What happened
On September 6, 1964, during a flight day at Beromünster airfield, a Bücker-Jungmann (model Bü 131), registration HB-USL, was parked in an area outside the standard parking zone due to high traffic volume. At approximately 16:00 UTC, the pilot began taxiing the aircraft toward the taxiway. During this maneuver, the pilot received hand signals from a ground official, which the pilot interpreted as instructions to proceed straight ahead. As the aircraft moved forward, it struck a drainage pipe protruding roughly 12 cm from the ground, which had been marked with yellow flags. The impact caused the left landing gear to bend backward and resulted in the propeller striking the ground and breaking.
The investigation
The investigation examined the interaction between the pilot and the ground official, as well as the visibility of the obstacle marking. It was noted that the Bü 131 has inherently poor visibility during taxiing because the pilot sits low in the fuselage, with significant blind spots created by the engine and lower wings. The ground official stated he had attempted to warn the pilot of the obstacle by pointing at it and then forming a cupped shape with both hands. However, the pilot, having looked in that direction and nodded, appeared to believe he understood the instruction for taxiing. There was conflicting information regarding whether the pilot actually saw the yellow flags marking the obstacle or if he mistook them for taxiway markers.
Findings
- The primary cause of the accident was a misunderstanding between the pilot and the ground official regarding hand signals.
- The ground official's signals were misinterpreted as clearance to proceed rather than a warning of an obstacle.
- The marking of the obstacle was insufficient, as the yellow flags used for the drainage pipe were similar in color to the yellow flags used for taxiway boundaries.
- The pilot failed to maintain adequate attention or verify the path ahead while taxiing without supervision.