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On the CRJ-900 AIR-CONDITIONING panel there is a RAM AIR switch light which opens a valve and allows air from the ram air scoop to enter the mixing manifold for distribution to both the flight deck and passenger cabin.

What would happen if the valve were opened at high altitude? Would this cause depressurization?

When I received CL-65 type rating I was taught that there was a protection that would prevent the valve from opening. I have also heard this was a mistake in the manuals and that it would open and cause depressurization. I have also been told that there is a one-way check valve inline with the ram air duct to prevent depressurization, but I’ve never seen a source for any of this information.

If anyone has an answer I’d appreciate citing some schematics or Bombardier manuals to back it up.

Update

I heard something. It's second-hand information from someone who worked at Bombardier when the CRJ was certified. This person said that there is no check valve on the RAM AIR valve, but the cabin outflow valves will close, compensating for the loss of pressure from the RAM AIR valve being open.

The cabin outflow valves should close completely as they are designed to prevent the cabin altitude from exceeding approximately 14,500 feet. This wouldn't prevent higher cabin altitudes if the ram air valve stayed open at a high altitude.

I still want a cited answer, but this is the first answer I've heard that makes sense to me.

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  • $\begingroup$ If the check valves close to prevent cabin alt exceeded 14.5k ft, why would the ram air valve staying open make any difference after that. If the check valves are closed, they are closed. $\endgroup$
    – Juan Jimenez
    May 23, 2019 at 11:28
  • $\begingroup$ The ram air port is a hole in the pressure vessel. If it's used when it is intended to be used, the cabin is at ambient pressure and it will provide some air flow. If it is used while the cabin is still pressurized, high pressure air will exit the ram air port. $\endgroup$
    – ryan1618
    May 23, 2019 at 16:25

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No RAM AIR VALVE can open if the cabin pressure exceeds the outer pressure. For the CRJ the RAM AIR VALVE is directed so that a dynamic component of the aircraft speed helps its opening, but this is much lower than the difference of pressure between cabin pressure and the outside pressure at high altitude. So no danger of depressurization whatsoever.

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    $\begingroup$ did you mean it can't open if the cabin pressure exceeds outer pressure? also, what's your info source? $\endgroup$
    – ryan1618
    May 27, 2019 at 22:00

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