Skyfox HEDARO CA25N, VH-KKM, 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.