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In article <gZAT5.59553$vc3.12437587@typhoon.tampabay.rr.com>, Bob Moore
<rmoore16@tampabay.rr.com> writes
>
>In older aircraft (B-727), the flight engineer simply dialed-in 200' below
>sea level on the pressurization control panel just before takeoff or just
>before landing in the MIA case, or 200' below the field elevation for any
>other airport. It was all done manually.
>In new aircraft, there is a TO/LDG switch set by one of the flightcrew
>members. On the ground before takeoff with the switch in the TO position,
>the pressurization system senses the field elevation (pressure) and auto-
>matically pressurizes the cabin down 200' and it remembers this setting
>should the aircraft need to return to the takeoff field prior to reaching the
>cruise altitude set in the automatic controller. Prior to landing, the auto
>controller is set to the landing field elevation and the TO/LDG switch to LDG
>and at touchdown, the cabin will automatically be pressurized to field
>elevation minus about 200'.
>
Bob,
Thank you for that information, I now understand the methods involved.
However, I still seem to have a mental block for the practical
implementation of this for field elevations +200' down to 0'. I
understand if the field elevation was +1000' you (lets stick with a B-
727) set the pressurisation system on the control panel to +800'. On
landing the cabin pressure will be +800' relative to sea level, a small
positive pressure.

If the field elevation was +10' you set the pressurisation system on the
control panel to -190'. On landing the cabin pressure will be -190'
relative to sea level, a small negative pressure (vacuum). Can the
pressurisation system produce a vacuum? I don't think it can. As the
B-727 has been working fine for many years, in works in practice, I must
be missing something that is obvious to everyone else! Can you help?

Alan Huckerby
C-150 through C-182 and a few others
PPL-IR/ME (on the FAA system, despite being English!)


--
Alan Huckerby. www.huckerby.net
Flying, Sailing, Family Tree
I prefer newsgroup replies,
but answer email at: alan@huckerby.net

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