What happened
On May 16, 1998, a Cyclone AX3 microlight, registration G-BUTC, was conducting a private cross-country flight from Waterford Airport toward the New Ross area. While attempting a touch-and-go landing at a private airstrip in Donan/Donanore, the aircraft entered an uncommanded left spiral dive at approximately 800 feet. Although the pilot managed to recover the aircraft and climb to 1,000 feet, a second reduction in power triggered another left spiral dive. During this third descent, the aircraft spiraled toward the south and struck a barley field in a heavy, nose-down attitude. The impact caused extensive damage to the airframe, including the destruction of the nose area, the cockpit, and the undercarriage. There were no fatalities or injuries to the two occupants.
The investigation
The AAIU examined the wreckage and the structural integrity of the wing assembly. Investigators found that the right wing tip had impacted the ground, leaving a distinct mark. Analysis of the right wing revealed that the drag wing cable had become slack and was permanently elongated by 2.5 cm. Crucially, the investigation identified that two rivets securing the aluminium fitting in the rear spar of the right wing had disappeared. This allowed the fitting to rotate within the spar, which increased the angle of attack at the wing tip by approximately 11 degrees. Furthermore, the wing covering fabric showed signs of progressive tearing, suggesting it had been flapping in the airflow for some time prior to the final impact.
Findings
- The primary cause of the accident was the loss of lateral control resulting from a significant reduction in the torsional stiffness of the right wing tip.
- This loss of stiffness was triggered by the loss of rivets in the rear spar end fitting, which allowed the wing section to flex.
- The resulting structural instability led to the progressive failure of the wing's fabric envelope.
- The uncommanded rolls and subsequent spiral dives subjected the aircraft to aerodynamic loads that exacerbated the structural damage.
- The investigation noted that the aircraft's proximity to the coast may have contributed to corrosion of the rivets.
- It was noted that there was no established method or manual instruction for pilots to verify the tension of the wing cables, making the detection of the slack cable extremely difficult.