Mine works the same as Jasons. Actually, they’re pretty crude and simple things. When I bought mine I was over thinking it and agonising over the settings endlessly etc. It was all new to me. Today I don’t even look at it most of the times. Have it set to 5000ft cabin alt always. That means that up to 5000ft the cabin is essentially unpressurised. As I climb past 5000ft, controller will try to maintain that cabin alt with diff press. Cabin will stay at 5000ft until I max out differential at which point it will climb at max differential (4.5psi). Only time I’d have to change any setting was if I planned on landing at an airport that has an elevation higher than 5000ft. Then, like Jason, I’d set it at field elevation + 1000ft. Other than that, never touch the damn thing.
Adam, In my personal opinion you are not getting the best out of your aircraft like that. It is noticeably less tiring if you can fly with the cabin as low as possible. Maybe some of your flights would not exceed a sea level cabin and it certainly makes me feel better.
Good point, Neil. I set it to 5000ft just because above that I’d heard the body starts to feel fatigue and reduce oxygen to organs etc.
Adam, mine is set to 1000ft unless I plan to land at an airport above that. And there are two ways to use it.
1. Leave it and it will go to max differential pressure then climb.
2. after departure set to your cruise altitude (on the alt scale) and it will steadily climb you. Then at TOD set to field plus x (I use 1000).
I use method 1. but it does lead to a more rapid climb after max delta p. But you can never forget to set it.
I suppose I subconsciously also don’t want to rob engines of bleed air or pressurise the bulkheads unnecessarily, that’s why I’ve kept it at 5000ft. Less of a problem in a turbine of course. The P Baron has a 10000hr limit on the fuselage after which you must scrap it. And the P210 has only been tested to 10000hrs, although they don’t have a hard limit.
Have it set to 5000ft cabin alt always.
The previous owner of our plane did the same. Sure it works but it can be very unpleasant when doing a fast decent below 5000ft as you are basically flying an unpressurized cabin below that. One of the great advantages flying a pressurized cabin is being able to decent at over 1000ft/min and more without any bad effects on the ears etc.
I suppose I subconsciously also don’t want to rob engines of bleed air
I am not sure about all designs but usually the cabin cntroller only regulates the outflow valve. Thus the engine always pushes air into the cabin and the only thing which changes is the outflow rate. So with a depressurized cabin maybe the backpressure felt by the engine will be different but I doubt it will be very much.
An interesting thing is landing at an elevation over 8000ft as in that case before landing you will have to climb the cabin instead of let it decent. As far as I remember the Airlines have special aircraft versions which do not drop the oxygen masks in this situation…
It depends how smart your controller is. On the 737, it works fairly well, despite the pressurisation system being an excellent way to kill yourself due to awful ergonomics.
You input your cruise altitude and landing field elevation on the control panel, leave the system set to AUTO, and then it will climb the cabin altitude at a comfortable rate to cruise altitude. 3 different max diff pressure depending on how high you go. This means that if you climb 1000’ you get the strange sensation of the cabin altitude decreasing! When you put your arrival and so on into the FMS, it calculates a required cabin rate, which is usually around 300-400fpm, and the cabin will descend at this rate to slightly below landing field elevation. The aircraft lands very slightly pressurised (to avoid big changes when you’re dropping like a stone with it all hanging out to make the profile). Once on the ground, the outflow valves drives fully open to depressurise the aircraft.
The modern Lears have fully automatic pressurisation, but perversely are less comfortable. Landing field elevation is a manual input, but the cabin climbs and descends automatically, but in descent the cabin rate is around 500fpm, which at FL350 with only 6000’ cabin altitude means if you have sensitive ears like me the change in cabin altitude is very noticeable.
I believe the 737 also uses inputs from the ADC to figure out when the descent has started. From memory it is whenever you are 0.25psi below the selected flight level on the pressurisation panel.
