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Citation ECU: Keeping Your Cool

August 2011

With the oppressive heat that most of the nation has experienced this summer, it is imperative to check for blockage on the Environmental Control Unit (ECU). A blockage on the ram air inlet side of the heat exchangers can be caused by a number of things, including dirt, debris, cottonwood seeds, etc., and will result in inefficient heat transfer in the ram air circuit. This will prevent the ECU from cooling properly.

When “Max Cool” means warmer cabin temps

An inefficient heat transfer can result in a hot cabin, even with the temperature control in auto mode with “Max Cool” selected. When “Max Cool” is selected, most of the bleed air passes through the Air Cycle Machine (ACM), causing a heat buildup. While in “auto mode,” the temperature control incorporates a 410-degree switch that senses when the ACM is approaching an overheat condition. To prevent overheating, it will begin to bypass hot bleed air around the ACM to help cool it. This hot air will mix with the cold air, resulting in the conditioned air into the cabin being hotter than desired to maintain a comfortable cabin temperature.

In an attempt to further cool the cabin, flight crews will often transfer to manual temperature control and select full cold position. In manual mode, the system deactivates the 410-degree switch and the ACM may reach an overheat condition, which is not annunciated. Once the ACM temperature reaches 450 degrees, the overheat switch shuts down the ACM and automatically activates Emergency Pressurization.  

The bleed air provided by the Emergency Pressurization system comes directly from the LH engine and is not conditioned. When this occurs, the only way to control the temperature is by reducing power on the LH engine.

Preventive maintenance of the ECU

The inlet side of the ram air circuit to the heat exchangers can be easily checked by removing the small nylon aspirator tube on the side of the ECU plenum and inserting a borescope. The entire surface of the ram air inlet to the heat exchanger will be visible through the borescope. 

If debris is present in sufficient quantity to block, or partially block, the ram air circuit of the heat exchangers, the ECU should be removed, disassembled and cleaned. This should result in a much more efficient heat transfer to make sure passengers and crew have a comfortable flight.