I don't really know what static pressure is. Does the ASI (Airspeed Indicator) use cockpit pressure as static pressure, to find the dynamic or differential pressure?

I just found this statement :

if the instrument static pressure line becomes disconnected inside a pressurized cabin during cruising flight the altimeter and airspeed indicator will both read low.

Could anyone help me understand static pressure and this statement?


2 Answers 2


There is a static pressure port, often on the side of the aircraft, that senses the air pressure at the altitude the aircraft is flying and unaffected by the aircraft's passage through the air.

The pitot tube, facing into the relative wind caused by the aircraft's passage through the air, measures the ram air pressure, which is higher the faster you go.

The indicated airspeed is the difference between the ram air pressure and the static pressure, subject to some adjustment due to sensor positioning.

The altimeter operates off the static air pressure.

So, if the static line is somehow opened to the inside of a pressurized cabin, the system will read that higher pressure and believe the altitude is what it is inside the pressurized cabin, which will be significantly less than the actual altitude.

And the difference between the ram air pressure and the false higher static pressure (the pressure inside the cabin) will be much less and the system will read this reduced different as a much lower speed.

This is a somewhat simplified answer.

  • 1
    $\begingroup$ A minor note: on most light aircraft the static inlet is a small metal disk about the size of a Canadian $2 coin, with a hole in the center. They are quite distinctive and you can see them in most photos. $\endgroup$ Apr 23, 2018 at 14:41

For any fluid that is not compressible, (as air flowing at subsonic speed is) the total pressure must remain constant. This is the essence of Bernouli's principle, which is based on conservation of energy. So since the total pressure must remain constant, when air is flowing at some velocity (less than subsonic) relative to the measuring instrument, the pressure measured parallel to the flow (the dynamic, or ram, pressure) increases as the velocity increases, so the pressure measured perpendicular to or across the flow must decrease in order that the total remain the same. This pressure, measured perpendicular or across the flow, is referred to as the static pressure.

In an aircraft we measure these two pressures by simply orienting two measurement sensors appropriately, one (the pitot tube) parallel to the flow, and which displays on the airspeed indicator, and the other (a static port), with the opening perpendicular to the flow, which displays on the altimeter.

Because the airspeed indicator measures the dynamic pressure pressing on the pitot tube, even at zero airspeed there would be some pressure there, in order to make the displayed value measure airspeed, you must subtract the zero airspeed pressure, so the membrane inside the airspeed indicator has the dynamic (ram) air pressure on one side,, and the static pressure from the static port on the other side, and therefore moves only according to the difference between them, i.e., according to that amount of Ram air pressure which is greater than the static pressure, i.e, the pressure in the pitot tube due to motion through the air.


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