Originally posted by Machinbird ...
Yes, if you block the pitot inlet only, the indications will drop. But, if you block both the inlet and the tiny bleed holes drilled into the pitot tube to clear moisture from the tube, you end up with trapped air in the pitot system and that is a different ball of wax. Once you have trapped air behind the blockage, you can build pressure. Both kinds of blockages can and have happened.
In addition to arguments against your pitot "pressure cooker" scenario made by
TheShadow, there is another unticked item on the ACARS list which to my mind thows a "spanner in the works".
Simply put; in your proposal the the AP compensates for the apparently real increase in Pstag by calling on the A/THR to reduce engine power and commanding nose up to maintain FL350. This all happens unobtrusively (especially if you are monitoring the Fuel Management page), but when the apparently real Vmo M0.86 is exceeded, the AP decides its current configuration doesn't make sense and disconnects. Apart from the a/c being left in a perilous situation with CAS probably less than M0.70, the control mode then changes from Normal Law to Alternate Law.
The crux of my argument is that when changing from Normal to ALT Law, the RTLU is locked at the maximum allowable travel associated with the aircraft's current level and CAS, and the BEA reported -
1.12.3.5.5 Examination of the Rudder Travel Limiter Unit (RTLU)
The RTLU was found in its place in the fin and disassembled. An examination was performed at the manufacturer’s and showed that it would allow travel of the rudder measured as 7.9° +/- 0.1°. As an example, at FL350, this travel is obtained for Mach 0.8 +/- 0.004, corresponding to a CAS of 272 +/- 2 kt.
This means that when the proverbial hit the fan, the a/c was proceeding in light chop (cobble-stoning) and the airspeed had dropped slightly from M0.82 to M0.80. Just prior to the AP disconnect the RTLU was last adjusted to represent the valid computated CAS, and if this was the apparently real Vmo M0.86, the RTLU would have been set at 4.7° per the following graphic.
Obviously if we knew that M0.82 was the set CRZ speed, then the RTLU max angle could actually be showing the prelude to the pitot disagreement, i.e. pitot blockage was occurring and an overspeed event was in the making. If the speed had been set at M0.80, then whatever happened at AP disconnect would seem not to be related.
The ACARS F/CTL RUD TRV LIM FAULT was sent after the pilot induced warnings, i.e. when they went looking for the AoA "bird" page. There is a vacant 9 sec slot between those warnings, and they are then followed by the RTLU fault, which could be as a result of a rudder pedal being depressed and held - a possible indication of a stall/spin?
mm43