Sequence of Events
The pilot, assigned to tow gliders at Luzern-Beromünster airfield, had completed sixteen towing flights that day. During the seventeenth flight, the tow cable was only partially retracted after the glider released at 1200 m MSL over Vogelsang. The pilot entered a left base leg for runway 34. The approach was short and high. The start director informed the pilot that the cable was not fully retracted. The pilot decided to fly a shortened circuit (cable drop circuit) and turned left over the runway. During this left turn, the aircraft stalled and spun from about 40 m AGL over the left wing. It completed a full spin and struck the ground in an almost three-point attitude in a wheat field. The impact threw the aircraft back into the air, and it ended up inverted on its nose.
Witnesses reported normal engine sounds but noted no increase in power during the go-around. The aircraft was observed to be very unsteady in the approach.
Aircraft and Pilot Information
The aircraft was a Piper PA-18-180 (Super Cub), registration HB-PAY, built in 1952. It was powered by a 180 HP engine and had a fixed four-blade Hoffmann propeller. It was privately operated and used for VFR day flights. At the time of the accident, the aircraft had 5354:39 hours on the airframe, 2986:00 on the engine, and 1334:27 on the propeller. The weight and center of gravity were within limits (approx. 652 kg, max 794 kg).
The pilot, born 1957, held a Swiss private pilot license valid until 22 April 1993, with a glider towing endorsement. He had 83:32 hours total flight time, including 11:26 hours on the accident type. He started flight training on 5 May 1989. His last medical examination on 22 April 1991 found him fit without restrictions.
Meteorological Conditions
Weather at the accident site around 1510 LT: 4/8 cumulus clouds base 1600 m MSL, visibility more than 15 km, wind from NE to E at 10 kt with gusts to 25-30 kt. Temperature 22°C, dew point 11°C, QNH 1014 hPa. Bise turbulence was a hazard. Witnesses reported beautiful weather with strong, variable, and gusty winds.
Wreckage Examination
The aircraft was found inverted with landing gear and cockpit severely compressed. Wing struts were broken. Both wing leading edges touched the ground at the tips. Fuel remained in wing tanks. The engine hung free in its cowling; the engine mount was torn from all attachments. Two propeller blades were broken. The fuselage was slightly bent upward behind the cockpit. A cable remnant of about 2.5 m was attached to the aircraft. There were no skid marks. The flap position was extended. The throttle was full forward. The mixture was rich. The RPM gauge indicated 700. The magneto switch was set to left, key broken. The propeller damage indicated the engine was producing only low power at impact. Examination of the engine and magnetos in the USA found no pre-existing defects. The pilot's seat belts were worn and held.
Conclusions
The accident was due to falling below the minimum flight speed during a go-around, turbulence, and inappropriate control inputs by the pilot.