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I have always thought that there is a probe or sensor kind of thing inside the static port so that it "measures" the static pressure and compare to other pressure sources for airspeed, altitude, and VSI reading.

But my other pilot friend recently told me that the way static port works is because of the air actually flows in and out of it.

For example, when you climb, the ambient pressure decreases so the air will flow out of the static port until it reaches equilibrium, which causes the static pressure inside the casing of altimeter to decrease and the altimeter shows an increase in altitude.

Tried to find answers about how static port work online but did not find a satisfying explanation...

If anyone knows more on this topic please share your opinion.

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Your friend is correct. It's situated in a spot where the dynamic pressure is non-existent or minimal such as on the rear side of the fuselage on a Cessna 172 where it's able to measure the ambient pressure. This pressure is then either fed by tubes to their respective instruments (ASI, Altimeter, and VSI), or on modern aircraft, it is read by a transducer (or analog to to digital interface units - A/DIFU) which converts the reading to electronic data which is then fed to the air data computer.

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  • $\begingroup$ Thanks for the replies! But this brings up another question of mine... how about pitot tube? I am sure that the air flows into the pitot tube but does it goes out of the tube as well? If it does, where is the exit? If it doesn't, isn't the ram air pressure going to keep accumulating inside the tube? $\endgroup$
    – alisonCY
    Jun 26, 2019 at 18:27
  • $\begingroup$ I don't think so. The pitot tube is designed to measure that ram air pressure (static + dynamic pressure) so it wouldn't make sense to drain it. It would only flow out if you slowed down. $\endgroup$
    – Speedalive
    Jun 27, 2019 at 1:07
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Your friend is right. There is no sensor inside a static port. It is just a little tube connecting the static system (meaning all the tubing connected to altimeter, and the vertical speed indicator, and the static side of the airspeed indicator), to the outside.

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  • $\begingroup$ Thanks for the replies! But this brings up another question of mine... how about pitot tube?I am sure that the air flows into the pitot tube but does it goes out of the tube as well? If it does, where is the exit? If it doesn't, isn't the ram air pressure going to keep accumulating inside the tube? $\endgroup$
    – alisonCY
    Jun 26, 2019 at 18:27
  • $\begingroup$ The pitot tube works similar to the static port except that its opening is in the free airflow. Meaning: The air does not flow through the pitot tube but rather into the opening and therefore creates an increased pressure inside the tube which is equivalent to the static pressure + dynamic pressure. Therefore, if the airplane slows down, the dynamic pressure decreases and air is flowing out of the pitot tube to the front until the pressure inside the tube is equivalent to static pressure + dynamic pressure again. $\endgroup$
    – rauberdaniel
    Jun 26, 2019 at 18:30
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There’s not a whole lot to a static port on an aircraft, though it is located at a point chosen where the boundary layer is at or very, very close to, the ambient air pressure. The port opens to a static line which connects to the pilot static instrument or air data computers and this, along with the ram air (stagnation) pressure provided by the pilot tubes allows these instruments to calculate airspeeds, altitude and rates of climb or descent.

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Static ports and pitot tubes do not measure anything. They are just holes with tubes leading to the interior of the aircraft. Instruments do the measuring.

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Air will flow in and out of both static-port and pitot-tubes - you have to take all internal volume into consideration (mainly lines, instruments): As long is the "outside" pressure (static and/or dynamic) is greater than the internal one, air will flow in, compressing (and doing the work inside the instruments) until both pressures are equal - than flow stops. When external pressure is lower - air will flow out until it is once again balanced. But this happens quite fast so in normal operation you can consider these ports closely linked to the instruments and disregard any flow.

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