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that moisture inside the cabin of your airplane could cause damage and premature failure to the interior soft goods. To prevent that, the air leaving the ACM is sent through a water separator where, in the words of the French person who wrote Dassault’s manuals, the air is “demoisturized by centrifuging.” Wow. I really need to step up my descriptions.
Dassault means that by “demoisturizing by centrifug- ing,” the air enters a canister where it is expanded, caus- ing the moisture to condense. The air is swirled, and any moisture is slung to the outside of the canister, where it’s collected by gravity into a drain. From there, the water is vented overboard, or, in the case of the Falcon I maintain, it’s sent into the ACM to assist in the cooling process.
When the air leaves the water separator, it goes into the cabin via ducts that distribute cooled and conditioned air to keep everyone comfortable. You also have a posi- tive air source to keep the cabin pressurized as you blast through the flight levels. The only problem now is that the engines produce far more air than you need to pressurize the cabin, especially down low, so you must modulate it.
Someone much smarter than I am – a low bar, I know – decided that the answer to keeping a cabin pressurized at a steady and comfortable rate was not to modulate how much air comes in but rather to modulate how much air goes out. This is accomplished by using the aptly named “outflow valves.”
Typically installed somewhere at the rear of the pres- sure vessel, these valves are controlled much like the temperature. A computer looks at the pressure inside the cabin and modulates the outflow valves to allow excess air to escape. Also, like the temperature control system, the outflow valves can be operated manually in case of a computer malfunction.
There isn’t much in the way of preventative measures for the environmental / pressurization system other than keeping the temp sensors clean and unblocked. One additional note of caution: It pays to perform a thor- ough preflight inspection after any maintenance event, but pay special attention to the environmental controls. Besides the temperature control knobs, we rarely touch any other controls, so it’s very easy to miss something that has been moved. More than a few airplanes have departed a maintenance event with the pressurization system turned off. That in itself isn’t terribly dangerous, but the distraction it causes can be.
The environmental system is one of the greatest un- sung heroes in an airplane because when it’s working correctly, it is invisible, and that’s how we like it. Happy flying, and stay cool.
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Elliott Cox is a pilot and the Director of Maintenance for a Part 91 Corporate Flight Department in the Southeast. You can reach him at his website TheWritingFlyer.com or by email at elliott@thewritingflyer.com.
May 2023 / TWIN & TURBINE • 21