Free Understanding Air France 447 by Bill Palmer

with the integration of the vertical accelerations to calculate the vertical speed, known as baro-inertial (Vzbi), which is that displayed on the PFD.

    The A330 static ports are located below the fuselage mid-line forward of the wing. On the A330-200 in particular, as a result of the position of the static pressure sensors, the measured static pressure overestimates the actual static pressure. One of the first effects after AF447’s pitot tubes became obstructed was that internal altimeter corrections were recalculated as if the airplane were flying at the lower speeds. This resulted in false indications of a 300 foot decrease in altitude and a downward vertical speed approaching 600 feet per minute.

    18
    The pitot icing lasted for about a minute and five seconds. But 30 seconds later the airspeed indications again fell to extremely low levels.
    Consider that in normal operation, the angle of the airflow along the fuselage is no more than a few degrees. At 02:11:45, as the airplane was descending through 35,000 feet, the angle of attack started to exceed 45° on a regular basis. At the same time the indicated airspeed fell to values that were well below its actual forward speed. If it were only a matter of the air striking the pitot tube at the 45° angle, geometry tells us that the resulting ram pressure would be 70% of its actual value, but the indicated airspeeds were often below 60 knots for the number one air data system and near zero for the standby instrument.
    This created a situation where the air was pushing into, in addition to flowing over, the static ports. Dynamic pitot pressure is only calculable by subtracting the static pressure component. If the air is directed at the pitot inlet and the static port inlet at the same angle then the differential will fall to zero, or perhaps beyond. This dynamic accounts for the repeated falling of the airspeed indications to invalid values.
    In addition to airspeed, altitude and vertical speed indications were also compromised because of this effect. At this same time, the recorded vertical speed indications become erratic, and changed at rates that had no corresponding change in the vertical g load.
    Further evidence of this phenomenon is that sometime during the 02:11 minute (the ACARS-transmitted fault reports were not recorded more precisely than to the minute) the comparison between the static and pitot pressures were “out of bounds.” That is the static pressure was greater than the pitot-tube sensed pressure. This caused a hard speed/Mach function error in the standby instrument (“hard” meaning that it persisted over a period of time). In normal flight regimes this would be a nonsense situation, where static pressure was greater than pitot pressure, even if the pitot tubes were completely blocked. But the correlation of this message with a time period where the angle of attack became consistently excessive lends credence to the explanation that the angle of attack was responsible for the airspeed values at ridiculously low readings, long after the icing issue ceased to exist.
    Airspeed vs. Angle of Attack
    There have been many calls for the installation of angle of attack (AOA) indications on transport category airplanes. This accident would seem to be the perfect example to make that case. Unfortunately, it is not that simple.
    Airspeed/Mach is an excellent indication for a number of reasons. It provides a direct indication of limit speeds for the airframe and flaps/slats. In cruise flight, it provides a higher degree of precision for performance than AOA alone, and an indirect indication of AOA within the normal envelope. Cruise performance is more related to Mach number than AOA. Lift is increased and stall AOA decreased with increasing Mach number, even at the same airspeed and AOA.
    Angle of attack indications are no panacea. In cruise, one degree of angle of attack change is equivalent to up to 25 knots of airspeed change. The stall AOA is also not a