Capitão Bill Palmer, em seu livro " Entendendo a Air France 447 "tem uma seção dedicada a responder a essa pergunta no que se refere a AF447. Embora não haja como saber com certeza, ele apresenta algumas possibilidades.
Ele cita um comentarista em um site:
One commenter on the Weather Graphics website’s AF447 article provided this interesting observation: “I'm an aircraft icing specialist and wanted to point out a factor that hasn't been discussed much … high ice crystal concentrations. I've seen flight test data from power rollbacks due to flight in high ice crystal environments … In our case, the crystals collected within heated, aspirated Ram Air Temperature sensors, forming a 0 ° C slush…”
Ele observa que pouco antes dos tubos-piloto entupirem, o som de cristais de gelo batendo no pára-brisa podia ser ouvido no CVR.
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.
O tipo particular de gelo que pode ter sido responsável foi uma substância chamada graupel .
Graupel forms when tiny supercooled water droplets adhere to snow crystals to the point that they engulf the snow crystal itself.
Graupel Foto de Wikimedia commons
Cap. Palmer aponta fatores que tornam o graupel um possível suspeito:
- No airframe icing. The supercooled water theory is discounted by the fact that the A330's icing detectors were not triggered.
- 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.
Deve-se notar que a causa exata dos problemas de formação de gelo no A330 nunca foi completamente identificada, mas foi específica para a marca específica de tubos pitot originalmente instalados. A Airbus estava em processo de substituí-los todos por tubos-piloto de um fabricante diferente.