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.
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.