Europe's safety authority is to order Airbus A330 and A340 operators to upgrade flight-control computer software to prevent autopilot engagement should airspeeds become unreliable.

Autopilot and auto-thrust on the types will automatically disconnect, and the aircraft will revert to alternate law, in cases where significant differences emerge between the airspeed sources.

This condition can arise if, for example, the pitot system becomes contaminated with ice.

Previously the European Aviation Safety Agency warned that, under such circumstances, it was possible for two airspeed sources to show similar - but nevertheless erroneous - data.

This would permit the crew to re-engage the autopilot while the airspeed data was unsound, potentially resulting in the autopilot transmitting unexpected commands to the flight-control system - such as an abrupt pitch-up.

EASA had cautioned pilots, in a December 2010 directive, to resist re-engaging the autopilot until they had carried out a cross-check of speed indications to ensure that the airspeed data was reliable.

Since that directive was issued, new primary flight-control computer software has been developed which, EASA states, will "inhibit autopilot engagement under unreliable airspeed conditions".

In a proposed new directive for the A330 and A340 the authority is intending to instruct operators to upgrade the primary computers with the new software standard.

EASA has opened the planned directive to consultation but has yet to fix an implementation date. The requirements of the previous precautionary directive to pilots will "not be cancelled yet", it added, in order to allow time to finalise software certification and service bulletins.

Source: Air Transport Intelligence news