Ansett-ANA Flight 325

Investigation

As pieces of wreckage from VH-TVC were progressively recovered from Botany Bay they were laid out in a hangar at Sydney airport to allow investigators to search for the cause of the accident. The right tailplane and the outer section of the right wing had received almost no damage on impact with the water but the main wreckage in Botany Bay showed extensive disintegration, suggesting a very high speed of impact. No evidence was found of any fault or mechanical failure that might have existed prior to the accident.

It soon became clear that the spar in the right wing had broken and the outer section of the wing had been torn away by the airstream. The lower boom (or lower flange) in the wing spar had failed in upward bending at station 323 due to extreme overload. The wing had been overloaded while the aircraft was flying at very high speed, probably in excess of the maximum safe speed of 260 kn indicated airspeed. As an immediate consequence of the aircraft's gyrations during failure of the right wing, the right tailplane had also been subjected to excessive forces and had separated from the fuselage.

Engineers assisting the accident investigation calculated that for the wing spar to fail in the way it did in VH-TVC would require the aircraft to be flying faster than its maximum speed of 260 kn and, while being subjected to a severe recovery manoeuvre by the crew, to encounter a very strong gust, possibly as much as 100 ft/s. At the time of the accident investigation, gusts up to 72 ft/s had been measured inside thunderstorms by suitably equipped research aircraft. Investigators were aware of the crash of Capital Airlines Flight 75, a Vickers Viscount that crashed in Maryland, USA in May 1959 after encountering extreme turbulence associated with thunderstorms.

All the wreckage lay in a trail aligned north-south. By making assumptions about the likely terminal velocities of key pieces of wreckage, accident investigators were able to determine the approximate location, height and speed of the aircraft at the time it broke up. They believed the outer section of the right wing was torn away when the aircraft was south of its intended flight path, heading in a northerly direction, and at a height between 3500 and 5500 ft. They believed this occurred at a time when the aircraft should have been at about 9000 ft, heading west and passing over a radio navigation aid nine miles away. It was necessary for the investigation to find a rational explanation as to why the aircraft was so far from where it should have been.

Accident investigators studied a scientific report titled The Thunderstorm published in the USA in 1949. This report proposed that where two thunderstorms were separated, edge to edge, by less than 6 mi there was a likelihood of severe turbulence in the clear air between the two. The accident investigators believed it was likely that Flight 325, while flying west between two mature thunderstorms, encountered strong turbulence that caused the crew to lose control and the aircraft to lose a significant amount of height and enter the thunderstorm to the south of the airport. While flying north, possibly in an attempt to escape the thunderstorm, the crew encountered continuing strong turbulence that caused control to be lost again. The aircraft accelerated to its maximum safe speed or faster and while the crew were struggling to regain control the aircraft was suddenly subjected to extreme turbulence that caused the right wing and the right tailplane to fail.