Re: 747 forward cargo door

Date:         13 Dec 96 04:26:01 
From:         Jean.Ludovicy@ping.be (Jean Ludovicy)
Organization: EUnet Belgium, Leuven, Belgium
References:   1 2 3 4
View raw article
  or MIME structure

"David G. Davidson" <gerhard@onramp.net> wrote:

>I'm not familiar with the 747 but for discussion lets pretend that
>747=757.

>The packs don't know that there is a leak, the pack valves are designed
>to provide a certain volume of air. On the B-757 each pack valve
>(acutally called a Flow Control and Shutoff Valve) is designed to
>provide 75 lbs/min of air to the pack (normal flow schedule).

>The pressurization system on the other hand can see that something is
>wrong if there is an excessive leak in the pressure vessel. If the
>outflow valve is fully closed and the cabin altitude is climbing, a low
>flow signal is triggered in the pressurization controller and the LOW
>FLOW fault light is illuminated. This may be an option on the 757 as our
>airplanes don't have the LOW FLOW light but our MD-80s do.

>In short, the packs can't compensate for a pressure leak, only the
>pressurization system can do that and only up to a point.

That's exactly the way I understand it works. It doesn't matter the
type of aircraft.
All modern jet airliners work on the simple principle of a pressure
regulated outflow valve.
If there is a small additional air leak somewhere, the outflow valve
just closes a little bit further to compensate in order to maintain
the same differential pressure.
The air input by the packs is not aware what the outflow valve does,
that's what I wanted to point out when I asked John, on how the packs
would know about the air leak.
Your way of explaining it was better of course.

Best Regards,