Understanding Air France 447 by Bill Palmer Read Free Book Online

Temperature sensors, forming a 0°C slush…”
    Seconds before the pitot tubes clogged, ice crystals hitting the exterior or the airplane are heard on the voice recorder. Ice crystals bounce off the exterior of an airplane and cause no visible ice accretion, but they can enter the probe inlets. When highly specific climatic conditions exist in combination with certain combinations of altitude, temperature, and Mach, the concentration of ice crystals entering a probe can exceed its capacity to melt and evacuate the moisture through its drain holes. The result is that the ice crystals form a physical barrier within the probe that disrupts the measurement of total pressure.
    The final report states, “As soon as the concentration of ice crystals is lower than the de-icing capacity of the probe, the physical barrier created by the accumulation of crystals disappears and measurement of the total pressure becomes correct again. Experience and follow-up of these phenomena in very severe conditions show that this loss of function is of limited duration, in general around 1 or 2 minutes. 10 ”
    The type of particle that has been suggested is graupel . Graupel forms when tiny super cooled water droplets adhere to snow crystals to the point that they engulf the snow crystal itself.

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    The graupel theory is supported by the following evidence from the accident investigation:
     
No airframe icing. The supercooled water theory is discounted by the non triggering of the A330's icing detectors.
Graupel has large enough particles to be audible on the voice recorder. It takes a particle with enough mass and inertia (a given density) to hit the fuselage with a sound, instead of flowing around it with the relative wind, like snow.
Graupel has enough mass to temporarily overwhelm pitot anti-icing when concentrations are high enough. The pitot tubes are hot. But even if you put a snowball on a hot skillet it does not melt instantaneously. If there is enough mass in the blockage, and in combination with new particles being added to the blockage as the first ones melt, it may exceed the pitot tubes capability to melt the obstruction as fast as it is introduced. Graupel is of significantly higher density than snow.
Graupel has sufficient blocking properties to prevent efficient transmission of dynamic pressure within the pitot tube. For example, water can flow and transmit pressure within the pitot tube, though it too can alter pitot-static readings, a physical non-fluid blockage could shield the pressure sensing port.
The likely presence of snow or similar form, as evidenced by the St. Elmo's fire discussed by the crew. The accident report stated that the sound of ice crystals hitting the aircraft can be heard about 20 seconds before the airspeed loss and autopilot disconnect.
    The detailed inner workings of the ITCZ thunderstorms are not well known, and the specifics of high concentrations of ice crystals within them is one of the unknown factors. Pilots routinely try to avoid flying through thunderstorms, so it is no wonder there is not a great deal of experience of flying through them in this area.
    In regards to testing pitot tubes, the final report states:
There are many wind tunnels around the world in which this type of test can be performed. Each wind tunnel nevertheless has its limits and its own utilization envelope in terms of speed, minimum temperature possible and water or ice crystal concentration.
It is important to note that there are no wind tunnels capable of reproducing all the conditions that the crew may be confronted with in reality.
Furthermore, some scientific studies are under way to characterize the exact composition of the cloud masses above 30,000 ft. They show in particular that not all the phenomena are known with sufficient precision. This is particularly true concerning the nature of ice crystals (size and density) as well as the dividing level of supercooled water and ice crystals.

Chapter 5: Into the