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Old 12th Jan 2008, 17:37
Norman Stanley Fletcher
Join Date: Jul 2001
Location: 'An Airfield Somewhere in England'
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I may be repeating some of the excellent descriptions given by other Airbus pilots but for non-Airbus pilots it may be helpful to have a 'Noddy's guide' to Airbus fly-by-wire control laws. There are 4 recognised states of Airbus flight control capability depending on computer and hydraulic serviceability. They are known as Normal Law, Alternate Law, Direct Law and Mechanical Back-up.

'Normal Law' is the standard flight control mode that pilots see every working day. Most pilots will go their whole professional career and never see any other mode except in the simulator. It requires that most flight control computers are operating normally and all hydraulic systems are functioning correctly. That is the mode for which the Airbus has become famous, whereby such features as limited g-loading, a maximum bank angle of 67 and the inability to stall are provided. Alternate Law is a degraded mode that still provides certain protections but will allow the aircraft to stall and to overbank, for example.

There is a further sub-mode somehwere between Normal Law and Alternate Law known as 'Abnormal Attitude Law' which caters for a scenario where the aircraft is thrown into a flight regime way outside the situations normally catered for in Normal Law. I have no idea if that is what happened in this case, but to give you a feel for what the pilots may have experienced, these are the paramaters that would have to occur for Abnormal Attitude Law to be invoked:

Pitch attitude > 50 nose up or 30 nose down
Bank angle > 125
Angle of attack > 30 or < - 10 (- 15 for A319 and A321)
Speed > 440 knots or < 60 knots
Mach > 0.91 or < 0.1

The system applies an abnormal-attitude law in pitch and roll if the aircraft exceeds any of these limits in flight. The law in pitch is the alternate law with no protection except load-factor protection and without auto trim. In roll it is a full-authority direct law with a yaw mechanical.When the aircraft has recovered from its abnormal attitude, the flight control laws in effect are :

in pitch : alternate law without protection with autotrim.
in roll : full authority direct law with yaw alternate law.
There is no reversion to direct law when the pilot extends the landing gear.

I hope that may be helpful to non-Airbus pilots, but in no way suggests that is what occured in this case. It nonetheless gives you a feel for the severity of the situation required to enter this scenario.

Last edited by Norman Stanley Fletcher; 12th Jan 2008 at 18:33. Reason: edited for typo!
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