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airspeed events that had occurred between November 2003 and June 2009. According to Airbus these events are attributable to the possible “destruction” of at least two pitot probes by ice. Eleven of these events occurred in 2008 and ten during the first five months of 2009. Twenty six of these incidents (81%) occurred on aircraft fitted with Thales AA probes, two on aircraft with Thales BA probes, and one on an airplane equipped with Goodrich HL probes. 16
    Post accident wind tunnel tests with large concentrations of ice crystals were able to duplicate the issue in a controlled environment. The Goodrich manufactured probe behaved better than the Thales probes and was therefore the eventual replacement probe.
    Pitot Static Operation
    The pitot-static system is designed to determine airspeed and altitude by precisely measuring both the dynamic pressure resulting from forward movement through the air, and the ambient static pressure at that altitude.
    The air pressure measured in a pitot tube is the combination of the dynamic air pressure plus the static pressure. To determine the dynamic component (airspeed), the static pressure must be subtracted from the total pitot pressure. The dynamic component then directly relates to indicated airspeed. That is where the fun begins, as that value must then be corrected for temperature, pressure, and errors induced by the probe’s placement on the airplane to determine the airplane’s true airspeed.
    You may have seen photos of flight-test aircraft where the probes are mounted on long poles that project out in to undisturbed air in front of the airplane for greater accuracy. No doubt, if production airplanes had those long probes, they would be broken on a regular basis by all manners of ramp equipment banging into them. Instead, the pitot, static, angle of attack, and other air data sensors are mounted on the fuselage.
    Manufacturers attempt to position the sensors so that they can be reasonably accurate throughout the range of the airplane’s operating envelope. This can be quite challenging, as airflow around the fuselage changes quite a bit throughout an aircraft’s speed range. Sometimes the outputs of additional sensors, such as accelerometers, are used to tweak the air data to avoid erroneous readings.
    Static ports are located to get an accurate measurement of the outside atmospheric pressure. But the ports have little choice but to be mounted on the fuselage somewhere, subject to local pressure differences as air flows around the complex exterior of the airplane. Almost all airplanes have some error due to the position of the static ports. These errors are carefully recorded during flight testing. The manufacturers provide a correction table and in the case of modern airplanes like the A330, build the corrections into the air-data computer software, so that the values displayed to the pilot are accurate.
    The value of the measured static pressure must be corrected for this error before being used to calculate other parameters such as airspeed. The value of the correction depends in particular on the Mach, and takes into account the position of the sensors on the fuselage. Therefore, the correction performed for each static port is slightly different. 17
    For each airspeed system, the calculation principle is as follows:
    Knowing Pt (total pitot pressure) and Ps (static pressure) makes it possible to calculate a Mach value used to correct the Ps. The corrected Ps is then used to calculate the CAS (calibrated airspeed) and the standard altitude.
    With the known Mach value, the total air temperature (TAT) measurement makes it possible to determine the static air temperature (SAT), which in turn makes it possible to calculate the true air speed (TAS).
    The corresponding IR (Inertial Reference) then uses the true air speed to calculate the wind speed from its own internal ground speed and track values. It also uses the derivative of the standard altitude value that it combines