CYPRUS AIRWAYS (CY) said yesterday it had begun fitting its Airbus jets with a newer version of speed sensors, which have speculatively been linked to the downed Air France plane off the Brazilian coast.
Crash theories being considered by investigators include the possibility that external speed monitors – called Pitot tubes – iced over and gave dangerously false readings to cockpit computers in a thunderstorm.
The 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-over, 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 fashion.
Airbus has confirmed it had issued a “service bulletin” asking the plane’s 50 or so airline operators to consider changing the speed sensors, but said it was an optional measure to improve performance and not related to safety fears.
Cyprus Airways’ fleet comprises 11 Airbus planes: three A319s, six A320s and two A330s. Airline spokesman Kyriacos Kyriacou said they have already replaced the speed sensors on seven of the aircraft.
The remaining four aircraft would be fitted with the newer version of Pitot tubes “very soon,” he added. The replacement programme was initiated in March last year.
Airbus planes use sensors made by Thales.
At any rate, Kyriacou noted, Airbus’ advisory to operators was an optional measure, meaning the aviation giant does not consider the speed sensors to be a safety critical system. The recommendation had not changed to mandatory in the wake of the accident, he said.
“Nevertheless, I would like to say that whenever it comes to safety issues, Cyprus Airways does not spare a penny,” he added.
An aviation expert told the Mail the speed sensor was “an extremely important device,” but stressed it was too early to say whether these had played any role in the Air France accident.
“First they thought the plane was struck by a thunder, then it was the speed sensors, and the latest speculation is that an explosion may have occurred on board.”
The source, who preferred not to be named, said commercial airliners fly at a specified and limited speed band, which means it’s crucial for pilots to have an accurate velocity read-out at all times.
Should a plane fly into a storm, the standard operating procedure – which aviators are obliged to learn by memory – is to slow the jet down from the normal cruise speed of Mach 0.80.
The instruments are the focus of attention after automated messages sent from the Air France Airbus 330 revealed that the three external sensors were giving different air speed readings in the final few minutes before the disaster on Sunday night.
The plane’s autopilot switched off after it received the conflicting velocity data, though it was not clear whether it cut out automatically or had been overridden by the pilots.
Flight 447 disappeared four minutes later, with the loss of 228 people on board.
The string of 24 automatic error messages sent to Air France headquarters indicated that the plane was flying too slowly as it was buffeted by violent storms inside 50,000 ft high “thunderclouds” over the Atlantic.
Airbus issued another memo last week for pilots on the correct speed and angle for flying the A330 model in bad weather if speed indicators give conflicting readings and the autopilot cuts out.
Peter Goelz, a former managing director of the National Transportation Safety Board, said the Airbus A330-200 has a “rudder limiter” which constricts how much the rudder can move at high speeds. If it were to move too far while travelling fast, it could shear off and take the vertical stabilizer with it.
“If you had a wrong speed being fed to the computer by the Pitot tube, it might allow the rudder to over travel,” Goelz told the Associated Press.
