Re: 747 forward cargo door

Date:         08 Dec 96 04:12:31 
From:         "David G. Davidson" <>
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Jean Ludovicy wrote:

> I can't follow you John.
>  How does the packs, wich are responsible for an eventuel increase in
> fuel consumption, know that there is an additional air leak at the
> cargo door.
> Do I miss something?

I'm not familiar with the 747 but for discussion lets pretend that

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.