A320 Flight Controls - stall or no stall?
Dear all. For all non pilots and those who do not know the Airbus, please let me shed some light on its built-in automation:
The Airbus is different from conventional aircraft by having two flight augmentation computers (FAC) which prevent the plane from leaving its normal flight envelope. This includes over- and under speed, g-load, bank angle and the angle of attack (AOA). If all works normally, the aircraft is in “normal law". It cannot be deliberately overbanked, flown at a higher or lower than certified speed and this excludes a stall by definition. And it really doesn’t. Should two flight relevant systems such as a FAC and an air data computer fail simultaneously, the flight control system degrades to “alternate law”. This is true for the A320, the A330 is slightly different in this respect. In “alternate law”, only a g-load protection is retained. The aircraft can be stalled. Adding certain other failures or lowering the landing gear will engage “direct law”, where stick deflections translate directly into proportional surface movement. No protection whatsoever. The Airbus then behaves like a conventional airplane again, but is a bit more difficult to fly in this mode.
The flight augmentation computers do a good job when fed the right information by the sensors. Problems start, when these fail: malfunctioning pitot tubes, for example, might suggest an over speed condition where there is none. In this case, the Airbus is programmed to lift the nose slightly, should the pilots take no corrective action by reducing the speed. This cannot be overridden in “normal law” by pushing the “magical red button”, as has been suggested in an earlier post. The instinctive action of reducing the speed though would definitely be the wrong action at FL 360 and at normal cruising speed. So it doesn't take too much imagination to see how much confusion even such a relatively simple malfunction of automation could potentially cause. There are quite a few other examples as well.
Flying an attitude on the stby-adi and a power setting which together produce an expected airspeed is the initial fix to most problems. Then, diagnosis can start. That's why more basic flying skills are needed with these new generation airplanes, not less. Although the new flight control system was – and probably still is – seen as a bridge to full automation without any human intervention at all, Airbus have recognized the problem and recommend to airlines to increase the training of basic flying skills. Training costs a lot of money, though, and I’d very surprised indeed if many cash-strapped airlines would heed a manufacturer’s recommendation and train more than is required by the regulators.
And yes, I personally have come across quite a few young colleagues who have never flown anything else than the Airbus and for whom interpreting a VOR needle poses a major problem. Not their fault, they just weren’t taught. And sadly, many airlines just seem to accept the remaining level of risk. As long as the whole rest works as it should, no problem… !?