History of the Flight
The aircraft had recently undergone an engine service and a new instrument panel. Several weeks before the accident, a test flight was conducted by the pilot after engine ground runs. Subsequently, a Permit to Fly check flight revealed an intermittent radio issue. A further flight was required to complete the requirements for the Permit renewal. During this flight, the aircraft and radios were tested with no faults, and the aircraft returned to Clench Common Airfield. During the final stages of the approach to landing at approximately 1605 UTC on 1 February 2022, the flaps retracted uncommanded at about 40 ft. The pilot immediately recognized what had happened and attempted to raise the nose but was unable to prevent the aircraft from striking the ground heavily. Both the pilot and passenger were injured.
Accident Site
The aircraft was severely damaged. The first impact was on the nose undercarriage leg, which sheared off, followed by the main undercarriage legs, which failed at some point. The aircraft came to rest on its fuselage the right way up, and the starboard wing was dislodged, held only by the trailing edge bolt.
Aircraft Information
The Jabiru UL-450 kit is supplied with manual flaps operated via a lever mounted on the side wall to the left of the pilot's head. The lever has a button that latches it in three positions: flaps up, mid-flap, or full flap. The flap lever is pulled laterally to release the peg from the hole, enabling the flaps to be moved to a new position. According to the Light Aircraft Association's type acceptance data sheet, it is common for the attach/pivot bolt of the flap actuating handle to not be tightened sufficiently, which can lead to the flap disengaging on approach with potentially serious consequences if not caught quickly by the pilot. The document emphasizes the importance of setting and maintaining the tightness of the flap lever pivot bolt to provide a suitable pre-load to hold the peg in place; otherwise, there is a risk of the flap control jumping the gate. Uncommanded flap retractions on this aircraft type are usually caused either by the flap lever not being engaged with the detent sufficiently or by exceeding the flap limiting speed. The pilot noted turbulence on the approach a few seconds before the flaps retracted, which may have dislodged the flap lever. The aircraft was flying below the flap limiting speed. The flaps are large, about 70% of the span of the wing, so any uncommanded retraction causes a significant loss of lift and change in aircraft attitude.
Analysis
The design of the flap system relies on the pivot bolt being correctly adjusted so that the flap lever does not come out of the selected gate. In the case of G-CBGR, turbulence on the approach may have dislodged the flap lever, causing the uncommanded flap retraction. This occurred at a height at which the pilot could not recover the aircraft before it struck the ground.
Conclusion
The flap handle in G-CBGR became dislodged from its selected gate, leading to the flaps retracting uncommanded by the pilot. Despite the pilot immediately recognizing what had happened, it was not possible to recover the aircraft before it struck the ground. The issue of the pivot bolt on the flap handle in this aircraft type not being tightened sufficiently is well-known and publicized by the LAA in their type acceptance data sheet.