Cockpit airpressure

ToiletDuck

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I know that if the static port is blocked you can use the alternate static port in the aircraft. However it will react because the cockpit has a slightly lower airpressure on the inside than there is outside. I understand it's there just fine but i'm prepping for my CFI and want to make sure I have it down and I'm not exactly 100% on this question. Is it because of the high speed at which the air is flowing around the cockpit and this causes the pressure on the inside to drop? The Bernoulli Principle?
 

avbug

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Air pressure in the cockpit/cabin of an unpressurized airplane may be higher or lower than ambient. In most cases, but not always, it's lower. Higher velocity airflow around the cabin decreases pressure.
 

groundpointsix

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avbug said:
Air pressure in the cockpit/cabin of an unpressurized airplane may be higher or lower than ambient. In most cases, but not always, it's lower. Higher velocity airflow around the cabin decreases pressure.

Avbug-

Can you explain why the pressure would increase inside the cabin? I've never heard this and can't come up with an explanation as to why it would work.
 

avbug

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Sure...strictly a function of cabin design. As an example, many ag airplanes have very pronounced cockpit structures which drastically alter the airflow about the cockpit. However, despite an increase in airflow over the canopy and cockpit area, airscoops are provided to assist in pressurizing this airspace. This serves for ventillation and also to provide some degree of slightly positive pressure for safety. The empennage of many such aircraft is also slightly pressurized to help prevent, to any degree possible, the collection of corrosive or explosive chemicals and vapors.

The battery compartment on your airplane, big or small, is ventillated and supplid positive pressure for the same reason, both for cooling, and to provide a way to eliminate and vent explosive and corrosive gasses.

A cockpit may or may not provide positive pressure. Clearly a pressurized cockpit does, but an unpressurized cockpit may have higher or lower pressure than ambient, depending on it's consturction and ventillation. Ram air ventillation can provide a cockpit pressure slightly higher than ambient.
 

NYCPilot

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Remember that generally speaking, in an unpressurized A/C, the pressure instruments will react one way, but will show a completely opposite indication when in a pressurized aircraft while utilizing the alternate static source.

Unpressurized
ASI - Higher than actual
ALT - Higher than actual
VSI - Momentary climb


Pressurized
ASI - Lower than actual
ALT - Lower than actual
VSI - Momentary descent
 

avbug

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Remember that generally speaking, in an unpressurized A/C, the pressure instruments will react one way, but will show a completely opposite indication when in a pressurized aircraft while utilizing the alternate static source.

Again, no. That is entirely dependent upon the aircraft, and the plumbing of the aircraft. In an aircraft that uses an external alternate air source, it doesn't matter if the aircraft is pressurized or not.
 

NYCPilot

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avbug said:
Again, no. That is entirely dependent upon the aircraft, and the plumbing of the aircraft. In an aircraft that uses an external alternate air source, it doesn't matter if the aircraft is pressurized or not.
Did you miss the part where I wrote "generally speaking," i.e., depending on the aircraft. I'm just parroting what the FAA books state. I agree that it is dependent on the pressure plumbing specific to the aircraft being flown, but in many cases what the FAA books state, are what really occurs under these situations and circumstances.

avbug said:
Air pressure in the cockpit/cabin of an unpressurized airplane may be higher or lower than ambient. In most cases, but not always, it's lower. Higher velocity airflow around the cabin decreases pressure.

...seems like you state the same thing as well.


Your posts are very informative though. They have pointed out what isn't included in FAA books.
 
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Gutenberg

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Refer to section 5 in your POH for airspeed while using alternate static. It depends on make and model. Also, if you've ever turned on alt static while on the ground, you will still notice (slight) changes in the 3 PS instruments. So what accounts for that? Temperature. High to low, look out below. The same reason you should be careful on a bitter cold day on a NP approach in the mountains.
Altimetry, my good man.
 
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