When there are significant differences between all airspeed sources, the
flight controls of an Airbus A330 or A340 aeroplane will revert to alternate
law, the autopilot (AP) and the auto-thrust (A/THR) automatically disconnect,
and the Flight Directors (FD) bars are automatically removed.
It has been identified that, after such an event, if two airspeed sources
become similar while still erroneous, the flight guidance computers will:
- Display FD bars again, and
- Enable autopilot and auto-thrust re-engagement
However, in some cases, the autopilot orders may be inappropriate, such as
possible abrupt pitch command.
The directive is issuing a new procedure to make sure the speed is correct before engaging again the autopilot.
The probability of this event to happen is probably very thin, but I imagine under the same conditions it is possible that two probes are giving the same wrong information. Designers of the system probably have thought that this case would never happen and that it was useless to make a procedure to deal whith that...
I would be very interested to know if this event ( two speeds being equals while being wrong) was anticipated on other planes, with a specific procedure or a different behavior of the system. This directive doesn't apply to the A340-500 and 600 for instance... If someone in the know can enlighten us...