Summary
On 27 November 1966, at approximately 1530 MEZ, a Bücker 131 (registration HB-URB) collided with a parked Piper Cub (registration HB-ODG) while taxiing on the parking area at Pruntrut airfield. Both aircraft sustained damage.
Investigation
Aircraft
HB-URB held a valid airworthiness certificate and was in airworthy condition.
Pilot
The pilot, born in 1909, held a valid private pilot license and had approximately 3300 hours of flying experience. He had held an instructor endorsement for private pilots since 1949.
Weather and Terrain
Weather had no direct influence on the occurrence. The ground was wet and partially frozen. The incident occurred on the parking area between the hangar and the runway, on flat, grass-covered ground.
Sequence of Events
The pilot was taxiing from the parking area to the runway and needed to pass through a gap between two parked aircraft. The steerable tailwheel was engaged but did not provide sufficient directional control on the wet and slippery ground, so the pilot used the wheel brake to steer. Intending a slight left turn, he applied brake pressure. The left brake, which was hard to actuate, initially did not respond. When additional foot pressure was applied, the left brake suddenly engaged sharply, causing the aircraft to yaw approximately 45 degrees. The pilot could not stop the aircraft on the wet, partially frozen ground, and the nose of HB-URB struck the right wing tip of the parked Piper Cub HB-ODG.
Damage
HB-URB suffered damage to the propeller, minor damage to the canopy, and minor damage to the left upper wing. HB-ODG sustained significant damage to the left wing tip.
Discussion
It is known that the wheel brakes on the Bü 131 type often operate hard and respond irregularly. In this case, the taxi speed, although likely not high, may not have been adequately reduced for the unfavorable ground conditions.
Conclusion
The taxi collision occurred because the aircraft deviated sharply from the intended taxi path due to unintended hard braking and could not be stopped in time.