As I study for my IRA written, this (VFR) question resurfaced and continues to irk me. The annotations cite the Venturi effect as the sole reason, but I don't follow.

From Wikipedia: "The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe"

Do we consider the parcel of air in the cabin to be the fluid moving through the constricted section of some imaginary/abstract pipe? If so, why? Can anyone shed more intuition on this?


1 Answer 1


The air moves around the fuselage, which causes a localized increase in velocity around the aircraft. This increase in velocity causes a slight decrease in the outside air pressure, which effectively sucks air out of the fuselage, making the cabin at a slightly higher pressure altitude.

The effect is localized.

While the reduction in pressure, and increase in pressure altitude inside the cabin is small, many GA aircraft display a 40 foot or so difference in pressure altitude between the alternate static port (inside the aircraft) and the pressure altitude with the factory static port.

  • 1
    $\begingroup$ Opening the air vents can change the pressure inside to be higher than outside. Barry Schiff gives a good demonstration in a Wonderful World of Flying clip. youtube.com/watch?v=eHnhRrbyEIE $\endgroup$
    – JScarry
    Apr 21, 2017 at 0:08
  • $\begingroup$ The original poster may be helped by imagining the fuselage as a carburetor venturi turned inside out. That is to say that the constriction is on the outside of the aircraft going through the air, rather than inside the cabin. $\endgroup$
    – mongo
    Apr 21, 2017 at 0:24
  • $\begingroup$ The opening and closing of air vents is a different effect, largely the introduction of ram air into the cabin. A window could increase the pressure with a slip into the open window, and decrease the pressure with a slip towards the side with the closed window. $\endgroup$
    – mongo
    Apr 21, 2017 at 0:26
  • $\begingroup$ Diagram would help here. $\endgroup$
    – Notts90
    Apr 21, 2017 at 7:02
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    $\begingroup$ Sorry, @Notts90, I am not a good artist. If it helps visualize, think of a wing, somewhat symmetric along the chord, with "lift" on the top and on the bottom and in this example, on the sides. That "lift" is created by a small pressure differential (Re: Bernoulli Principle) caused by a small increase in air velocity as the air moves around the fuselage. I will see if one of this term's aerodynamics students would do a diagram with calculations for extra credit. $\endgroup$
    – mongo
    Apr 21, 2017 at 11:31

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