Kapten Bill Palmer, i sin bok "
Förstå Air France 447 "har en del dedikerad till att svara på den här frågan som den avser AF447. Även om det inte finns något sätt att veta säkert ställer han fram några möjligheter.
Han citerar en kommenterare på en webbplats:
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…”
Han noterar att just innan pilotrören var igensatta kunde ljudet av iskristaller som träffade vindrutan höras på 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.
Den speciella typen av is som kan ha varit ansvarig var ett ämne som heter graupel .
Graupel forms when tiny supercooled water droplets adhere to snow crystals to the point that they engulf the snow crystal itself.
Graupel. Foto från Wikimedia Commons
Capt. Palmer noterar faktorer som gör grabben till en möjlig misstänkt:
- 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.
Det måste noteras att den exakta orsaken till isbildningsproblemen på A330 aldrig identifierades helt, men den var specifik för det speciella märket av pitotrör som ursprungligen installerades. Airbus var på väg att ersätta dem alla med pilotrör från en annan tillverkare.
