What happened
On April 11, 2022, at approximately 12:40, a Bölkow Waggon- und Maschinenbau Phoebus C glider departed from the Oerlinghausen glider airfield for a planned thermal flight. Shortly after 12:45, the airfield manager received an urgent distress call. While the initial message was unclear, a subsequent transmission indicated that the aircraft was experiencing rudder control issues.
The airfield manager observed the glider in an uncontrolled flight state, which briefly stabilized before the aircraft entered a second period of uncontrolled flight following reports of elevator control problems. Video evidence from a witness and a local webcam captured the aircraft in various states of instability, including a spin, a series of undulating turns, and a final, steep descent where the glider pitched over its right wing and plunged into trees. The aircraft struck a path in the Oerlinghausen area, resulting in the death of the pilot and the total destruction of the aircraft.
The investigation
The BFU investigation focused on the mechanical integrity of the rudder control system. Investigators examined the rudder pedals and the cables connecting them to the control lever. During the technical examination, it was discovered that turnbuckles had been installed in the rudder control cables, a feature not standard for this aircraft model. These turnbuckles appeared to have been retrofitted by the owner.
Microscopic analysis of the hardware revealed that the right turnbuckle had failed. Specifically, the steel screw of the turnbuckle had stripped the internal brass threads of the nut. The investigation also found that the safety wire used to secure the components was improperly installed.
Findings
- The primary cause of the accident was the loss of rudder control due to the mechanical failure of the rudder control system.
- The failure was triggered by the use of non-standard turnbuckles that had been retrofitted to the control cables.
- The steel threads of the turnbuckle screw were insufficiently engaged with the softer brass threads of the nut, providing only minimal holding strength.
- Under the tension of rudder pedal input during flight, the harder steel threads stripped the brass threads from the nut, causing the connection to separate.
- The safety wire was not applied according to regulations, which allowed the connection to fully detach once the threads failed.