Originally Posted by
DutchRoll
When the air data side of the system decides that all its pitot-static data is unreliable it automatically displays the BUSS on both PFDs in the A330. The BUSS uses only data from the AOA vanes on the airspeed tape side of the display. That's a "normal" reversionary mode on the aircraft built into the system software.
That's not how it works on our A330s that have BUSS; it's not displayed until the flight crew turn off all three ADRs. In the case of unreliable speed, the
pilot decides the airspeed is unreliable and then applies the UNRELIABLE SPEED INDICATION procedure. That procedure leads the flight crew to select all three ADRs off when below FL250, which causes the BUSS to be displayed.