Re: Passenger cabin pressurization

Date:         06 Oct 97 02:14:23 
From:         luisma@spainmail.com (Luis Manuel Perez Llera)
Organization: Unisource Espana NEWS SERVER
References:   1 2 3 4
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El día 17 Sep 97 02:49:16 , en el grupo sci.aeronautics.airliners dijiste:

The highest prority is aircraft structure (the maximum diff pressure for
this A/C should not be achieved). The second priority is passenger comfort,
which means:

1) The cabin equivalent altitude should not be greater than 8000 ft (this
condition, together with the structural condition stated above, imposes
a limit to the aircraft flight level, limit which is known as SERVICE
CEILING).

2) The cabin pressure rate of climb (which is controlled by the crew)
should not be greater than, say, 300 fpm.

3) If the aircraft is flying at a lower altitude than the service ceiling,
the cabin pressure is higher (the altitude is lower) than the 8000 ft
equivalence.

4) In order to avoid bumps when in flight, the max differential pressure
is NOT exactly achieved. A 500 ft margin is left (so that the outflow
valve does not open and no bump is created if the pressure-altitude
inadvertidely increaes (for example, flying through a hotter air mass).

5) For the same reason, the pressure is slightly INCREASED during take-off
(this is called pre-presurisation). The amount is not a lot, 150 to 200 ft.

A typical max differential pressure is 8.5 PSI for DC-9 family. An
emergency relief valve is open if the normal outflow valve locks and the
diff pressure exceeds the maximum allowable. Another relief valve open if
this blockage occurs and the differential pressure gets negative values
of 1.0 PSI.

Hope this helps