Larvatus Prodeo

The media reports aren’t particularly informative, but the Australian Transport Safety Bureau’s news release contains a fairly good explanation of why a Qantas Airbus A330 airliner suddenly decided to dive a couple of hundred meters (not thousands of metres as some of the more colourful passenger interviews state).

All Airbus airliners since the A320, and the more recent of Boeing’s aircraft models, are “fly-by-wire” craft. That is, there is no direct mechanical or hydraulic connection between the control stick, and the aircraft’s control surfaces, at all. So, even when the plane is not on autopilot, there is a computer system that translates the pilot’s commands on the controls into movement of the various movable bits on the wings and tail. This is by no means a new thing – the A320 first went in to service in 1988, and the F-16 fighter had such a system way back in 1979. Obviously, to get regulatory approval for such systems, the manufacturers had to demonstrate that the systems wouldn’t malfunction and cause the plane to dive into the ground. So all flight control systems implement multiple, redundant control computers, wiring, and whatnot, and the software is developed to the very highest standards, with highly rigorous testing and using the most advanced software engineering techniques to ensure reliability. This isn’t just marketing guff, either; I’m no expert in aviation, but I am a published academic in the area of software reliablility. And so, I’ve read one or two technical papers that came out of Airbus work. They do some very clever stuff (as, I’m sure, do Boeing).

One of the basic tenets of designing reliable systems is redundancy; the aircraft should be able to survive the failure of any single component, and critical components often have triple or quadruple redundancy. And so it is the case with the A320’s flight control system. The first relevant bit was the “angle of attack” sensors on the plane’s exterior, of which there were three. These measure the angle at which the plane is pointing. These are fed into three Air Data Inertial Reference Unit (ADIRU) units, which translate the raw readings of the sensors into processed data, which is then fed to the three, redundant flight computers which end up controlling the aircraft.
Continue reading ‘The Qantas nosedive – what happened’