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When the static ports are blocked, pilots use the alternative static soure and it measures the static pressure inside the cockpit. And it is said that the static pressure inside is lower than the outside static pressure. I wonder why does this happen? Because you know cockpit is blocked from outside and how does the venturi effect applied inside the cockpit and static pressure inside?

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    $\begingroup$ Related question $\endgroup$ – DeltaLima Jan 29 '15 at 11:02
  • $\begingroup$ @DeltaLima Good find! $\endgroup$ – SentryRaven Jan 29 '15 at 13:41
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    $\begingroup$ On some light aircraft without alternate sources, breaking the glass on the VSI or altimeter will give you close enough readings to get downstairs safely. $\endgroup$ – Simon Jan 29 '15 at 13:43
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    $\begingroup$ @Simon the VSI is usually preferred: When you smash the glass there's a good chance that whatever you used will also smash the face of the instrument, and losing your VSI is less critical than your altimeter. (VSIs also generally cost less to replace.) $\endgroup$ – voretaq7 Jan 29 '15 at 16:57
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You're referring to this statement in the Pilots Handbook of Aeronautical Knowledge, Page 7-12:

Some aircraft are equipped with an alternate static source in the flight deck. In the case of a blocked static source, opening the alternate static source introduces static pressure from the flight deck back into the system. Flight deck static pressure is lower than outside static pressure.

The reason for this error with alternate static source is that the air flowing around the airframe creates a slight vacuum inside the airframe, thereby lowering the barometric pressure and generating erroneous readings in the pitot-static system.

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The physical effect that causes this drop is call the Bernoulli Effect. To compute anything reasonable, you would need to use the Compressible flow equation. From this point of view, it would be easier to measure the difference and put it in the instrument failure manual.

In short for the effect: gas with the same density presses against a non-moving object less than against a moving one.

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