# Understanding Aircraft Pressurization

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I am working towards flying the CaptainSim C130 line of aircraft with a better understanding of it's system's. In a few tutorials it makes reference to setting the aircraft's pressurization. This question is not about how to do it in this aircraft, but rather the logic behind determining the values to set.

Here is the portion of the A/C - Pressurization panel responsible for pressurization.

My questions:

1) Rate ft/min - what does this relate to? What would this value be based on? The rate of climb perhaps? Is this set only once, or something that would need constant adjustment?

2) Cabin altitude - What is this set for? Is it set only once initially on the ground? Or something that requires adjustment through the flight.

3) I'm assuming setting 1) and 2) above provides automatic pressurization? When would one use the manual pressurization switch? I think in real flight one would have an idea, but other the the physical symptoms, are there any instruments to monitor that would suggest a problem?

4) When would one have to use the emergency depressurization switch? When and where would you use it?

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Have no specific experince with the C130, but will try my best to answer your questions based on experience with other planes.

1) This relates to the rate at which the cabin pressure can change. A higher value will allow the cabin pressurization system to adapt more quickly to rapid changes in altitude (and avoiding risks for hypoxia or blackout). The ability to adjust this can be especially important for military planes required to make high-peformance climbs and descents due to threats. Rapid changes in pressurization, however, could lead to passenger discomfort. Most modern airliners now adjust this value automatically. Only plane I have with this settings is the turbine duke, which climbs like a bat out of hell... so I usually max this setting out. Gotta love that Duke.  ^_^

2) Cabin alt = The altitude at which the cabin is, relative to air density/atmospheric pressure, e.g. it's "as though" the cabin were at x ft... 12,000ft is the maximum a human can go safely without acclimation. This would theoretically be the limit at which you would set cabin altitude, but more realistically they are set 6000-8000ft. As you climb higher and higher it will be harder to maintain dense air, so a cabin altitude of 0ft is unrealistic, and also causes more stress on the airplane skin due to the increased pressure difference between the inside and outside of the airplane. Some planes will ask you to set desired cruise altitude and cabin altitude via a connected dial, the cruising altitude being pivotal for deciding cabin altitude.

3) I can imagine a few circumstances, but off the top of my head lets say you have high altitude parachuters jumping out of your C130. They will be jumping on oxygen tanks. The cargo/seating area will need to be depressurized and remain depressurized for the length of the jumping process. Same might hold true for cargo drops. You would need to control this manually.

4) Again, in a military situation, imagine if the c130 was attacked in cruise and suffered damage to the skin. They might hit the emergency decompression switch and put the crew on oxygen to avoid a rapid or explosive decompression due to the air pressure differences and compromised structure.

I haven't studied these system in relation to military, but the above dangers have factored in to commercial incidents, and the same system demands apply - although more of these systems tend to be automated on commercial airliners. Hope that helps.

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