Cabin Pressurization

[USMCmech]

Another gap in my areonatuical knowledge.

I understand how the cabin is pressurized, but how does the pilot control it durring flight.

Air is constantly beign pumped into the cabin (either bleed air or air from the Turbocharger), and the outflow valve is adjusted to maintain cabin altitude.

There is a cabin altimiter, and a cabin VSI.

How do you set the controlls and monitor the instruments durring climb and descent?

 

[EatSleepFly]

I think most aircraft you just set it and then monitor it.

King Air- Has a 3 position switch: TEST (allows pressurization on the ground for maint.), PRESS (pressure), and DUMP. Leave it set to PRESS for normal ops. Then dial in the cruising altitude before takeoff. Before landing, dial in the landing elevation, rounded up to the nearest 100'. There is a rate knob where you can set the cabin FPM from 50-2000-ish. You've got three gauges giving you status: cabin altitude, cabin rate, and cabin pressure/pressure differential.

The ATR is completely automatic, you just put in the destination landing elevation before you even leave the gate for departure. You can also control it manually via a knob, but its super sensitive and only really for use in abnormal situations. Same thing, three gauges: rate, differential, and altitude.

For anything pressurized, you're watching the PSI-D (differential) to be within min and max limits, cabin altitude to be below it's limit, and cabin rate to agree with knob position and what the other two are saying.

 

[USMCmech]

Correct me if I'm wrong.

After engine start, and before take off.

You would set cabin alt based on your planed cruise alt.

You would set cabin rate of climb to the rate you want.

Before descent,

You would set destination altitude.

 

[chris]

The a/c should have an "outlfow valve" which helps to control and maintain the pressure within the cabin. They basically control the escape of air from the cabin, back outside.

A "positive pressure relief valve" acts as a safety valve, to release any excess pressure to prevent an over-pressurization problem. They help prevent exceeding Max. Diff (i.e. the max pressure ratio between inside the a/c, relative to outside).

A "negative pressure relief valve" ensures the cabin pressure does not fall below the outside atmospheric pressure. (this will kick in automatically usually, if the pilots forget to reset the pressure controller prior to descent).

A "dump valve" can be opened to rapidly depressurize the a/c.

 

[B767Driver]

[ QUOTE ]
Correct me if I'm wrong.

After engine start, and before take off.

You would set cabin alt based on your planed cruise alt.

You would set cabin rate of climb to the rate you want.

Before descent,

You would set destination altitude.

[/ QUOTE ]


Most newer aircraft have an electronic pressurization system...this system requires almost no knowledge of the system to use...it's completely automatic. Before takeoff you simply set the landing field elevelation in the controller and that's it. It automatically schedules the cabin during all facets of flight.


In older aircraft such as the 727, DC9 etc, they incorporated manual pressurization systems. They were a pain! The required constand tweaking after power changes and had to be reset for climbs and descents. For landing with this system you had to know if the airplane could be pressurized on the ground or not. If it could be pressurized on the ground you would set the controller for 100' below field elevation. If it could not, then you would set if for 500' above field eleveation. For cruise flight you would set the controller for cruising altitude + 1000'. This would prevent the system from "riding" on the relief valves and causing uncomfortable pressure bumps.