Static Port Blockage

berge7f9

berge7f9

Well-Known Member
Static Port Blockage

If you climb above the blockage altitude, airspeed becomes slower than it should be

If you descend below blockage altitude, airspeed becomes higher than it should be

• Why is this the case? How would you explain it to someone?
 
HUH??

I was under the impression that Static port would effect your Altimeter, and VSI not Airspeed.

I am also confused as to how you would climb above or descend below a blockage?

Or do I need to study some more?
 
desertdog71 said:
HUH??

I was under the impression that Static port would effect your Altimeter, and VSI not Airspeed.

I am also confused as to how you would climb above or descend below a blockage?

Or do I need to study some more?
Click to expand...

well remember that the airspeed indicator takes in both ram air and static pressure, and the difference between them gives you the reading on the instrument. so you have the pitot tube, giving total pressure, and the static port sensing true static pressure of the air. the difference between gives you dynamic pressure, or airspeed.

if you're not alt static equipped, you gotta break someone's face!
 
You get the variation in airspeeds due to the pressure differential that is trapped in the static lines v. the outside pressure.
 
I just read it again. I see what you are saying now, climb above or descend below the altitude at which the blockage occured. Ice for example, I understand now. I have to look at my text book, I don't recall this being in there, but it is possible I missed it. You just made me more knowledgable.
 
Here's the way I think of it...

When the static port is blocked at a certain altitude, the pressure of that altitude is trapped inside the static line. Therefore, when you are at an atitude above where the blockage occured, the indicated airspeed shows slower than True Airspeed vice versa, just as you said...think about this statement for a minute...

The higher up you go, the less dense, the air is. In other words, the same amount of air (number of molecules) will take up a larger volume, because they are not packed as tightly. So, when above the blockage altitude, the trapped air in the static line is higher than at that altitude, so it takes up less volume than the ambient (outside) air.

Now think of it as a smaller volume of air in the a/s indicator diaphram than would normally occur at that altitude, resulting in it not "inflating" as much, resulting in less needle displacement. :) . Let that sink in and it should click! It's perfectly logical.

Hope that helped. If not, let me know and I have another way of explaining it...
 
KBUF said:
So, when above the blockage altitude, the trapped air in the static line is higher than at that altitude, so it takes up less volume than the ambient (outside) air.
Click to expand...
That makes no sense at all.

The FAA is here to help.
BLOCKED STATIC SYSTEM
If the static system becomes blocked but the pitot tube
remains clear, the airspeed indicator continues to
operate; however, it is inaccurate. Airspeed indications
are slower than the actual speed when the airplane is
operated above the altitude where the static ports
became blocked, because the trapped static pressure is
higher than normal for that altitude. When operating at
a lower altitude, a faster than actual airspeed is
displayed due to the relatively low static pressure
trapped in the system.
Click to expand...
 
The OTHER effects that you guys are forgetting about with a blocked static port is the one that you will probably notice first. The altimeter holds the altitude at where ever the blockage occured and the VSI shows zero. So you could be climbing or descending with no indications of it! IMHO, these effects are probably more insidious and more likely to cause you harm, especially if the blockage occurs at low altitude.

A great page that explains the effects of pitot and static blockages with discriptions, recommendations and war stories is: http://www.avweb.com/news/airman/184279-1.html
 
:eek: Okay, read my explanation over (and over), and its kind of wack...

The faa source you quoted is the same one I read, but that doesn't make much sense to me. Im sure it's correct, but I have to come up with a way to understand it myself...

Reading a little deeper now.

Looking at how the the a/s indicator works in the trusty ol' inst/comm text from jep...The airtight case in which the diaphragm is housed of the a/s indicator is vented by the static line. I was thinking the diaphragm was vented!! In that case, the faa explantion makes perfect sense.

DISREGARD MY PREVIOUS POST, and thank you Mike!

New and improved...please tell me if im wrong.

Since the trapped static pressure is relativley high at an alitude above where the blockage occured, it simply doesn't allow the diaphragm to expand as much, resulting in a lower than normal airspeed indication. Reverse is true for lower than blockage altitude.
:nana2:
 
No problem. The way the airspeed indicator works is by comparing ram air to the static air. If the static source is blocked, the static pressure won't change with altitude and the ratio will be messed up.

Mike
 
Just think of it this way. The air gets thinner as your ascend and thicker as you decend, but with a blocked static port, the ASI isn't sure how. With the constant pressure stuck in the lines, as the plane descends and ram air pressure increased, the ASI indicates you are going faster (for a given airspeed) when in fact you are not. And vice versa.
 
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