Airbus A330 and A340 pilots are being issued with a new procedure for dealing with unreliable airspeed indications, which instructs them to resist immediately re-engaging the autopilot if it disconnects.

Unreliable airspeed data, sourced through pitot tubes mounted on the aircraft, is still a consideration in the investigation of the loss of Air France flight AF447, an A330-200, over the South Atlantic last year.

In an airworthiness directive the European Aviation Safety Agency states that a significant difference between airspeed sources causes the aircraft's autopilot and autothrust to disconnect, and the flight controls revert to 'alternate' law. The flight-director bars also vanish.

But if two airspeed sources nevertheless show similar, incorrect, results the aircraft will restore the flight-director bars and allow the autopilot and autothrust to be re-engaged.

EASA says that this can result in the aircraft giving misleading instructions to the autopilot.

"In some cases the autopilot orders may be inappropriate, such as possible abrupt pitch command," it says.

This, in turn, could result in unsafe aircraft behaviour. EASA is directing operators to amend their aircraft flight manuals to instruct pilots, in similar circumstances, not to engage the autopilot and autothrust immediately, and not to follow the flight-director orders.

EASA says crews should instead perform a cross-check of all speed indications to assess whether the airspeed information is reliable, and to apply relevant 'unreliable airspeed' checks if necessary.

At least two air data references must provide reliable speed indications for at least 30s, it adds, with the aircraft stable on its intended course, before pilots attempt to re-engage the autopilot and autothrust.

EASA has ordered that the changes be made within 15 days from 5 January 2011.

Source: Air Transport Intelligence news