What happened
On July 9, 2020, a PZL-Bielsko SZD-59 ACRO glider, registration OY-RUX, was performing a cross-country flight from Vesthimmerland (EKVH) under Visual Flight Rules. Following a winch launch earlier that morning, the pilot had been flying for approximately three hours and twenty minutes when they began a descent toward the airfield.
During the final stages of the flight, the pilot discovered that the aircraft's battery was depleted. As the pilot approached the grass runway (runway 29), they attempted to extend the landing wheel but found the control handle would only move about 25% of its required range before jamming. In an attempt to resolve the issue, the pilot used both hands to force the handle forward, but this rotation actually worsened the jam.
With the altitude reaching a critical level and the landing wheel stuck in the retracted position, the pilot elected to land on the asphalt runway. The aircraft performed a wheel-up landing, resulting in substantial damage to the lower fuselage, frames, and landing wheel doors.
The investigation
The Danish Accident Investigation Board conducted a technical examination of the landing wheel mechanism. Investigators found that while the handle functioned normally when positioned horizontally, any slight rotation of the handle tube caused the system to bind.
Upon detailed inspection of the landing wheel handle tube, investigators discovered that the nut intended to secure the rod end to the tube was loose. Furthermore, the tab washer was not properly bent to lock the nut in place. This allowed the handle tube to rotate around its longitudinal axis, which obstructed the forward movement necessary to extend the wheel. When the nut was properly tightened and secured, the mechanism functioned correctly.
Additionally, the investigation noted that the pilot had reinstalled the battery after a two-hour charge, assuming it was sufficient based on the charger's indicator light. The investigation also found that the pilot's ability to communicate the emergency was hindered by the dead battery and a lack of awareness that radio transmissions were impossible without power.
Findings
- The primary cause of the accident was a mechanical failure of the landing wheel extension system caused by a loose and unsecured nut on the handle tube.
- The rotation of the handle tube during the pilot's attempt to force the handle forward exacerbated the jamming mechanism.
- The pilot's decision-making regarding runway selection was likely impacted by the depleted battery and potential lack of familiarity with switching battery sources.
- The battery was likely insufficiently charged due to the pilot's reliance on a simple red/green indicator light on the charger.