What aircraft was involved and where did it happen?

The accident on 1966-07-10 involved a AG FÜR DORNIER-FLUGZEUGE BÜ 131, registration HB-UTG, at Lenk, BE, CH.

What was the probable cause of the accident?

The accident was caused by the pilot attempting a takeoff with a weight and configuration that were not suitable for the limited runway length and existing obstacles, leading to a collision with high-voltage power lines during a steep climb.

Bücker 131 crashes into power lines during departure in Lenk

A Bücker 131 aircraft struck high-voltage power lines during takeoff from an airshow field in Lenk, resulting in the total destruction of the aircraft.

What happened

On July 10, 1966, a Bücker 131 (Jungmann), registration HB-UTG, was performing a departure from an airfield in Lenk, Switzerland, following an aerial display. The pilot was attempting to depart for a return flight to Grenchen, carrying one passenger.

During the takeoff roll, the aircraft traveled further along the ground than anticipated. The pilot initiated rotation approximately 80 meters before the end of the available runway. To clear a 10-meter-high power line located shortly after the field boundary, the pilot entered a steep climb. While the aircraft successfully cleared the first line, it failed to clear a second, slightly higher line located 75 meters further along the path. The landing gear struck the 16,000-volt copper wires, severing them. The impact caused the aircraft to lose significant momentum and drop heavily onto the ground, where it slid on its belly and caught fire. Both occupants escaped the wreckage without injury.

The investigation

The investigation examined the aircraft's performance capabilities, the airfield conditions, and the takeoff parameters. Investigators analyzed the impact of a modified exhaust system, known as a "Frankfurter Topf," which had been installed on HB-UTG in 1964. This modification resulted in a slight reduction in engine power, which increased the required takeoff distance by approximately 5%.

Technical data regarding the takeoff roll and climb gradient were compared against the official airfield regulations. The investigation also reviewed the pilot's prior reconnaissance of the field and the weight of the aircraft, which included a passenger and a limited fuel load of approximately 50 liters. The engine was found to be in good working condition, and no mechanical defects were identified in the aircraft.

Findings

The pilot utilized a payload that was inappropriate for the available takeoff distance.
The aircraft's climb profile was forced into an excessive attitude due to the proximity of the power lines to the takeoff path.
The installation of the modified exhaust system reduced engine performance, thereby lengthening the takeoff roll.
The pilot relied on personal experience and a visual reconnaissance of the field rather than a precise numerical estimation of the required takeoff distance.
The takeoff site's obstacles did not align with the safety margins required for the aircraft's weight and performance under the specific atmospheric conditions.