What happened
On March 7, 2015, a private flight training session was underway in Kägiswil, Switzerland, under calm weather conditions. While performing the sixth landing of the session, the pilot was executing a crosswind approach when the left cockpit door unexpectedly opened upward. Initially, the door remained fully open against the airflow. As the aircraft turned into the final approach, the door began to flutter violently. This aerodynamic force caused the door to tear away from both its forward and rear attachment points, resulting in the door falling downward. Despite the loss of the door, the RANS S-10, registration HB-YGY, remained fully controllable, allowing the pilot to complete the landing safely. The incident resulted in one person being slightly or not injured and caused damage to the aircraft's cabin canopy and fabric covering.
The investigation
The investigation focused on the mechanical failure of the door's securing mechanism. The aircraft features a lightweight Lexan door with an aluminum frame, which is held in place by a clamping mechanism located at the fuselage level near the wing. To ensure the door remains closed, a rubber strap is intended to secure the clamp via a bracket. Investigators determined that the rubber strap must have become detached prior to the event. This detachment allowed sufficient negative pressure to build up on the exterior of the door, enabling it to swing open during the approach.
Findings
- The primary cause of the door detachment was the failure of the rubber strap to secure the clamping mechanism.
- The loss of the securing strap allowed external aerodynamic pressure to open the door during the crosswind leg of the approach.
- Subsequent aerodynamic fluttering during the turn to final approach led to the structural failure of the door's attachment points.