An Environmental Control Unit in a Cessna Citation XLS+
Photos by elliott cox
Spring is the time of year here in the Carolinas when we have ice on the windshields in the morning and heat shimmers from the roads in the afternoon, and we’re constantly adjusting the settings on our thermostats to stay comfortable. A properly functioning environmental system can make flying in the heat of the summer much more enjoyable.
There are two types of environmental systems commonly used in airplanes. Vapor cycle systems are used in piston aircraft, turboprops, and smaller turbine airplanes. Air cycle systems are used in most turbine airplanes equipped with an APU.
Vapor cycle systems operate like the air conditioning systems in your house or car. These use refrigerant, typically R-134a, in a closed system to remove heat from one area (the cabin) and exhaust it to a different place (overboard.) The refrigerant is compressed, causing it to heat up and become vapor. The superheated vapor is sent through a condenser, where a fan blows air over the coils and cooling fins to cool the vapor inside. If you’ve ever held your hand over a running home air conditioner, the hot air you feel blowing out is the product of the fan blowing air over the condenser.
When the refrigerant vapor cools, it returns to a liquid state and flows through an evaporator. The evaporator is also made of coils and cooling fins and works like a condenser. The difference is instead of outside air blowing over the coils, recirculated cabin air is blown over the evaporator and returned to the cabin as colder air. The liquid refrigerant inside the evaporator’s coils absorbs heat and turns to vapor again before returning to the compressor to continue the cycle.
Of course, that’s a very simplified description of how the system works. I don’t want to go into too much detail because unless you have the proper certification(s), there isn’t a lot you can do yourself to maintain the system.
Air cycle systems have the advantage of not having to contain a pressurized gas to condition air, but the tradeoff is that these systems utilize more moving parts to make everything work. These systems use the same thermodynamic principles as vapor cycle systems, but instead of compressing and expanding a refrigerant to remove heat from the air, they compress and expand the air itself. Gay-Lussac’s Law states, “The pressure of a given mass of gas varies directly with the absolute temperature of the gas when the volume is kept constant.” The pressure and temperature of a gas mass are directly proportional, so when we compress the air inside an air cycle machine, it heats proportionally to the amount we compress it, and vice versa when we expand it.
Air cycle systems typically use a combination of outside air and bleed air tapped from the engines and/or the APU. If you drilled a hole in your leaf blower and stuck a tube through the hole so it blew air over your face while you used it, you’d be using the same concept. When bleed air is pulled from an engine or the APU, it’s at high pressure and high temperature, so the hot bleed air is plumbed through a “precooler,” which looks and works like a car’s radiator. The hot air passes through the inside of the precooler, and cold ram air is forced over its cooling fins, pulling a lot of heat from the bleed air. Bleed air leaving the engines can be over four hundred degrees Celsius and may lose a hundred degrees as it goes through the precooler. So how do we turn three-hundred-degree air into super-cooled air to dump into the cabin? We run it through an air cycle machine, which is nothing more than a big heat exchanger.
Each manufacturer calls this piece of magical equipment something different. Air Cycle Machine (ACM) and Environmental Control Unit (ECU) seem to be the leading nomenclature in my experience. For simplicity’s sake, I’ll use “ACM” from here on.
The slightly cooled bleed air comes out of the precooler, and some of it is routed directly into the cabin via an emergency pressurization valve, while most of it is piped into the ACM. Allowing the capability to pump unconditioned hot air into the cabin might sound counterintuitive, especially if you’ve ever operated out of South Florida in the summer. However, if the ACM fails and won’t allow any conditioned air into the cabin, you still have a positive air source for pressurization. It’ll get hotter than South Florida in the summer inside the cabin, but everyone aboard will be conscious while the crew gets the airplane on the ground as soon as possible.
Some pilots use empty legs to check the function of the emergency pressurization. It can be alarming for the co-pilot, especially when he or she is staring out the window and not expecting a loud rush of air. I’m not condoning it – I’m just saying it happens, in theory.
Now that hot air is coming into the ACM, we must decide what to do with it. To control the rate of air released from the ACM, we use temperature control valves which are modulating valves, meaning they can be in any position between fully closed and fully open. The more open these valves are, the more the air is expanded and thus cooled to varying degrees. The temperature control valves are normally operated by a temperature controller or computer, but they can be operated manually if there’s a failure.
When in automatic mode, the temperature controller/computer looks at the temperature in the cabin and compares it to the temperature of the air coming out of the ACM. The controller will modulate the temp valves to adjust the air coming into the cabin accordingly. When I worked for one of the Cessna-owned service centers, we saw a lot of environmental write-ups throughout the year, and the fix for most of them, maybe 80 percent now that I think about it, was simply cleaning the temp sensors in the cabin and cockpit. If you want to save yourself a little time and money, find out where the temp sensors are in your airplane, lay your hands on them, and you’ll see what I’m talking about. They’re typically very easy to access because they must be in the open air to do their jobs.
The other favor you can do for yourself is to know where your temp sensors are so you’ll know where NOT to throw a pile of coats. If the sensors can’t get a good reading of the cabin’s temperature, the computer’s automatic function will be completely inoperative.
When the air leaves the ACM at the desired temperature, that air still contains a lot of moisture, especially if you’re operating in humid climates, and spraying all 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 centrifuging,” the air enters a canister where it is expanded, causing 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 positive 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 pressure 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 thorough 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 unsung 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.
