What happened
On June 27, 1971, at approximately 10:30 AM, a Piper PA-18-150 Super Cub, registration HB-OYD, was performing a glider tow at Beromünster airfield. The aircraft was towing a Ka 7 glider, registration HB-652, which carried a flight instructor and a student. During the takeoff roll on runway 34, the aircraft failed to accelerate to the required liftoff speed. After a roll of roughly 280 meters, the pilot attempted to rotate the aircraft at approximately 55 mph. This resulted in the aircraft banking sharply to the right, touching down on its right landing gear, and veering off the grass runway through the airfield perimeter fence.
In response to the instability of the towing aircraft, the instructor in the glider released the tow rope, bringing the glider to a halt on the runway. The pilot of the HB-OYelD then released the tow rope. Continuing with full engine power, the aircraft traveled across uneven, marshy terrain and lifted off again at a high angle of attack. This second attempt led to an aerodynamic stall, causing the aircraft to bank right, strike the ground with the right wingtip and wheel, and eventually roll over approximately 200 meters outside the airfield boundaries.
The investigation
The investigation focused on the mechanical state of the aircraft and the sequence of the takeoff attempt. Investigators examined the wreckage and determined that the carburetor heat was engaged during the takeoff roll. This setting caused a significant reduction in engine performance, specifically a drop in maximum RPM of approximately 300 to 400 revolutions per minute.
Findings
- The primary cause of the accident was that the pilot initiated the takeoff with the carburetor heat engaged.
- This setting caused a substantial loss of engine power, preventing the aircraft from reaching its necessary takeoff velocity.
- The pilot failed to abort the takeoff attempt in a timely manner once the loss of performance became apparent, leading to the subsequent excursion and rollover.