What happened
On 8 August 2010, during a gliding event at Bicester Airfield in Oxfordshire, an SZD-24-4A Foka 4 with registration G-DBZZ experienced a catastrophic structural failure during its second winch launch of the day. While the aircraft was climbing, the left wing bent upward at an angle of approximately 20 degrees, followed shortly by the right wing. This caused the wings to detach from the fuselage. The wings remained connected to each other at the main spar joint but separated from the aircraft body. The fuselage entered a steep, nosedown attitude and struck the ground with high speed. The pilot sustained fatal injuries in the impact.
The investigation
The AAIB investigation focused on the wing attachment mechanism, specifically the wing main fitting. Examination of the wreckage revealed that the wings had become detached due to a failure in the lower bevel bolt. Metallurgical analysis showed that the lower bevel bolt had fractured in shear.
Investigators found that during the rigging process, the lower attachment lugs were not correctly aligned within the lug stack. The use of a non-approved, modified speed brace—rather than the manual-specified hand-operated T-wrench—had reduced the tactile feedback necessary to detect resistance. This allowed the rigging team to inadvertently complete the required number of turns without realizing the lower bolt had not fully engaged. Furthermore, the investigation noted that the upper bolt could appear correctly seated even when the lower joint was compromised, providing a false sense of security.
Findings
- The lower bevel bolt of the wing main fitting had not fully engaged with the lower lug stack during the rigging process.
- The use of a modified speed brace instead of the approved T-wrench prevented the pilot from feeling the resistance that would have indicated improper alignment.
- The structural integrity of the joint was significantly compromised, leaving it with less than half its normal shear strength.
- The wing bending moments experienced during the second, faster winch launch exceeded the reduced capacity of the partially secured joint.