What happened
On May 8, 2011, a HK 36 R Super Dimona motor glider was performing a training flight at the Bensheimer Stadtwiesen glider airfield. The flight was intended as an instructional session for a student pilot accompanied by a flight instructor. After completing three previous flights that day, the crew attempted a fourth takeoff from runway 14.
During the takeoff roll, the aircraft reached a speed of approximately 80 to 90 km/h before lifting off. Immediately following rotation, the aircraft entered an uncontrolled climb. The crew attempted to correct the pitch by applying full forward elevator control, but the aircraft remained unresponsive. During this maneuver, the occupants heard a loud bang. At an altitude of only 10 to 15 meters, the aircraft entered a stall, pitched forward, and struck the ground heavily. There were no fatalities and no injuries to the two occupants, though the aircraft sustained heavy damage.
The investigation
The BFU examined the wreckage and the control mechanisms of the aircraft. Investigators focused on the elevator control linkage, specifically the bellcrank connecting the elevator pushrod. During the inspection, the BFU found that the connecting bolt and self-locking nut were missing from the bellcrank.
Investigators recovered an 18 mm bolt from the rear fuselage segment. Microscopic analysis of this bolt revealed horizontal wear marks, indicating lateral movement within the connection. The investigation also revealed that during maintenance work performed in November 2010, the existing hardware had been reused rather than replaced with new components.
Findings
- The primary cause of the accident was the failure of the elevator control linkage connection due to the use of inappropriate hardware.
- The connection between the elevator pushrod and the bellcrank was secured with an 18 mm bolt that was too short to engage the locking mechanism of the self-locking nut.
- An additional washer had been used in the assembly, which further prevented the bolt from protruding sufficiently through the nut to ensure a secure lock.
- The self-locking nut gradually loosened and was lost, allowing the remaining bolt to work itself out of the assembly laterally until it finally dislodged during the takeoff roll.
- The use of improper hardware was linked to previous maintenance work where old components were reused instead of being replaced with the manufacturer-specified 24 mm bolt and appropriate nut configuration.