PuraVida:
When the plane reverted to Alternate Law (2B if I remember correctly), engaging the autopilot was not an option. Once latched into alternate law, the autopilot can not be reengaged until the system is reset on the ground.
Thank you for the more precise and more accurate statement.
I had forgotten the latching in 2B.
algo, what that means in layman's terms is that not only will AP not function when certain data is unreliable, like airspeed, (the computer for the A/P has a logic function that detects conflicting data and at best avoids applying bad data to flight control inputs), but in the case that the degradation takes the plane into a control law called alternate 2B, you can't even put A/P on until you get to the next landing and maintenance facility and have the various features checked and reset.
Again, you don't actually need Auto Pilot to fly. You can fly the plane without it. (It's a bit more work for the crew to do so, however).
In other control laws and modes, you could reset it in the air once the airspeed data problem gets sorted out.
You are welcome to search Control Laws in the series of threads on this based on the link I gave to you for page one of this thread.
A great deal of very educational discussion was posted here regarding how A330 Control Laws work. These aren't the AutoPilot per se, but the compbination of mechanical, electric, hydraulic, and computer driven inputs to the positioning of the flight controls as a response to pilot action, or level of AutoPilot pilot assistance selected for a given phase of flight.