Originally Posted by
Diversification
By chance I found a Pitot Tube simulator on the internet. Using this you can block the drain port, etc, and study the effect. I don't know how accurate the simulation is, but the pitot tube is rather well described by bernoulli's eqn.
Try it. Here is a link:
luizmonteiro - Online Simulators - Pitot Static System Simulator
Perhaps it will make all agree to on what can happen.
It doesn't simulate above 20,000ft or 220knots as far as I can tell.
At alts/speeds in its range it doesn't seem to model the small over-read due to the drain blockage, but this is likely to be specific to the particular design of probe. The model includes drain because if both ram plus drain blocked the total pressure is locked in and indicated speed just becomes function of altitude.
The AA probe was originally introduced to fix unreliable measurements, the critical period of flight is during climb after take-off, blocked drain+ram = false indicated speed increasing with increasing altitude, during which period pitot tube heater failure has been fatal. This model can be used to see how this can happen.
I believe there is a possibility of 3 identical probes in same airstream could suffer common mode fault of slowly blocking drains at same time. With auto-thrust the indicated air speed would not be seen to rise if all drains blocked - just true air-speed would fall (by small unquantified amount). Calculated wind speed would rise and engine thrust would be reduced. Would 30-50 knots be an issue?
At 02:10 there was an ACARS report
PROBE-PITOT 1X2 / 2X3 / 1X3
Which requires, for example one working correctly, one blocked ram+drain (stuck), one blocked ram (drain open =read zero).
So any false speed readings were just before this point, and have only an impact on the state of the a/c when the auto control gave up.