Skyfox CA25N, VH-KKM, on 1 July 2000

The instructor and student pilot were conducting circuit training. The instructor reported the aircraft was on late downwind when there was a sudden loud bang and the aircraft began to roll to the left. He immediately took control of the aircraft and requested a priority landing. As the aircraft was descending he noticed that all engine indications were normal, however the aircraft required excessive right aileron and right rudder control. A visual inspection revealed the entire right aileron had separated from the aircraft. The descent was continued at a higher than normal speed and the aircraft landed safely.

The separated section of aileron was not recovered. Examination of the remaining aileron to wing attachment (wooden strut) fittings, shows all failures were because of overload. There was no evidence of wood rot or previous damage to any of the fittings.

The ailerons are actuated by a steel control tube from the cockpit, which fits into an aluminium tube (spar) aileron fitting. There is a small shim spacer collar between the inner and outer tubes. The tubes are secured by aluminium pop rivets with steel mandrels.

On the incident aircraft, the inner section of the steel tube shows extensive areas of corrosion (rust), particularly around the securing rivets. There does not appear to be any corrosion proof material present between the tubes or spacer surfaces.

A small broken piece of the aileron (aluminium tube) fitting was still attached to the steel control tube from the cockpit. Part of the broken surface indicates a dark area similar to a pre-existing stress crack, which emanates from a rivet hole. There is also evidence of exfoliation corrosion on this section. The evidence indicates the aileron failure set up at this point. It is probable the aileron then deflected and the air loads tore it from the wing attachments. The initial investigation was able to examine 3 other similar aircraft. Of these, 2 were found to have a significant degree of rust on the inner steel control tube and extensive exfoliation corrosion around the rivet section of the outer aluminium tube.