Accident Overview
On Saturday, 1 April 1961, two gliders from the Schaffhausen gliding group were engaged in ridge soaring at Schlossranden near Siblingen. The aircraft involved were a Grunau Baby GB II (registration HB-170) and a Rhönlerche RL II (registration HB-669). Both were painted yellow and had no reported technical defects.
Pilots
Both pilots were born in 1941. The pilot of HB-170 held a glider pilot license since 11 May 1960, with 13 hours and 6 minutes of flight experience and 88 flights. The pilot of HB-669 held a license since 1 August 1960, with 32 hours and 27 minutes of flight experience and 120 flights. Both were considered to have good flying qualifications, with no previous accidents or violations. The investigation found no evidence of health issues at the time of the accident.
Flight Sequence
Around 1410, the pilot of HB-170 took off from Schaffhausen-Neunkirch airfield behind a Piper J3C tow plane, being towed to Schlossranden and releasing at about 900 m above sea level after approximately five minutes. The pilot of HB-669 took off at 1432 behind the same tow plane and followed a similar flight path.
At about 1500, HB-669 was approximately 100 meters behind and 30–40 meters below HB-170, at an altitude of about 1100 m. Both were flying south toward the Randenturm. The distance between them decreased to about 50 meters due to speed differences. Near the Randenturm, the pilot of HB-669 saw HB-170 slightly to his left and above him, and observed a slight left bank, leading him to expect a left turn at the end of the ridge. However, HB-170 initiated a right turn. The pilot of HB-669 immediately also turned right, losing sight of HB-170. After a few seconds, he suddenly saw HB-170 approaching from his left rear. He steepened his turn and pushed the nose down, but could not avoid a collision.
The pilot of HB-170 had not seen HB-669 before the collision. When he had completed about three-quarters of his right turn, he suddenly saw HB-669 on his right side, and the collision occurred.
The two gliders collided laterally: the left wing tip of the Rhönlerche struck the nose of the Grunau Baby, breaking off a 165 cm section of the wing that remained embedded in the other aircraft.
Aftermath
After the collision, the pilot of HB-669 was able to fly back to Schaffhausen airfield and land. HB-170 initially entered a dive but was recovered and landed approximately two kilometers away at Hinterranden. Both pilots remained uninjured.
Damage
HB-170 suffered severe damage to the canopy, instrument panel, and nose. HB-669 had major damage to the left wing.
Investigation
The investigation concluded that the immediate cause was not a lack of airspace surveillance but rather the control inputs of the pilot of HB-669. The critical situation developed when HB-669 approached HB-170 at close range and reacted to HB-170's right turn with a right turn of his own, losing sight while the distance decreased. The pilot's actions were deemed inappropriate; a more appropriate response would have been to descend slightly to underfly HB-170's turn. However, the pilot's limited experience was considered a factor in his instinctive reaction. The pilot of HB-170 was not found at fault, as he could not have anticipated the other pilot's reaction.
It was uncertain whether the collision could have been avoided once both pilots saw each other again immediately before impact.