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Pressurization Terms

uwochris

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Hey guys,

Can anyone please explain the following terms and their significance:

1. Normal Differential Range
2. Isobaric Range.

Thanks in advance,
Chris.
 

uwochris

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If it's not too late, I'd like to add another question to the thread :)

It's my understanding that engine bleed air is supplied to the cabin for pressurization. I also know that the level of pressurization is maintained by the outflow valves, which restrict/allow air to escape.

Out of curiosity, how exactly do the outflow valves work? For instance, if you set a cabin altitude pressure of 6,000ft on the pressure controller, how does the outflow valve "know" how to react to maintain this altitude? Furthermore, what is occuring when you change the settings (i.e. from 6,000 to 4,000ft)? ie) what happens to the outflow valves so that they "know" to maintain 4000 instead of 6,000 ft now?

Thanks again,
Chris.
 

Dangerkitty

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uwochris said:
If it's not too late, I'd like to add another question to the thread :)

It's my understanding that engine bleed air is supplied to the cabin for pressurization. I also know that the level of pressurization is maintained by the outflow valves, which restrict/allow air to escape.

Out of curiosity, how exactly do the outflow valves work? For instance, if you set a cabin altitude pressure of 6,000ft on the pressure controller, how does the outflow valve "know" how to react to maintain this altitude? Furthermore, what is occuring when you change the settings (i.e. from 6,000 to 4,000ft)? ie) what happens to the outflow valves so that they "know" to maintain 4000 instead of 6,000 ft now?

Thanks again,
Chris.

OK, I am bored so I will give it a shot.

1. Normal Differential. If you are asking what the normal pressure differential is for most aircraft it would top out around 8.5 - 9.0 PSI. That is the max for most of the aircraft I have flown. On the ground the PSI is 0 and then it will slowly work its way up as you clmib.

2. As to your second question I have no idea. Sorry

As for the outflow valves. They are usually 2 modes of operation. Automatic and Manual. In Automatic mode you just set the altitude you are climbing to or the field elevation landing at and the Automatic Pressure controller operates the outflow to get the desired PSI and cabin altitude.

If the automatic pressure controller goes inop then you can still pressurize the aircraft manually. It's really not that big of a deal. Now you (the pilot) becomes the Automatic Pressure Controller. You manually turn a knob (or in the case of the MD-80 a big yellow wheel) which manually positions the outflow valve. You then keep a close eye on the gauge and correct from there. You will most likely feel a little pressure bump from time to time but after awhile it is somewhat easy to get the hang of it. I can thank all those years at COEX for flying around in Manual mode for my pressurization training. :rolleyes:
 

typhoonpilot

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Hi Chris:

1. Differential pressure. The difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure. The normal maximum is between 7 and 9 PSI in commercial jet aircraft. Some of the higher altitude capable Lears and Gulfstreams would be higher, while most General Aviation aircraft would be slightly lower.


2.Isobaric Control refers to the condition where the cabin altitude is maintained at a constant altitude or pressure as the ambient pressure decreases. Isobaric Range is when the pressurisation system is working to maintain the cabin altitude at the preset level, which it can only do up to it's max differential. Here is an example with a system that has a max differential of 5 PSI: As the aircraft climbs, the aircraft is unpressurized to an altitude of 8,000 feet. In this example the aircraft isn't pressurizing until reaching 8000 feet for simplicity sake. Upon reaching 8000 feet the outflow valve starts to modulate closed as the aircraft continues to climb in order to keep the cabin altitude at 8000 feet. From 8,000 feet to approximately 23,000 feet, cabin pressure remains at 8,000 feet (isobaric range). The pressure differential increases up to my stated max of 5 PSI by 23,000 feet. From 23,000 feet up to the ceiling of the aircraft, the cabin pressure is maintained at a pressure differential of 5 psi but now the cabin would have to climb as well to maintain the maximum of 5 PSI.

For instance, if you set a cabin altitude pressure of 6,000ft on the pressure controller, how does the outflow valve "know" how to react to maintain this altitude? Furthermore, what is occuring when you change the settings (i.e. from 6,000 to 4,000ft)? ie) what happens to the outflow valves so that they "know" to maintain 4000 instead of 6,000 ft now?

The aircraft has a cabin altitude sensor and the system will modulate the outflow valve to maintain 6000 feet up to the maximum differential. Most systems have a climb schedule so that the aircraft starts pressurizing after takeoff ( unlike my example ). This results in a slow ( cabin ) rate of climb up to the set altitude. If you changed the level from 6000 feet to 4000 feet while remaining in level flight at a given cruise altitude, the outflow valve would close further to descend the cabin to 4000 feet. Upon reaching 4000 feet it would modulate as necessary to maintain 4000 feet as you climb or descend. Again, it can only do that within the pressurization cabability of the aircraft, which might answer question 1 for you.


Typhoonpilot
 
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