European safety authorities have ordered modified Airbus flight procedures, intended to deal with blocked angle-of-attack sensors, be extended to include A320-family aircraft.
The emergency measure was initially introduced for Airbus A330s and A340s following an incident in which an aircraft suffered blocked sensors during climb, resulting in autopilot disconnection and activation of stall-protection systems.
Investigators are trying to understand the reason for the blockage and the possible role of conic plates installed on the sensors.
Airbus drew up a technique to counter the problem, centred on switching off air data reference units to revert to alternate flight-control laws and disengage the stall-protection system.
While the European Aviation Safety Agency had previously instructed A330 and A340 operators to include the technique in the aircraft flight manual, it has broadened the directive to include A320-family jets.
Conic plates of "similar design", says EASA, are fitted to angle-of-attack systems across the Airbus narrowbody family.
Ironically the installation of conic plates had been required by EASA to improve icing protection of the sensors, as part of an effort to prepare for forthcoming changes in certification requirements.
EASA had ordered the installation on Thales and Goodrich sensors.
The authority has been tackling a separate issue relating to Thales sensors after final assembly line flight tests revealed incorrect angle-of-attack data on two different aircraft.
Airbus and Thales discovered that oil residue between the stator and rotor sections of the angle-of-attack vane became viscous at low temperatures and obstructed the sensor's movement. The problem was traced to incorrect removal of machining oil during the manufacturing process.
EASA subsequently ordered the identification and replacement of all suspect units with serviceable ones.