CIRCUMSTANCES: "The aircraft was engaged in glider towing operations. After a normal glider release at 3 300 ft, the tug aircraft was seen to enter a left descending turn at about 45 degrees angle of bank. At about 2 000 ft, the direction of turn rapidly reversed and the right wing was observed to have collapsed in the vicinity of the wing strut's attachment points. The rate of turn and rate of descent increased rapidly before the aircraft impacted the ground in a high-speed spiral. Examination revealed that the right wing of VH-SUT had failed at the rear spar inboard of the lift strut attachment point between rib numbers 8 and 9. The failure had commenced at a weathered compression shake area. There was no evidence that the aircraft had been used in aerobatic flight or had been subjected to excessive flight loads immediately prior to the failure. The aircraft was in a steady left, descending turn when the wing collapsed. VH-SUT had suffered minor damage to both wings in an overturn accident in late 1987. As a result, the wings were subjected to inspections in accordance with Airworthiness Directive AD/CHA/23 which required inspection of the wing spars for compression failures. No spar damage was found. Shortly after this accident, another Bellanca Scout wing became available for examination and a number of compression failures were found. The aircraft log book indicated that the wing had previously sustained minor damage. A study of foreign accident reports and articles in aircraft magazines also indicated that what was often perceived as minor, superficial wing damage from a main landing gear collapse, overturn, or windgust on the ground, could cause extensive internal damage to the wing spars. It was evident that compliance with AD/CHA/23 would not guarantee that all existing compression shakes would be found." RECCOMENDATIONS: The Bureau made a recommendation to the Civil Aviation Authority in the early stages of the investigation that Airworthiness Directive AD/CHA/23 should be revised to incorporate an inspection of the the whole spar. The Civil Aviation Authority issued an Airworthiness Advisory Circular article on 21 March 1991 and Amendment 1 to AD/CHA/23 on 16 May 1991.