Originally Posted by
Chris Martyr
Using a B.747 as an example ; Both outflows operate independently of each other and the chances of both of them failing completely are almost zero . Don't forget , they can also be toggled manually via both AC & DC motors As BAeng. says , in the crz they are just 'pulsing' and are almost closed anyway.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
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as I mentioned in another thread, I have actually experienced a dual outflow valve failure, or to be more precise, a dual outflow valve controller failure in a B747! Sitting in cruise somewhere over Canada on a flight from AMS to LAX, both controllers failed and both outflow valves went fully open. Can’t remember the exact details but I think we were at approx 35.000 with the cabin around 6.000(somebody with a 747 manual can probably look up if that’s about right). In the timespan of about 45 seconds the cabin went up to about 13.000 before we got it to go down again in manual mode. In the stress of the moment we closed the outflow valve completely and when the cabin got below 10.000 we took of our mask off, congratulated ourselves of a situation well handled but than the next warning went off! Can’t exactly remember the actual EICAS message but we were overpressurizing due to the outflow valve being completely closed and the overpressure relief valve kicking in! This is where that valve kicks in. At a certain PSID it opens to keep the aircraft from overpressurizing. Continued the flight to LAX in manual mode, which wasn’t too bad but it was a 3 pilot operation. Apologies for not remembering the exact numbers and messages but fly a different airplane these days!