Differtential and Isobaric control?

[Sandesh]

Dear fellow aviators,
I am currently learning about pressurization, And got confused on the idea of Diff and iso control? Can somebody please explain this to me?



Your time is greatly appreciated.
Sandesh

[SpiraMirabilis]

Uh, its been a while since my high altitude endorsement but... maybe someone who has more first-hand experience will answer better. However:

You set the isobaric pressure as to what you want the pressure to be in the cabin (usually a few hundred feet above your destination altitude.) Isobaric means constant. This is the only type I know about.

I would assume that a differential pressure system has a changing pressure inside the cabin where the system attempts to keep the pressure difference between ambient outside and cabin inside within certain parameters.

[SpiraMirabilis]

Looking at the pilot training manual for a CJ2 525A, section 12 is pressurization and this citation has 3 modes: AUTO, MANUAL and ISOBARIC.

In AUTO you select the destination elevation prior to taxi and the system will automagically generate a schedule of pressure changes so you arrive at your destination completely depressurized.

If the switch is in the MANUAL position you must control the pressure with the manual toggle valve (a red cherry picker knob.)

If while in AUTO there is an interruption of data the system will automagically switch from AUTO to ISOBARIC control. You will have to set the desired cruising altitude (in FL.) The controller uses the cruising altitude specified to control cabin rate of pressure change to maintain optimal differential pressure. You can flipflop to display cabin altitude and set that and the controller will maintain the specified cabin altitude.

So I guess I was wrong, it seems that most of the time the computer is maintaining a differential of pressure automatically -- and then if the computer fails it reverts to isobaric control where you have to enter some of the data in for it. Atleast on this aircraft.

[fish314]

Originally Posted by Sandesh

Dear fellow aviators,
I am currently learning about pressurization, And got confused on the idea of Diff and iso control? Can somebody please explain this to me?



Your time is greatly appreciated.
Sandesh

I think that Spira's first post was pretty much the answer that you were looking for. In the pressurization systems that have been on the aircraft that I've flown, you had a readout that gave you the cabin altitude, and also the differential pressure. In the KC-135 you'd set the pressure altitude that you wanted, which I think is probably what "Iso control" means. So you could specify a cabin altitude of 8000 feet, for example. But if you got too high for the pressure altitude that you've set, the differential would approach a limit, and the system would raise the cabin altitude so that the differential wouldn't exceed limits.

My guess as to "differential control" is probably a system where instead of setting the cabin altitude that you want, you set the differential that you want to maintain. BTW, the differential is just the difference between the outside air pressure and the cabin pressure. So if you have sea level set as your cabin altitude, that's 2193.8 psf (pounds/square foot). If you are flying at 10000 MSL (standard day) that's 1455.6 psf. That's a differential of 2193.8-1455.6=738.2 psf. Most systems measure in pounds/square INCH, though, so the differential would be 15.23 psi at sea level - 10.11 psi at 10000 MSL= a differential of 5.22.

In the KC-135 the system limits were differentials of about 8.6 using the automatic system or approximately 9 using the manual system. In the T-6 that I fly now, it tries to maintain 8000 feet or 3.6 psi differential, whichever comes first.