As a part of its investigation of AF447, the BEA examined other incidents of unreliable airspeed readings in Airbus A330 and A340 fleets. They found 13 events at cruise altitude among five operators that supplied data, including four at Air France. None resulted in an accident, but crews reported being confused.
There is a drill in the crews' quick reference handbook (QRH) for managing unreliable airspeed incidents, but the AF447 crew did not refer to it and, in several of the other incidents studied, the crews did not use it either. In all cases, like the AF447 crew, the aircraft were in blind flying conditions, were experiencing turbulence, the autopilot had disengaged, and the flight control law had changed from normal to alternate. However, there were no unintentional altitude variations exceeding 1,000ft (300m), with the exception of four cases where the crews deliberately descended following stall warnings.
Essentially the QRH drill, if the event occurs while cruising at a high altitude as AF447 was, is to maintain the attitude and the power setting and hold these until the airspeed readings become reliable once more.
If in doubt, states the drill, adopt a +5˚ pitch attitude and select the power levers to "climb". In the case of AF447, the cruising pitch attitude that had been maintaining stable flight before the incident was +2.5˚, and when the auto-thrust disengaged it did not change the power setting, so if the pilot flying had not reacted by pitching up dramatically to +12˚, as the BEA observed, the crew would have avoided trouble.