SOD-PUD-SUC-PUC

[Ben2000]

Hello +TSRA,

Big thanks, my confusion a typo caused by a lack of understanding. This is fantastic.

So, the SOD-SUC-POC-PUD only applies to the ASI. Got it. I am also confused by the wordings of various questions which further messes up my understanding, e.g.

If the power supply to the pitot heater failed during flight in icing conditions and the aircraft subsequently descended, the readings on the Altimeter/the VSI/the ASI would, if ice had blocked the pitot (Total Pressure) tube:

1. [A] Read Correctly / Read Correctly / Over-read.
   
2. [B] Read Correctly / Under-read / Over-read.
   
3. [C] Under-read / Read Correctly / Over-read.
   
4. [D] Read Correctly / Read Correctly / Untrustworthy.
   

As I see it, the options should be either option A, D (as the pitot and total pressure does not affect the Altimeter or VSI). However, when it comes to Untrustworthy vs. Over-read, I am not sure what the correct answer is.

Here's my logic. Pitot Tube is blocked. The aircraft is descending, which means the static pressure from the static port will start to increase due to the increasing density of air as it descends. This is turn means that the Total Pitot Pressure should relatively be pushed back, i.e. it should under-read and not over-read. So, by that logic, Option D. This I am not sure!

Another example:

In icing conditions, if a static vent became blocked during level flight and the aircraft subsequently climbed, the readings on the Altimeter/the VSI/the ASI would:

[A] Remain unchanged / Under-read / Over-read
[B] Over-read / Over-read / Under-read
[C] Remain unchanged / Remain unchanged / Under-read
[D] Under-read / Remain unchanged / Over-read

So: Static vent blocked. My logic: The static pressure from inside the cabin (alternate cabin source) would take care of it. Of course, IF there is no alternate static source, then it should be option C as there would be no change in the static pressure (for the Altimeter, VSI).

However, for the ASI, the total pressure from the Pitot is still active, and the static pressure from the static port is stagnant, but static pressure from the pitot has reduced, which reduces the overall pressure. I.e. makes ASI work as though it were at a lower altitude on the static side with less total pressure from the pitot. Therefore, it would under-read due to the inbalance between the static port pressure and the static component of the total pressure from the pitot side. Therefore it would under-read.

I wonder if I am making sense.