What happened
On 6 June 1999, a Cuby II, registration G-BVNA, was participating in a group flight with two other microlight aircraft near Mullaghmore Airfield. The flight, consisting of two crew members, was intended to route north over the Giant's Causeway. While the aircraft were flying at approximately 1,200 feet, the crew of G-BVNA turned right and descended toward the cliffs behind the Causeway.
Witnesses on the ground and in the air observed the aircraft entering a descending spiral. Observers noted that one of the wings appeared to be 'folded' or 'flipped up' during the descent. The aircraft impacted the ground approximately 70 metres downwind from the cliff edge at a high speed and a nose-down angle. The accident resulted in one fatality among the crew.
The investigation
Investigators examined the wreckage at the site and later at Farnborough. The examination of the airframe revealed that the right-hand outer wing panel had suffered an upward failure in flight. Specifically, the failure originated in the main wing spar, which experienced a crippling failure of the upper cap due to high compressive loads. This was followed by the failure of the rear spar just inboard of the wing strut attachment point.
Evidence from the left wing showed incipient buckling in the main spar, suggesting that a similar structural collapse was imminent on that side. The investigation also looked into the meteorological conditions, noting that while the surface winds were around 20 knots, the cliffs could generate significant turbulence. The investigation also reviewed the aircraft's flight envelope, which indicated a 4g limit load factor at 86 mph.
Findings
- The primary cause of the accident was the upward failure of the right-hand outer wing panel due to structural instability.
- The failure was triggered by a combination of upward wind gusts from the cliffs and potential wing loading from control inputs.
- The main wing spar failed due to the buckling of the upper cap under compressive loads.
- The left wing was also at risk of similar failure, as evidenced by incipient buckling in its main spar.
- The structural design of the wing, specifically the lack of torsional stiffness and the simple construction of the wing ribs, may have contributed to the vulnerability of the spar.