HN39
If the pitot intake and drains are blocked, the pressure rise due to heating depends on the volume of trapped air, which includes that in the tubing up to the pressure transducer in the ADM, and for the STBY pitot that in the pneumatic line up to the STBY ASI on the instrument panel. Wouldn't that difference result in less pressure rise in the STBY system?
The same question bothered me, and you are completely correct when the air trapped in the pitot tubes piping is dry air.
When you are at ~FL350 and you have air and moisture trapped in the pitot tubes in contact with heated metal, the moisture will flash to steam and the steam will try to hold a pressure dependent on the temperature of the trapped water. Now this isn't hot steam that you are acquainted with from your tea kettle. It is really just cool water vapor. The situation is very analogous to the newer steam catapults with the boiling water type steam receivers. As the launch valves open and begins driving the catapult pistons, water flashes to steam and tends to hold the pressure constant.
With 3 identical pitot tubes that have just experienced common mode plugging due to weather, you are going to have 3 identical steam generators that will attempt to keep the pitot tube interior at a pressure characteristic of the trapped water temperature. As long as there is water to flash to steam, the pressure in all 3 pitot systems should be relatively constant. The transition from open to plugged pitot tubes should look like a step function on a pressure graph with the system with the greatest volume lagging behind the other two systems by a second or two and all stabilizing at the same (higher) absolute pressure value.
I see the calculation in the QF72 report. I'd like to do some homework before I comment on the second part.