instructor trainee and a glider pilot initiated a takeoff from the grass runway 25 at Grenchen Airport in an ASK 1lag 16 motor glider, registration HB-2033. The flight was intended as an instructional session. During the takeoff roll, the aircraft lifted off shortly after passing the midpoint of the runway. However, the aircraft failed to achieve the necessary climb speed and continued to fly at an extremely low altitude.
As the aircraft approached a road crossing the runway axis, approximately 115 meters from the end of the runway, it struck a cyclist. The impact caused the cyclist to sustain fatal injuries. The pilots of the HB-2033 remained uninjured, though the aircraft sustained light damage. The aircraft eventually came to rest in a cornfield after the pilots attempted to retract the landing gear in a desperate effort to clear the road.
The investigation
The investigation examined the takeoff configuration, the engine performance, and the runway conditions. Investigators found that the engine was functioning perfectly and that no mechanical failure was responsible for the loss of climb performance. The aircraft was configured with the propeller set to a high pitch (cruise setting) for noise abatement, a practice recommended by the local flying school.
Analysis of the takeoff roll revealed that using a high propeller pitch increases the required takeoff distance by approximately 40% compared to a low pitch setting. The investigation also noted that the pilots utilized an incorrect takeoff technique, pulling the control stick fully back before achieving sufficient airspeed, which caused the aircraft to enter an aerodynamically stalled state immediately after liftoff.
Findings
- The primary cause of the accident was the failure to abort the takeoff in a timely manner.
- The takeoff was performed with the propeller in a high-pitch setting, which significantly lengthened the required ground roll.
- The pilots employed improper takeoff technique, leading to an excessive angle of attack and a loss of climb capability.
- The short length of the grass runway at Grenchen made a takeoff with high propeller pitch unsafe under the prevailing conditions.
- The pilots failed to recognize the danger of the approaching road in time to execute an emergency stop.