One of my club's small planes apparently has an intermittent problem with moisture in the static port line, so the recommendation is to always fly with the alternate static port open. That leads me to wonder why (1) the plane in question (a 172) was not simply designed to receive static pressure from inside the cabin, thereby avoiding this kind of problem and (2) if there's any downside (other than slight miscalibration of speeds and altitudes) to doing so.

I'm aware of the venturi-effect related pressure diff between an external static port and the cabin pressure. That just seems like a calibration issue for airspeed and altitude measurement.

tl;dr: Why are/were GA planes designed with external instead of cabin-side static ports, other than to give the examiners a tricky thing to ask us about?

  • 3
    $\begingroup$ Internal air pressure changes slower than external, so things like the climb rate indicator may be inaccurate or slow responding. $\endgroup$
    – Ron Beyer
    Jan 11 at 20:07
  • $\begingroup$ @RonBeyer -- It seems unlikely to be a rate-of-change issue-- considering that the effect seems to be worse with the windows open (as noted in the second sentence of this answer aviation.stackexchange.com/questions/91204/… ) $\endgroup$ Jan 12 at 2:39

The problem with the alternate static source is it will over or under-read depending on the status of windows, vents and heating. Look at this excerpt from a Cessna 172 PoH and you'll see there's very significant differences, especially with the windows open at low speeds.

C172 Airspeed calibration

If you use a static source inside the cabin your airspeed your altimeter will over-read, how much again depends on vents, heating and windows. This means you'll be flying lower than you think, how much depends on these factors.

You cannot calibrate for these variables as they change dynamically. How much do you need to adjust your airspeed for heating halfway on, or a window cracked, and how would your instruments compensate? The primary static port is much more consistent.


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