Qantas Airliner Plunges Out of Control.



A terrifying headline to all who fly in aircraft and it is a very real risk in the age of modern air travel. The infrequency of this occurrence is thankfully rare and the majority are weather related and avoidable.



So, what went wrong with the Qantas Airbus A330-300 aircraft? During the flight the aircraft had several departures from its assigned altitude, the result of these changes were injuries and panic in the cabin as people who were not wearing their seat belts were thrown around.



The majority of modern aircraft are flown by a fly by wire system; this is in response to the development of flight control computers which control the aircraft's altitude and direction of the flight. These computers are designed to keep the aircraft within its design limits. If the aircraft is going too slow, too fast, or approaching the design limits it will take action to correct the situation.



So, how does the computer know whats going on? There are sensors on the aircraft which measure the outside air temperature, air speed, altitude and angle of attack, (the amount of nose up or nose down), all these inputs, and many others, are sent to an Air Data Inertial Reference Unit (ADIRU) which converts them to signals for the Flight Computer so they can operate the flight controls the way the pilots require.



On the Qantas aircraft the ADIRU gave incorrect data to the Flight Computers which made the aircraft climb to reduce an incorrect overspeed and then dive to avoid a non existent stall due to low airspeed. The aircraft autopilot was engaged until the unexpected climb when it disconnected and the crew flew the aircraft themselves. Even though the crew was flying the aircraft the inputs from the ADIRU still continued to affect the aircraft. The design of the system is to keep the aircraft within its limits and the ADIRU was giving erroneous information to the Flight Control Computer that these limits were being approached. The effects were minimised by the crew but they were still bad enough to cause inuries to people who were in the cabin.



This is the first known occurrence of this type of failure with the ADIRU. Airbus have issued a Telex to all the operators of this type of aircraft to provide operational actions to minimise the effects in the event of this happening again. There are 3 ADIRU’s and 3 Primary Flight Computers on the aircraft control system and the investigation will now concentrate on how a failure with one had such an effect on the aircraft.