Ansett-ANA Flight 149

Investigation

On the morning after the crash, a team of 22 members of the Department of Civil Aviation reached Winton to investigate the accident. The crash site on Nadjayamba Station was flat and dry, with only a few trees. The investigation was difficult because most of the aircraft was destroyed in the impact and subsequent fire. After two weeks of investigation at the crash site, most of the wreckage was catalogued and secured in crates. The crates were then transported to Melbourne, where an empty wool store was hired for the purpose of laying out all items of wreckage in their original position in the aircraft.

The aircraft was equipped with an early-model flight data recorder, so this was the first accident investigation in Australia to be aided by information from such a recorder. Housed in the forward belly locker, the recorder was damaged in the crash and subsequent fire, but it provided enough information to allow reconstruction of the Viscount's flight path until the moment of impact. The aircraft was not equipped with a cockpit voice recorder.

The investigation eventually determined that the rotors in the cabin pressurisation blower on number 2 engine began to break up, resulting in severe vibration that loosened the nuts securing the oil metering unit to the blower and allowed oil to escape freely. The rear bearing of one of the rotors also came free, so the rotor gyrated, causing metal-to-metal contact and great heat. The blower was located aft of the firewall, and a fire started in the rear of the nacelle when the escaping oil was ignited by contact with the hot metal in the damaged blower. The fire burned the engine control rods, preventing feathering of the propeller. Burning oil flowed into the wheel bay and from there into the leading edge of the left wing, where the fire breached the wall of a fuel tank. The abundant supply of fuel caused the fire to spread throughout much of the left wing and become so intense it caused softening of aluminium alloy and loss of strength of the upper boom (or upper flange) in the wing spar. The spar was critically weakened in the region between number 1 and 2 engines.

With the aircraft at a height between 3,500 feet (1 067 m) and 4,000 feet (1 220 m) the outer part of the left wing folded upwards and the remainder of the aircraft rolled to the left to meet it. The propeller of number 1 engine slashed the roof of the cabin before the detached part of the left wing separated from the remainder of the aircraft. With the roof slashed open, the airstream entered the fuselage and peeled away large segments of the cabin roof. The fuselage disintegrated aft of the propeller slash. Passengers and passenger seats from the rear of the cabin were ejected into the airstream, some passing through the ball of fire created by fuel from the severed left wing. The tail and rear fuselage aft of the rear cabin door remained intact but broke away from the rest of the fuselage.

The investigation was the longest and most detailed of any aircraft accident investigation conducted in Australia. It concluded that the probable cause of the accident was:

The means of securing the oil metering unit to the no. 2 cabin blower became ineffective and this led to the initiation of a fire within the blower, which propagated to the wing fuel tank and substantially reduced the strength of the main spar upper boom. It is probable that the separation of the oil metering unit arose from an out-of-balance condition induced by rotor break-up but the source of the rotor break-up could not be determined.

When an oil metering unit was installed on a cabin pressurisation blower, locking wire was used to ensure the five securing nuts did not rotate and become loose. In the wreckage of number 2 cabin blower accident investigators found none of the nuts and no locking wire. The investigators believed that when the blower was last overhauled the oil metering unit may have been re-attached without locking wire securing the nuts. The blower was overhauled and then installed on VH-RMI in April 1966.

The investigation discovered that a couple of years earlier, fires had occurred in one of the cabin pressurisation blowers on a Vickers Viscount in Canada and another in the British West Indies. Both fires broke out after loosening of the nuts securing the oil metering unit to the blower. Both occurred during testing of the engine on the ground. At the time, the blower manufacturer was not advised. It was not until the crash of Flight 149 in Australia that the blower manufacturer became aware of the need for these cabin pressurisation blowers to be modified.