French accident investigators said the cockpit instruments were receiving conflicting speed data at the time of the incident.

Air France has said that icing of the external monitors at high altitude has previously led to a loss of crucial flying information.

'Loss of control'

An internal memo obtained by The Associated Press, written by the Alter Union which represents about 12 per cent of Air France pilots, calls on the pilots to refuse to fly A330s and A340s until at least two of the three Pitot sensors on the planes are replaced.

Investigators continue to search for clues on why the plane went down [AFP]
The memo said the airline should have grounded all A330 and A340 jets pending the replacement, and warned of a "real risk of loss of control" of an Airbus experiencing problems with its Pitot tubes.

The L-shaped metal Pitot tubes jut from the wing or fuselage of a plane, and are heated to prevent icing. The pressure of air entering the tubes lets sensors measure the speed and angle of flight.

An iced, blocked or malfunctioning Pitot tube could cause an airspeed sensor to fail, and lead the computer controlling the plane to accelerate or decelerate in a potentially dangerous manner.

'Possible factor'

However, the secretary-general of another French pilots' union, said on Monday that the tubes were not likely the cause of the crash.

Julien Gourguechon, head of the SNPL, conceded that they could have been "a possible contributing factor".

But even without the Pitots, he said, "there is a procedure that exists to pilot the plane", a procedure all pilots are trained in.

"We can make the plane fly'' even if "we are not very precise in the piloting," he said.

Meanwhile, several other airlines using A330-200 planes said on Monday that they would wait for directives from manufacturer Airbus before making any equipment changes.

And several airline executives at the Iata meeting stressed that the Airbus A330-200 was a safe aircraft and they had no reason to investigate the speed sensors on their own planes at this stage.