Yesterday the world saw the disappearance of an A330 Air Frane during a trans Atlantic flight between Rio to Paris

Airspeed inconsistency

The airspeed numbers which are presented to pilots on the Primary Flight Display (PFD) are derived from pitot and static probes, which measure the pressure of the air that passes by the nose of the plane. That pneumatic information is then converted to numerical data by the Air Data Modules (ADMs), and then fed into the ADIRU units, which supply the numbers to the pilots on their respective PFDs.

For redundancy, the plane has 3 pitot/static probes, 8 ADMs, and 3 ADIRUs. Each Air Data and Inertial Reference Unit (ADIRU) is divided into two parts, which can operate separately in the event of the failure of one of the parts.

The Air Data Reference (ADR) part of the ADIRU supplies barometric altitude, speed, Mach, angle of attack, temperature and warnings for too much speed. The Inertial Reference (IR) part of the ADIRU, supplies attitude, flight path vector, track, heading, accelerations, angular rates, ground speed, vertical speed and the position of the plane.

Diagram of a pitot/static probe that uses a transducer to indicate pressure created by airspeed

Paul-Louis Arslanian, of France's air accident investigation agency, confirmed that F-GZCP previously had problems calculating its speed as did other A330 aircraft stating "We have seen a certain number of these types of faults on the A330 ... There is a programme of replacement, of improvement".^[104] One specific event mentioned was an Air France A340-300 (F-GLZL) on level flight at FL310 (~31000 ft/9450 m) from Japan to France, in which airspeed was incorrectly reported and autopilot automatically disengaged. Later, the drainage holes of all three pitot tubes had been found clogged. Weather and obstructed drainage holes were implicated in the cause of the incident.^[105] In the second incident, an Air France A340-300 (F-GLZN) enroute to New York encountered turbulence followed by the autoflight systems going offline, warnings over the accuracy of the reported airspeed and two minutes of stall alerts.^[105] Investigations of accidents with other aircraft type involved Pitot tube malfuction. An Austral DC9 crashed as a result be an erroneously low airspeed reading which was exxacerbated when the pilots attempted to compensate.^[106] Clogged Pitot tubes are suspected as the cause in the failure of a Birgenair Boeing 757.

On 6 June 2009, Arslanian said that Air France had not replaced pitot probes as Airbus recommended on F-GZCP during its last major overhaul, saying that "it does not mean that without replacing the probes that the A330 was dangerous."^[107] The problems primarily occurred on the Airbus A320, but, awaiting a recommendation from Airbus, Air France delayed installing new pitots on A330/A340 yet increased inspection frequencies.^[107] Air France issued further clarification of the situation after the nature of the Pitot tube service bulletin was incorrectly interpreted:

"1) Malfunctions in the pitot probes on the A320 led the manufacturer to issue a recommendation in September 2007 to change the probes. This recommendation also applies to long-haul aircraft using the same probes and on which a very few incidents of a similar nature had occurred."

Since it was not an airworthiness directive (AD) the guidelines allow the operator to apply the recommendations at its discretion. However Air France implemented the change on its A320 fleet where the incidences of water ingress were observed.

"2) Starting in May 2008 Air France experienced incidents involving a loss of airspeed data in flight (see two incidences above) in cruise phase on A340s and A330s. These incidents were analysed with Airbus as resulting from pitot probe icing for a few minutes, after which the phenomenon disappeared."

After discussing these with the manufacturer, Air France sought a means of reducing these incidents, and Airbus indicated that the new pitot probe designed for the A320 was not designed to prevent cruise level ice-over. However in 2009 tests suggested that the new probe could improve its reliability prompting Air France to initiate and accelerate the replacement program^[108], however not before F-GZCP underwent its major overhaul on April 16.

The problem created by faulty indicated airspeed increases with altitude, particularly for aircraft with relatively high weight loads. The term coffin corner is used to describe the dangerous portion in the flight envelope when the stall speed comes close to the critical Mach and can lead to a loss of control of the aircraft. Managing aircraft velocity is particularly important for pilots navigating thunderstorms at high altitude: not enough speed and the plane can lose lift and stall or lose altitude; too much speed and the aircraft can suffer damage or even break up. French Transport Minister, Dominique Bussreau, said "Obviously the pilots [of Flight 447] did not have the right speed showing, which can lead to two bad consequences for the life of the aircraft: under-speed, which can lead to a stall, and over-speed, which can lead to the aircraft breaking up because it is approaching the speed of sound and the structure of the plane is not made for resisting such speeds".^[109] On 4 June, Airbus issued an Accident Information Telex to operators of all Airbus models reminding pilots of the recommended Abnormal and Emergency Procedures to be taken in the case of unreliable airspeed indication.^[110]

Flight AF 447 has some parallels with incidents involving A330 aircraft flown by other carriers. Three similar reports are on file at the Australian Transport Safety Bureau (ATSB), with two incidents relating to Airbus A330 with the flight computer problems, plus one which involved a Boeing 777.^{[Note 3][111]} In the October 2008 incident, this fault caused injuries to passengers and damage to the aircraft on Qantas Flight 72, en route from Singapore to Perth, Western Australia, which was forced into a dive by a malfunctioning ADIRU. These incidents often started with the autopilot disengaging and sending ADIRU failure messages. Incorrect speed indications were also observed.^[111] The airframe and the ADIRU involved in the QF 72 incident was also previously involved in another incident on Qantas Flight 68. Pitot probe malfunction was not a problem of QF72 and precedes ADR and ADIRU failure within the same "electrical control flight system".^{[clarification needed]}