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SOD-PUD-SUC-PUC

Old 25th Mar 2024, 17:45
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Question SOD-PUD-SUC-PUC

Hi all
Related to a previous post, can somebody help a bit more with the an easier way to remember this SOD-PUD-SUC-PUC? Are these right?

If the Pitot is blocked:
  1. While descending, ASI would under-read, as it cannot register a change in the total pressure. I.e. you are likely going faster (higher density of air), but the ASI reads slower.
  2. While climbing, the ASI would over-read, i.e. nose-up, but the pitot being blocked does not register a change, so while you are already IAS is steady, the speed may be slowing
If the static is blocked
  1. While climbing or descending, the Altimeter and VIS should not register any change EXCEPT that the static instruments (at least the Altimeter) might register a small change from the cabin pressure.
  2. I.e. the altimeter won't register any change, so if you are at 1000 ft, and then climbing it would under-read, and if you are descending from 1000 ft, it would over-read.
  3. The VSI might register a small change and then return to zero.
Are these correct? Many thanks for the advice.
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Old 2nd Apr 2024, 16:29
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Hi Ben2000,

I've typed up a response a few times in answer to your post, only to delete it, mostly because I'm only somewhat sure of where your misunderstanding is, and every time I type a response it went off in a direction that may have made your misunderstanding worse. Instead, I'm going to keep this brief, let you respond, and we can go from there.

First, watch the video below by flight-club. It gives an excellent description of how things work, although you may have to watch the video from 3:34 to 4:12 a few times because the author does not animate the pitot blockage description in the same way she animated the first part of the video.

Second, whether by accident or not, you mistyped the acronym. You have typed it to show a pitot blockage always underreading (PUD and PUC). The way you have it written it should be SOD-PUD-SUC-POC, although I've mostly seen it written as PUDSUC POCSOD. Ultimately, it doesn't matter how it's written if you remember the basic principles come exam time.

Third,(and this is where I think your misunderstanding may come from) the acronym only applies to the airspeed indicator. By adding the altimeter and vertical speed indicator in the static block part of your post, you are confusing the issue. The acronym only applies to the airspeed indicator because it is the only pressure instrument that will be affected by a pitot or static blockage. As all other pressure instruments are supplied static information only, they cannot over-read or under-read in the same manner that the airspeed indicator will. With the altimeter and vertical speed indicator, there is either a change in static pressure or their isn't, so they can only ever show a change in altitude or vertical speed, or not.

This is normally where I would add a bunch of paragraphs to explain the information, but I'll let you respond first just in case you've had a light-bulb moment.


Last edited by +TSRA; 2nd Apr 2024 at 16:34. Reason: update to better reflect where the video was placed in the body
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Old 2nd Apr 2024, 20:19
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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.
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Old 2nd Apr 2024, 23:14
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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.

Last edited by +TSRA; 2nd Apr 2024 at 23:15. Reason: edited because Ben2000's reply finally showed in the thread, meaning I could remove it from this post
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Old 2nd Apr 2024, 23:27
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Big thanks +TSRA, that clears it up for me. Much appreciate it!
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