Re: Cabin pressure profile?

From: (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         23 Sep 93 00:11:25 PDT
References:   1
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In article <airliners.1993.598@ohare.Chicago.COM> (Clemens Emmanuel Tillier) writes:
>This is a somewhat vague question, but here goes: 
>How does the cabin pressure in a 'typical' airliner vary with
>altitude? Of course it decreases, but how? Is there a proportional
>decrease as you go up or does it follow atmospheric pressure to a
>certain altitude and stay at this final pressure as you go higher?
>Even more basic: what is a 'typical' cabin pressure at say 35,000
>feet, and to what real altitude does it correspond?

Since the 727-200 is representative, and since I know a great deal about it:

The 727-200 pressurization system is all-electronic.  It is comprised of an 
outflow valve, located in the right aft fuselage, under the exit.  The airflow 
entering the cabin from the engines is offset by the air leaving the airplane 
through this valve.  The valve rotates to maintain an appropriate 
pressurization schedule.

The valve is normally AC controlled, but there is a standby and manual 
mode, which use alternate AC paths, and even a DC path, to actuate it.  

The system is normally programmed with the anticipated flight altitude and
ground altitude, prior to takeoff (auto mode).  A switch is flipped prior to 
takeoff (and after landing) to switch between the in-flight and on-ground 
modes of the system.  After this (auto mode), it's hands-off.

The optimal cabin differential is 8.65 psi.  IF this maintained, then the
airplane can maintain sea level pressure up to 22,500 feet.  At 40,000 
feet, the differential yields a cabin altitude of 7,000 feet.

The system attempts to maintain the anticipated landing field altitude.
Therefore, after takeoff, it will initiate a rate of climb of up to 
500 feet per minute, but in actuality, the rate of climb is proportional to
the final cruising altitude.  The system will accept a rate of descent
of up to 350 feet per minute.

Underpressure relief (outside air at higher pressure than the internal air) 
is provided when the differential exceeds 1.0 psi.  Overpressure relief is 
set at a differential of 9.42 psi, via two independent relief valves.

The cabin itself can withstand a pressure differential of 1.5 x 8.6 psi,
or about 12.9 psi.

Standard instrumentation includes the auto, standby, and manual mode select-
ors, a cabin altimeter, a cabin rate of climb indicator, and a cabin/ambient 
differential indicator.

A "differential" is the difference (in psi) of the internal pressure, com-
pared to the ambient (flight altitude) pressure.  Therefore, an airplane
sitting on the ground at sea level has a differential of 0; one cruising at
40,000' with a cabin altitude of 7000' has one of 8.6.  Standard atmosphere.

There are many other fun quirks; it's a complex system to model.  Boeing
aircraft all use the same basic principles; more modern airplanes have greater
degrees of automation.

Robert Dorsett!!rdd