Mig_Driver wrote:That makes sense, thanks, Esa! I was wondering how and at which altitude or pressure it tripped. I've been digging pretty deep into the 777 lately but I couldn't remember if they gave any specifics.
No problem. I might as well add some further points.
Using the 737 as an example, there is a similar relay functionally parallel to the one that is signaled by the cabin pressure switch I mentioned. It is not actuated by the pressure switch but by a guarded overhead switch in the cockpit to manually deploy the masks. That way there are two independent ways to energize the mask door solenoids in case the pressure switch or the auto deploy relay malfunctions.
Moreover, it is good to notice that oxygen mask system is separate from the cabin pressurization system. The two cabin pressure controllers in the EE compartment have their own pressure sensors which have nothing to do with the pressure switch that operates the mask system. And further there is a yet separate pressure switch, or several switches (again, depending on the individual airplane), that make the cabin altitude warning horn to sound and in most cases the red CABIN ALTITUDE indicator lights come on (in some airplanes they are inoperative). Hopefully that warning prompts the crew to take action before the automatic mask deployment occurs. Note that the cockpit has no automatic masks nor oxygen generators, but the emergency oxygen is provided to the manually operated masks from a pressurized bottle under the floor behind the cockpit.
So both the warning and the emergency oxygen systems use their own switches and are independent from each other and from the cabin pressurization system.
CAPFlyer wrote:Light aircraft, however are usually very simple.
Yes indeed. To use late series Cessna P210 as an example (as I happen to have some stuff about it still on my own handy collections), there is, as you say, a pressure switch that closes when the cabin altitude exceeds 12400 ft Ã‚Â± 100 ft, as it is quoted, and resets at about 11700 ft. All it does is that it illuminates the red light labelled 'CABIN ALTITUDE'. An STC of adding a horn to the system would make much sense for those regular high-flyers.
The pressurization system controls are altitude selector and ON/OFF switch for the system (the airplane could as well be flown unpressurized). The altitude selector sets the altitude at which the pressurization begins and at which the cabin is maintained until maximum differential is reached. This is done by controlling the outflow valve at the rear wall of the cabin. Adjacent to it is a safety dump valve, which is similar but normally held close during pressurized flight and is controlled by a solenoid that is activated by setting the ON/OFF switch to OFF, thereby releasing the cabin pressure and preventing cabin from pressurizing. Both of the valves have a set safety differential at which they open to protect the structure from overloading. There is also a mechanical dump valve operated by a T-handle next to the master switch that needs to be pushed in for the cabin to pressurize.
As these planes are often flown unpressurized, I wonder how easy it is, in practice, to mismanage the system - especially if not routinely flown similar airplane pressurized.
Let me imagine what is impossible - and then do it.