Since the European Aviation Safety Agency issued an emergency airworthiness directive requiring airlines prepare their Airbus A330 and A340 pilots to cope with the effects of stuck angle of attack vanes, more detail has emerged about the nature of the event that sparked the new measures.
As the A330 was climbing through FL113 (11,300ft) at about 250kt indicated airspeed, with the outside air temperature at minus 12C, the angle of attack vanes became stuck at an indication of 5°.
Airbus says it is keeping its mind open as to whether this was icing-related or some other fault, but icing appears likely because the unit, manufactured by Goodrich, became unstuck during the descent.
The pilots had no direct indication of the stuck vanes and continued to climb toward cruise altitude. But at FL310, as the Mach number increased, the effect of the stuck vanes showed itself through the activation of the "alpha prot" stall-protection system, which pitched the nose down.
Effectively, the combination of high Mach number and a falsely-indicated 5° angle of attack misled the A330's flight-control system into concluding the aircraft was approaching a high-altitude stall, so it took automatic action to reduce the angle of attack.
At all times during the flight, says Airbus, the indicated airspeed was reading correctly, but the stall-protection system depends on angle-of-attack readings.
The crew levelled the aircraft and turned off all three air data reference units. This action took the aircraft out of normal flight law into alternate, which de-activated the stall-protection system. Then the pilots reviewed the situation and decided to divert. During descent, Airbus notes, the angle-of-attack vanes became unstuck once more.
When the event occurred Airbus notified its customers, via an all operators telex, as well as EASA, under the mandatory occurrence reporting system. But there has been no call for a formal incident investigation.
After consultation with Airbus, EASA issued an emergency airworthiness directive requiring airlines to amend A330 and A340 flight manuals to include a drill for pilots to adopt if this situation occurs. This drill largely reflects the actions of the incident crew: level out, taking account of safety altitude, maintain the same airspeed, and trip out two of the three air data units to achieve alternate flight control law and disengage the angle-of-attack protection.
Airbus has praised the pilots of the A330, commenting that their systems knowledge and airmanship was good. During the remainder of the flight to the diversion airport, which was uneventful, Airbus noted that they kept a close eye on their attitude, power and airspeed relationship, because neither the artificial horizon nor the airspeed indicator was affected by the angle-of-attack sensor problem.
Airbus says this is the only know occurrence of this type, but it is reviewing the design of its heated sensors and their resistance to icing.