Overview
On 9 August 1968, a glider accident occurred at Birrfeld airfield (AG, Switzerland) involving a Rhönlerche II (HB-594) and a tow aircraft, a D.H.C.-1 Chipmunk (HB-TUK). The glider was destroyed and the student pilot remained uninjured.
Aircraft and Pilots
The tow aircraft was a D.H.C.-1 Chipmunk (HB-TUK), a single-engine low-wing monoplane. The glider was a Rhönlerche II (HB-594), a two-seat strut-braced high-wing glider. The tow pilot, born in 1944, held a private pilot license with a towing endorsement and had approximately 231 flight hours. The student pilot, born in 1950, held a student license and had recently started glider training on 29 July 1968, with a total of 5 hours and 4 minutes of flight time, including 1 hour and 21 minutes solo.
Sequence of Events
The tow pilot took off at 16:16 local time with the glider in tow. About one minute later, at approximately 50 meters above ground, the tow aircraft's engine stopped due to fuel starvation. The tow pilot released the tow rope and performed a forced landing in a harvested cornfield about 1 km southwest of the airfield, landing safely without damage.
After release, the student glider pilot turned approximately 200° to the left, intending to return to the airfield. However, he realized he would be unable to safely clear a road (Hausen-Birrhard-Mellingen) running adjacent to the west side of the airfield. He decided to land parallel to the road and initiated a right turn. The altitude was insufficient for a normal turn. The student pulled the stick back and applied right rudder simultaneously, causing the glider to stall and drop from about 5 meters. The glider struck the ground, sustaining substantial damage: the nose was crushed, the right wing broke off about 2 meters from the tip, and other components were damaged. The student was not injured.
Further Findings
Investigation revealed that the tow pilot had refueled the Chipmunk earlier that day with 80 liters of aviation gasoline, filling both wing tanks to the brim. After refueling, the aircraft conducted 19 flights totaling 2 hours and 36 minutes of flight time. The aircraft's fuel consumption according to the pilot's manual was about 33.5 liters per hour at sea level, giving an endurance of approximately 2 hours and 26 minutes. The flight time since refueling closely matched this consumption. Both fuel gauges were reported to be functioning correctly. After the accident, the Chipmunk was refueled and flown away from the landing field without any technical issues.
The student pilot had completed his first solo flight two days prior on the same glider type. At the time of the accident, he had accumulated eight solo flights totaling 1 hour and 21 minutes.
Conclusion
The commission concluded that the accident was caused by the student pilot, who had limited experience, stalling the glider during the last turn of a forced landing at low altitude after the tow pilot released the rope due to fuel shortage at 50 meters above ground. The stall led to a loss of speed and subsequent crash.