Accident Overview
On June 27, 1971, at approximately 1030 hours, a Piper PA-18-150 Super Cub (registration HB-OYD) was attempting to tow a Ka 7 glider (HB-652) at Beromünster airfield. The takeoff roll proceeded abnormally slowly. After about 280 meters of ground roll, the pilot lifted off at a speed of approximately 55 mph. Immediately thereafter, the aircraft stalled, banked to the right, and contacted the ground with the right main gear before veering off the runway.
Sequence of Events
The glider instructor recognized the critical situation and released the tow rope while the glider was still rolling on the ground. The glider subsequently came to a stop on the runway. Meanwhile, the Super Cub left the grass runway at an angle of about 20 degrees to the runway axis and breached the airfield perimeter fence. The tow pilot then released the tow rope. With engine power still applied, the aircraft rolled across slightly rising, marshy terrain, became airborne again in a nose-high attitude, entered a stall, and banked right once more. It struck the ground with the right wingtip, then the right wheel, and overturned on the uneven terrain approximately 200 meters beyond the airfield boundary.
Injuries and Damage
The pilot of the Super Cub was uninjured. The aircraft suffered substantial damage. Only minor third-party damage occurred.
Pilot Information
The tow pilot held a valid private pilot license with a glider towing endorsement. There was no indication of any impairment to his health at the time of the accident.
Meteorological Conditions
At the time of the accident, winds were from 240 to 270 degrees at 15 knots with gusts of 25 to 30 knots. Visibility exceeded 20 kilometers. No precipitation was reported. Temperature was 15°C, and relative humidity was 65%.
Investigation Findings
Examination of the wreckage revealed that the carburetor heat was engaged on the Super Cub. This condition caused a reduction in maximum engine revolutions of approximately 300 to 400 rpm, resulting in a significant decrease in maximum available power.
Conclusion
The accident resulted from the pilot beginning the takeoff with the carburetor heat engaged, preventing the aircraft from reaching the necessary lift-off speed. The pilot did not abort the takeoff in a timely manner, leading to the aircraft overturning beyond the airfield.