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.
As a result of this occurrence and the subsequent discovery of two other aircraft found to have sustained similar structural deterioration due to moisture ingress, the aircraft manufacturer issued Service Bulletin No 24 on 27 February 1998. This Service Bulletin required an immediate inspection of the aileron hanger ribs of factory-built aircraft which had accumulated more than 300 flight hours or were more than 12 months old. The Civil Aviation Safety Authority issued Airworthiness Directive AD/CA25/7 requiring inspection in accordance with the manufacturer's service bulletin by 3 March 1998 or before further flight.
|Date:||08 February 1998||Investigation status:||Completed|
|Time:||1220 hours CST|
|Location:||20 km SE Darwin, Aero.|
|State:||Northern Territory||Occurrence type:||In-flight break-up|
|Release date:||01 February 1999||Occurrence category:||Accident|
|Report status:||Final||Highest injury level:||Fatal|
|Aircraft manufacturer||Skyfox Aviation Ltd|
|Type of operation||Sports Aviation|
|Damage to aircraft||Substantial|
|Departure point||Emkaytee, NT|
|Departure time||1200 hours CST|
|Role||Class of licence||Hours on type||Hours total|