Pressurization

[Nidalap]

I'm trying to figure out how pressurization works. In the manual, it stipulates the maximum cabin rate of change that the system can pressurize at but at what rate does pressurization take place if for example you are climbing at the same or lower rate? Does pressurization generally just pressurise the aircraft to 8,000 ft at the maximum rate regardless of aircraft altitude/climb rate?

[Martin_123]

I suppose it may differ from type to type, I fly Q400 and for us the computer schedules the cabin climb rate based on the actual rate of climb and differential pressure. Generally on a normal climbout we see cabin to climb at a rate of about 400-500 feet per minute through-out the climb. The computer initially keeps the cabin altitude low and allows the maximum differential pressure to be reached just above 10k feet, because or initial climb rate at low altitudes is high. Once we are higher up we can no-longer climb as fast, so the computer has a chance to catch up with the pressurization. It does not pressurize the cabin at 8000ft, it uses the differential of 5.46 psi instead, and based on this, your cabin altitude can be around 6000ft if you're flying at FL200 or 9000ft+ if you are above FL250.. (not certified, but just to illustrate a point )

It is a bit more tricky on descent, because q400 is not as slippery, like a jet, so you can, if needed, drop like a rock - in these cases you can notice the pressurization computer trying to guess what we are trying to do - and if it gets its calculation wrong, it will drop the pressure at about 1000 feet per minute or more when you're just 2000 or so feet away from the pre-selected landing altitude. It can get unpleasant for the ears

[Banana Joe]

This is a very type specific question. I fly the 737. Cabin altitude ROC is controlled by the cabin pressure controller. On the Classic at the maximum ceiling of FL370 you have a cabin altitude of 8000 feet and on the NG you have a cabin altitude of 8000 feet at FL410. The NG structure is capable of sustaining a greater differential pressure.
Depending on the system installed, at the beginning of the takeoff roll the cabin is already slightly pressurized or begins to pressurized past 60% N1. On the climb out, the controller maintains a proportional pressure differential. Sometimes if you are light you climb too quickly that you reach the maximum cabin differential pressure (8.65 psi on the Classic and 9.1 psi on the NG) and the pressure relief valves will open.
When you reach cruising altitude, the controller will modulate the outflow valve to maintain the lowest possible cabin altitude depending on differential pressure limits depending on the cruising altitude.
Upon descent, the controller will do the opposite, control the cabin altitude ROD proportionally.

You can find some more information here.

[Council Van]

This might be of intetest to you, it shows how the pressurization system works on the machine I fly, the B757.

https://www.google.co.uk/url?sa=t&so...5dIQvTtd9gzS9f

[Nidalap]

Thanks guys, was hoping there was some standardization happening.