Investigation Summary
The accident was traced to the number 2 cabin pressurization blower, located aft of the engine firewall. A rotor within the blower began to break up, creating severe vibrations that loosened the nuts securing the oil metering unit. As the unit separated, oil escaped and was ignited by the intense heat generated by metal-to-metal contact within the failing blower. The resulting fire was catastrophic. It burned through engine control rods, preventing the crew from feathering the number 2 propeller, and spread into the wheel bay and leading edge of the left wing. The fire eventually breached a fuel tank, creating an inferno that softened the aluminum alloy of the wing's main spar upper boom. Under the aerodynamic loads of flight, the weakened spar failed, causing the outer left wing to fold upward and detach. As the wing separated, the number 1 propeller slashed the fuselage roof, leading to the total disintegration of the aircraft's mid-section.
Final Conclusions
The investigation concluded that the probable cause was the failure of the means of securing the oil metering unit to the number 2 cabin blower. This failure, likely induced by an out-of-balance condition from rotor break-up, led to an internal fire that propagated to the wing fuel tank and critically reduced the strength of the main spar upper boom. While the separation of the oil metering unit was attributed to vibration from the rotor failure, investigators explicitly stated that the cause of the initial rotor break-up could not be determined and was not due to incorrect assembly during the last overhaul.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Cabin Pressurisation Blower
A Roots-type blower driven by the engine to provide pressurized air to the cabin; its internal rotor failure and subsequent oil leak initiated the fire.
Oil Metering Unit
A component attached to the blower that regulates oil flow; its separation due to missing locking wires allowed oil to ignite on hot blower components.
Main Spar Upper Boom
The primary load-bearing structural member of the wing, which lost its structural integrity when exposed to the intense fuel-fed fire.
Interesting Facts
- 01This was the first Australian accident investigation to be aided by a flight data recorder.
- 02The aircraft was not equipped with a cockpit voice recorder.
- 03The investigation discovered similar ground-based blower fires had occurred previously in Canada and the British West Indies, but the manufacturer had not been notified.
- 04The main spar failed between the number 1 and number 2 engines due to heat-induced softening of the aluminum alloy.
- 05The tail section and rear fuselage aft of the rear cabin door remained relatively intact and unburned after separating from the main wreckage.
- 06The crash remains the fifth-deadliest civil aviation accident in Australian history.
Safety Actions & Advisories
Blower Modification
Following the accident, the blower manufacturer became aware of the safety risk and initiated modifications to the cabin pressurisation blowers to prevent similar failures.