UPDATE 1-FACTBOX-Attention centres on Air France speed sensors

(Adds comment by Emirates and Etihad Airways airlines)

June 10 (Reuters) - Air accident investigators have said an Air France Airbus A330 was registering "inconsistent" speed readings before it crashed into the Atlantic Ocean last week.

Air France (AIRF.PA) has since said it noticed temporary loss of air speed data on previous Airbus flights due to icing up of the speed sensors, or pitot tubes.

Aviation experts and pilots' unions have asked whether the same thing might have happened on the doomed Airbus A330 as the pilots tried to navigate equatorial thunderstorms.

Here are details and background on the pitot tube:

CONTRIBUTING FACTOR?

Pitot tubes on aircraft are typically heated to prevent them becoming clogged with ice. Investigators are looking into whether this mechanism failed, but the head of France's air accident agency has said it is too soon to say if problems with the sensors were in any way responsible for the crash.

A French pilots' union, Alter, said on Tuesday that "there is a real risk of losing control of an Airbus" in the event of a pitot tube malfunction, but added that it was drawing no conclusions on the cause of flight 447's crash.

Another pilots' union, SNLP, said frozen pitot tubes could not be the sole reason for the disaster.

Widely used to gauge the speed of aircraft, pitot tubes are also used to measure wind and gas speed for industrial purposes.

Air France has said all its flights using long-haul Airbus (EAD.PA) jets will be equipped immediately with new speed sensors, a pilot's union said on Tuesday [ID:nL918170].

CAUSE OF PREVIOUS CRASHES?

Blocked speed sensors have been blamed for contributing to aircraft crashes in the past.

In 1996, a report into the crash of a Boeing 757 airliner off the Dominican Republic blamed a faulty reading by the pitot tube -- which was blocked by dirt or insect remains -- for the disaster, alongside pilot error.

HENRI PITOT, INVENTOR

The devices were invented nearly three centuries ago and still play a key role in 21st century high-tech avionics.

Invented in 1732 by French engineer Henri Pitot, they were used to measure the flowing speed of rivers and canals.

The device consists of a tube with two holes, bent at a right angle. The first hole is placed in the moving air (or in Pitot's days, water), with the mouth of the bent section aimed upstream to measure its velocity.

The side hole measures static air pressure, and the difference between the two is used to calculate airspeed.

Pitot tubes can be mounted on a plane in several different ways -- including from the edge of the wing, or on the exterior of the fuselage. The A330 Airbus has three pitot tubes.

MANUFACTURERS

The pitot tubes on Airbus planes are made by French firm Thales (TCFP.PA) and Goodrich Corp (GR.N) of the United States.

An older Thales model was on flight 447. This model is being replaced on all Air France's longhaul Airbus planes.

Emirates, which operates 29 A330s -- the largest such fleet in the world -- told Reuters its planes were fitted with the Goodrich Corp tubes, adding that these had not suffered the sort of problems reported by Air France.

Another big user of the A330, Abu Dhabi's Etihad Airways, said its speed sensors were also made by Goodrich "so there is no question of replacing or changing them".

Thales was not immediately available for comment.

(Reporting by Sophie Taylor, editing by Tim Pearce)