Outflow Valve
 

Is an aircraft outflow valve a one way valve or simply a butterfly valve that once open pressure inside and outside the aircraft is equal?

Its a bit more complex than that. I can only speak for my aircraft (737NG) but its an electrically driven motorised door that opens outwards. Its got several motors and controlled by several controllers to prevent failure. The pressurisation system is a bit more sophisticated and picks a cabin altitude depending on your cruise level, field elevation etc. It then modulates the outflow to maintain whatever its looking for, beit steady or say 500fpm cilmb/descend etc.


Not sure what we're allowed to link to here, but there's some good descriptions and pictures here - Pressurisation


There are also negative and positive pressure relief valves which are just mechanical devices.

Basically - outflow valves are designed to -
Cabin rate of climb or rate of descent control and
Maintain cabin pressure differential limits.
xxx
With some loss of pressurization, these valves close automatically.
They also are supplemented by relief valves in case limitations are exceeded.
xxx
:8
Happy contrails

on the 320, it 's automatically controlled (2 CPC cabin pressure controllers).
can override manually or close it (plus 2 safety valves in the back)for ditching like in new york!well they are not here for dichting, it s to relieve pressure if the valve stay closed.

so you have several choices:

automatic: you do nothing!
semi automatic: you set landing altitude in the computer.
manual: you maintain pressure on the switch(UP/DOWN) until you got cabin altitude required.
Ditching mode: close all valves(you don't want water to come inside the plane too fast), and bring me some fishes...:E

It is not a one way valve, it regulates the pressure in the cabin to a desired pressure, it is just that air does not flow in while the aircraft is in operation because of a higher relative pressure inside the aircraft.

The outflow Valve is a small door that opens and closes by a computed Automated demand or manually by a switch in the flight deck.

Three motors can drive the door open by its respected computer/controller or swich, but only one can be in command at any one time, generally x2 AC motors for Auto 1 or Auto 2 operation and a DC Motor for manual operation.

The contoller/computer works to a schedule derived by the outside ambient px to the inside cabin px and this rate of px/flow commands the valve to open or close, usually to a 500ftperminute climb or descent.

In cruise the valve is open slightly to allow a flow of air through the cabin, regulations stipulate each passenger must recieve xlbs/hr.

The flight crew report to engineers if the valve is closed more than a given percentage as this tells us the aircraft has a possible leak of air from other than the ouflow valve.

Hope this helps

Beeline

An outflow valve can be a hinged single rectangular flap or a double flap (with a common drive system).

Some aircraft also have an "overboard valve" or another outflow valve at the front of the aircraft to help balance the airflow in the cabin (you don't want all the air rushing to the back of the aircraft to the regular outflow valve/s ... as this airflow can annoy passengers). These devices can be butterfly valves or flap-type valves.

Rgds.
NSEU

What? All the airliners I have flown have multiple exits for cabin air - through the toilets, vents at floor level all the way along the cabin sides, and so on. In the event of a rapid depressurisation this avoids bending the airframe.

Exactly what aircraft are we talking here? Because a system in an A320 will be very much differ from the one on an A310... We must put things in context... (or did I miss something? :confused:)

TyroPicard said What? All the airliners I have flown have multiple exits for cabin air - through the toilets, vents at floor level all the way along the cabin sides, and so on. In the event of a rapid depressurisation this avoids bending the airframe.

Yes there are vents all along the floor to prevent it collapsing in the event of an explosive decompression, however they only allow the air to flow in to the freight holds thus equalising the pressure above & below, they do NOT go overboard.

tristar500