What happened
On June 1, 2019, a Schempp-Hirth Nimbus 4M glider, registration OY-XXL, was performing a solo landing at Arnborg (EKAB) under visual flight rules. Prior to the flight, the pilot and a technician had successfully tested the engine following previous starting difficulties. During the approach, the pilot noted the smell of exhaust gases at low altitude, which they attributed to the recent engine testing.
As the aircraft descended to approximately 5 meters above the runway, the flap control handle unexpectedly dislodged from its selected position. Due to the mechanical design and spring tension, the handle moved from a positive flap setting to a negative setting. This sudden change caused the glider to sink rapidly. Despite the pilot's attempt to pull back on the control stick and re-engage positive flaps, the low altitude left insufficient time to recover. The aircraft struck the runway with significant force, resulting in serious injury to one person and substantial damage to the glider's wings and landing gear.
The investigation
The investigation by AIB Denmark focused on the mechanical state of the flap control system. Technical examinations of the aircraft revealed several discrepancies. While the flange of the flap handle showed wear, the most critical findings involved the locking mechanism. The grooves in the handle's guide rail were worn, with the level of degradation matching or exceeding the limits illustrated in the manufacturer's technical guidance. Furthermore, the spring force intended to secure the handle in place was measured at less than 2 Newtons, significantly below the manufacturer's minimum requirement of 8 Newtons.
Investigators also noted that the technician responsible for the most recent annual inspection was aware of wear in the system and had previously replaced the guide rail. However, the technician noted that the manufacturer's technical note (SHK-M-01-11) did not provide specific, measurable criteria for acceptable wear, only illustrative images.
Findings
- The primary cause of the accident was the insufficient spring tension in the flap handle mechanism combined with wear in the flap control system.
- This mechanical failure allowed the handle to move from a positive to a negative flap setting during the landing phase.
- The resulting rapid loss of lift occurred at such a low altitude that the pilot could not execute an effective recovery.
- The wear on the guide rail and the handle flange had reached levels that compromised the stability of the selected flap position.
- Previous similar accidents involving the same type of flap control failure had occurred in Denmark in 2009 and 2010.