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.
Last edited by HazelNuts39; 11th June 2012 at 11:29.
Reason: wording clarified