Bleed air main switches
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On a couple of flights now I've noticed that after engine start when I switch both cockpit and cabin to ON the valves shows correctly as both open. Once in the climb with spare time to check the overhead I see that the cockpit valve is now shut although the switch is still selected ON.
I see no adverse impact or lights - temps and cabin pressure is all good. Once I reduce power in the cruise or for descent I see the valve switches open again. The manual doesn't mention how it works - my feeling is its correct, but just in case - mentioned here.
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That is correct behavior and it is simulating the bleed air auto shut-off system which automatically shuts off the air conditioning MAIN valves when additional air is required by the engines in order to achieve the desired thrust level.
A table showing the valve states with various RPM settings can be found on page 35 of the F28 Professional Operations Manual and we go on to explain a little about this system near the end of the "Bleed Air Supply & Air Conditioning" video that we posted a few months back: https://youtu.be/lVewFdk3y5I?t=248
Mark - Just Flight
