Originally posted by Machinbird ...
But if an aircraft were pitching up and then the blockages began to let go, which is what would have triggered the switch to Alt Law, then the last acceptable polled airspeed prior to the switch could have been almost anything including M.79. The trigger would be the 30 knot drop in 1 second. Yes it would be highly coincidental but not impossible.
You may have provided a valid reason for the airspeed disagreement, i.e as the TAS actually slowed, the pitot heating became more effective and the uniformly blocked pitots started to unblock in a disjointed fashion. However, if the CAS being polled was the median of the 3 x Pstag pressures, its a plausibility stretch that 2 pitots reverted smoothly to M0.80 before total disagreement of all 3 pitots. It may have happened though.
So, I interpret that if the "pressure cooker" scenario had developed, the NU attitude and loss of TAS had occurred, then the RTLU setting of M0.80 was the last "apparently" valid CAS. In other words, as
CONF_iture put it, "The RTLU can be duped".
The Pilotaydin SIM experience would seem to fit, and I daresay that Airbus have attempted and probably succeeded in duplicating such a configuration. Or was that experience a SIM instructor's "special"??
The +30° pitch attitude protection in Normal Law is not available in Alternative and Direct Laws, but as
PJ2 pointed out a few posts back -
There is at least one ECAM Abnormal which will occur if there is a difference, or rather a disagreement between the FMS CG/THS calculated trim position, and the actual THS trim position, "F/CTL PITCH TRIM/MCDU/CG DISAGREE).
- which I assume would have triggered if the THS had moved outside the accepted range (including change of CG with trim tank in use).
There is something about this whole situation that doesn't quite fit, i.e. a sophisticated FBW aircraft is being held by AP in an increasing nose up attitude and decreasing A/THR to maintain a BARO-ALT. The available inertia is disappearing fast, and somehow I think that that scenario would have been covered in the design algorithms as indicative of unreliable airspeed.
P.S. Your pitot tube experiment (which I have just seen) puts some "polish" on your argument, and no doubt Thales have done similar tests!
mm43