SOD-PUD-SUC-PUC

[+TSRA]

Hello again Ben2000,

So, yes you're making sense. But, you're adding a lot of unnecessary information which tends to confuse the issue.

Before I go too much further, remember that in any multiple choice exam, there is always a play going on in the answer stem. Two answers will be completely wrong (at least one of these will be very obviously wrong). One answer will be wrong, but only just - maybe by a word or two. The correct answer will either be written as you expect or will be written in a way to appear as though it is wrong. In both your exemplars, you have these two correct versions at play - one is very obviously correct and the other seems to be incorrect, but is actually true.

In your first question, [A] looks correct, as it gives the 'Read Correctly' answer for the Altimeter and VSI. However, we know that should the pitot tube freeze and a descent is begun, the airspeed indicator will under-read (PUD). Therefore, [A] is incorrect as it lists 'over-read.' The answer is wrong, but looks right because it is using the same terms you've come to learn. Answer [D] is correct because both the altimeter and VSI will 'read correctly' and the airspeed indicator does become unreliable. However, they've used a term that you're unfamiliar with, and this is on purpose as it makes the answer appear incorrect. It forces you to think critically about what is happening in the moment, rather than on the acronym you've come to learn. They know that you know you've learned the acronym, so they have to work around that knowledge. In the real world, you're not going to see "over read" or "under read" while you're flying along. The airspeed indicator will just look wrong for the pitch and power setting you have. It becomes "unreliable."

With this first question then, your analysis is correct. With a blocked pitot tube and a descending aircraft, the static pressure fed to the airspeed indicator case will increase above the total pressure located in the airspeed indicator diaphragm and, due to the linkage between the diaphragm and faceplate, this will be shown as a reduction in airspeed as the case pressure presses upon the diaphragm. Because it under-reads from what we expect, we can also say the airspeed becomes untrustworthy. And, because it is the pitot tube that is blocked, only the airspeed indicator is affected, meaning all other pressure instruments will read correctly.

In the second question, all three pressure instruments will be affected, given it is the static port that is being examined. This time, they've not attempted to confuse the issue - they're using all the terms you've come to learn. But in attempting to not confuse the issue, they intentionally make you question yourself because the correct answer, in light of the previous question, becomes "too correct." This is why, unless the question specifically tells you to refer to a previous question, we take all questions as individual items with no link to a previous or future question. I digress slightly.

Let's think it through, as you did: The static port becomes blocked during cruise flight. That means the pressure within the static pressure lines will not change. Ergo, the altimeter and vertical speed indicator will not register a change - they'll remain unchanged. The airspeed indicator, on the other hand, cannot now cancel out the static pressure between the case and the diaphragm. In a climb, this means the trapped static pressure in the case is higher than the static pressure in the diaphragm, which is reducing with altitude. Due to the linkage with the face plate, the airspeed indicator will under-read. The only correct answer to the second question is [C] - Remain unchanged / Remain Unchanged / Under-read.

In your analysis, however, you are adding information that is not present in the question. First, the question is not asking you what the indications will be if you use an alternate static source. Although you get there in the end by saying "if there is no alternate static source, then...", you're adding information not present in the question. You're over analysing the question. Take the question at face value. If they wanted to know what would happen if the alternate static source was used, they'd include that information. Don't try to get into the examiners head. If it's not relevant, ditch it. Second, you're attempting to think of the airspeed indicator in terms of altimetry, which is a problem often caused by ground and flight instructors who teach that a pitot tube blockage will make the airspeed indicator act like an altimeter. While this is mostly correctly, it can lead to an issue with some students who then have a problem when it comes time to think about a static source blockage. How can the airspeed indicator now be an altimeter, even though the altimeter is no longer working? But we forget that little line only applies if the pitot tube is blocked. It becomes very confusing, and often leads to students adding the altimeter and VSI to the SOD-PUD-SUC-POC acronym to try and resolve their misunderstandings.

I came to realize I'm much more mechanically minded years ago, and your posts read as though you are too. So, try thinking about the airspeed indicator in terms of how it works on the inside: the difference between the pressure in the case versus the pressure in the diaphragm. No tricks, just straight up mechanical knowledge, not how it would operate in a different space. In fact, go back to that video and really understand what she's saying in the first three minutes about how it works. Get an intuitive understanding, and you'll be off to the races.