Skyfox Aviation Ltd CA-21, CA21017

The pilot was flying the aircraft at an altitude of about 60 ft
in a steep left orbit near the Elizabeth River. When the pilot
reversed the direction of turn, two of four aileron segments, and
what appeared to be a small piece of fabric, fell from the left
wing. The pilot later reported that the control column locked after
he heard a loud bang. He retarded the throttle as the aircraft
dived out of control into the river. Impact with the water
destroyed the left wing but the cabin and right wing remained
largely intact. The pilot was able to extricate himself from the
submerged cabin, but his efforts to free his passenger were in
vain.

Several weeks after the accident, a helicopter search found two
aileron segments still joined by the interconnecting torque tube.
They were recovered from mangroves 80 m from the river. A third
aileron segment from the left wing was not found.

The investigation found evidence of deterioration of the timber
wing ribs/aileron hangers from which the ailerons were suspended.
The outer layer of plywood displayed "tide" marks where the
trailing edge of the wing ribs had been immersed in moisture. Some
ribs were stained by black mould and had deteriorated due to timber
rot. The aircraft had been standing under a shade cloth roof in
hot, humid, tropical conditions for an extended period. Moisture
had accumulated in the wing and had gone unnoticed.

About a year before the accident, two outboard ribs/aileron
hangers had been broken when the left aileron was pushed against a
hangar door. A subsequent repair had been carried out in accordance
with factory repair instructions. Small sections of fabric had been
removed from the upper wing surface to facilitate the repair and
had been replaced afterwards. Fabric covering the adjacent third
rib/aileron hanger had not been removed for inspection. The part
was not recovered from the river.

Examination by a specialist from the CSIRO Forestry and Forest
Products Division found that the rib/aileron hangers suspending the
inboard aileron segment of the left wing were profoundly affected
by two types of wood rot. The wood rot caused a significant
reduction in strength of the timber laminates. In addition,
over-tightened bolts holding the aileron hinge compressed the
timber laminates.

Witness evidence indicated that there were no birds in the
immediate area when the aircraft descended out of control. The
wreckage was examined for any evidence of bird-strike damage. None
was found. Two of three propeller blades were broken by impact with
water and rocks. The pilot's assessment that the control column
locked, was confirmed by discovery of a bent torque tube holding
the inboard segment of left aileron which had streamlined in the
airflow.