Re: Why aren't air cabins pressurised to 1 atmosphere?

Date:         26 Apr 98 03:44:19 
From:         hmt@cygnus.co.ukx (Hugo Tyson)
Organization: Cygnus Solutions, Cambridge, UK
References:   1 2
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In article <airliners.1998.627@ohare.Chicago.COM> kls@ohare.Chicago.COM (Karl Swartz) writes:
> >I've a vague recollection of reading that aircraft cabins are
> >pressurised to 0.5 atmosphere.
>
> They're pressurized to the equivalent of an altitude of about 8,000 ft
> above MSL.  I'm not sure what that works out to in atmospheres offhand,
> but I'd guess it would be more than 0.5.

My watch consistently says it's about 830mBar over various flights.

> >Presumably it would be simple enough to hold it at 1 atmosphere in which
> >case all this ear ache stuff at takeoff/landing would be eliminated?
> >
> >OK so where's the flaw in this?
>
> First, the greater pressure differential (assuming the same cruise
> altitude) would require a considerably stronger fuselage structure,
> which implies more weight.  Weight is anathema to airliner design.

Exactly.

> Second, it requires energy to pressurize the cabin, and more energy
> would be needed to sustain higher pressurization.

Not exactly; it's bypass air from the engine fans (AIUI, IANA aero
engineer) and the pressure is limited by the release valves of which there
are several, in particular:

 * one which opens at _absolute_ pressure inside of 1 Bar, so that below
   cruise/on the ground, the cabin doesn't go above 1 bar which would hurt
   the passengers even more and prevent the doors opening too, as it
   happens.

 * one which opens at _relative_ pressure of about 0.5 Bar above the
   outside (I think it's about 0.5, could be where the figure above came
   from) so keep the pressure sensible high up.

So the pressure inside is MIN( 1.0, (outside + 0.5) ); the hull can
withstand a pressure _difference_ of 0.5 Bar assuming that figure's about
right.

The air inlet is at the front, often in the cockpit, and the release valves
are at the back, generally.  Smokers to the back please.  This is the
explanation for the apparently odd fact that an engine fire can fill the
cockpit with smoke, or at least the pilots smell smoke.

	- Huge