Originally Posted by
HazelNuts39
AZR,
If the drain of a single pitot is blocked, and the intake of same pitot almost at same instant, then the total pressure inside the pitot is 'frozen', no change to detect. Same for a second and third pitot some time later. The IAS changes when the static pressure changes, i.e. the altitude. IAS increases with increasing altitude, and vice versa. The IAS from all frozen pitots change in the same manner, no differences to detect.
What IMO makes this scenario somewhat unlikely is the absence of turbulence in the simulation. Turbulence was present in all UAS incidents studied by BEA, and would cause altitude variations causing airspeed anomalies that would trigger detection unless several pitots froze simultaneously.
The total pressure will freeze, but remember that the static ports were available and working 100% all the time in the AF447 scenario. IAS is still equal to Pdynamic, which is derived by substracting Pstatic from Ptotal.
The pressure variations in the turbulent air will still find their way to the IAS via the static ports, and this is of course also the reason why climbing will give you the impression aircraft is accelerating. What was also significant in the AF447 case is that the turbulence will cover up any other cues that the aircraft has stalled (stall buffet), and the FBW will mask any change in roll response of the flight controls due to the extreme AOA, as I recall that the roll rate response vs. stick deflection is linear in alternate law. FBW being FBW, it will always try to give you what you are asking of it. So the aircraft will appear to respond normally around the longitudinal axis, and the only thing telling you otherwise will be the compass heading turning in the opposite direction. But trying to figure that one out when you are descending at 10000fpm through a CB at night with a nose-up attitude with your aircraft telling you to pull up even more is an exercise of futility.
With regards to Airbus vs. Boeing: the Airbus overspeed mode was commanding an increase in pitch through the flight directors, which was followed religiously at least until the aircraft reached its apogee at 38000ft. This continuous backpressure on the stick caused the THS to trim all the way up, leading to the deep stall. I doubt they even realised that the THS was at the position it was in at any point as you are not supposed to fiddle with it yourself but let the automatics handle this, even though the captain correctly remarked on his return to the FD that the aircraft was stalled. It has been argued that if they just let go of the controls it would have allowed the aircraft to recover itself, and this is probably correct. This tells you how a Boeing would have fared in the same situation: the lack of autotrim on Boeings leaves the THS in the same position unless you conciously tell it to do something else using the trim switches, and this leaves open your avenue of sensing that the aircraft is stalled as you are unable to maintain a nose-up attitude as the aircraft loses airspeed. It's become pretty clear that in the same situation, a Boeing would not have deep-stalled. Not unless the crew trimmed up by themselves while not looking at the stab trim indicators.