Correct pressurization tips?

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Benedettini
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Correct pressurization tips?

Post by Benedettini »

Hello again!
May I ask whether its correct to pressure the acft manually that during cruise,the diff. Ratio is only 1000ft behind the redline, then inside presure is comfortable?
Is it OK to start pressuring over 6000ft (or when diff reaches half way) instead direct after takeoff,just like automatic B737 or B727's do? Or pressure at relatively low rates (100-200fpm) from ground to TOC, without exceeding diff. Limit anyway?

I ask coz in DMFS Comet manual its stated you need 8000 cabinpressure if flying about FL370 however i once did it around 4000ft, at differential of 6800ft (red:8000).

As real as it gets. I ask if this would cause problems.
Anyway,I never let diff. go over red or yellow; and always aimed high comfort!
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Benedettini
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Re: Correct pressurization tips?

Post by Benedettini »

EDIT: Redline was 9k, I mistaked. anyway I keep only 1,1.5k to it
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MALTBY D
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Re: Correct pressurization tips?

Post by MALTBY D »

Hi Gökalp

The idea is to pre-set a cabin altitude before takeoff, according to your expected cruising level. Then just let it get on with it.
I can't remember how you work it out properly, I just have a rough guess.
18000ft cruise, cabin alt =4000
25000ft cruise, cabin alt = 6000
35000ft cruise, cabin alt = 8000
That kind of thing.

Then set cabin altitude back down to 2000 (or zero if you like) before you start the descent.

The rate of the pressure change should be set about 400 ft/min (default), which would be comfortable for passengers.
That rate of change should allow your Cabin altitude to roughly keep pace with the differential psi as the aircraft climbs or descends.
If you know you are going to climb or descend at a particularly fast rate, you can raise the rate of change for the cabin too. But if it's too high your passengers ears may explode.

By my reckoning, with cabin alt of 4000ft you can get to about 35000ft in the Comet before the differential psi hits the maximum allowed (about 8.8 in the Comet).
Above that point, cabin alt would rise naturally as the mechanical safety valve has no choice but to vent excess air. The psi can't go any higher.

Ideally you don't want the psi to get to the maximum, unless you have to (by being at max altitude). It's a measurement of the stress acting on the aircraft skin.
For a passenger aircraft, max cruising altitude is dependant on the max psi.

DM
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Chris Trott
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Re: Correct pressurization tips?

Post by Chris Trott »

I don't remember if the Comet has the combo gauge, but some of the aircraft show the maximum altitude allowed by a given cabin altitude (the CL-44 has this such gauge, as does the BAC 1-11). As a general reference, I set the pressurization so that the maximum "allowed" cruise altitude is 4,000 feet above my planned cruise altitude. This allows me "breathing room" to climb over weather or traffic and keep a decent safety margin without having to change the cabin pressurization.

Benedettini
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Re: Correct pressurization tips?

Post by Benedettini »

basically in BAC i was about 2500 cabpr. At FL230, diff. of 5.5k ft.
Well,what I really wonder: What differs between less and more different ratio?
I kinda consider up to 10k feet its no need to pressurize as long as diff.
lets you.
My dreamfleet 727 was getting close to redline at cruise alt,automatic control. Whereas in 737 its just about half or less! Hence I ask the difference. :dunno:
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Chris Trott
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Re: Correct pressurization tips?

Post by Chris Trott »

It's all about the programming used. In the newer 737's (as represented by the NGX) the pressurization schedule is computerized and thus can be more exactly controlled. In the 727, it's mechanical. Older aircraft with automatic systems typically maintain the maximum allowable differential (and thus the lowest possible cabin pressure), however the result of this is that it also is constantly changing and can cause passenger discomfort. With the newer stuff and computerized programming, it knows what cruise altitude you've set either in the FMC or the autopilot, allowing for a constant cabin altitude and better passenger comfort.

Benedettini
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Re: Correct pressurization tips?

Post by Benedettini »

Well,Chris. DF727 has pressure system that you can select your FL and landing alt. If you meant so (still mechanical?)
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Chris Trott
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Re: Correct pressurization tips?

Post by Chris Trott »

Yes, it's still mechanical. It uses a rheostat, microswitches, and relays to control the pressurization. There is no logic function to how it works, thus it's not strictly "computerized" even though it is electronic. The system the 727 uses was actually originally developed for the B-29 and only "adapted" to be used on the B-377 and later the 707 and 727. Later in the 1970s (around the time of the 737-300 initial development) computers had gotten small enough that adding computerized logic systems to the system was possible.

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