The Basics Of Preconditioned Air

Provding both power and air at a parking location can save energy and keep the APU ready for use when it’s really needed.

The low-cost airline I mentioned was one airline that figured this out early. And its solution was to put power and air in all its parking positions and standardize operations with the flight and ground crews to use these external power and air units to save significant money.

While I do not have specific numbers on what it saved each year with this solution, I have seen many studies done for airports where millions of dollars can be saved annually at hub airports and hundreds of thousands of dollars at medium-sized airports with the same improvements to power and air.

The difficulty, however, comes with different climates, different aircraft in one place and different restraints to infrastructure.

Having the exact same POU DX unit at each parking spot in the entire system is not normally going to work for most operators.

Let’s discuss some of the basic designs and the differences in technology to help us better understand the options.

DX units are the most common PC Air designs used today followed by AHUs. POU DX units range typically from 20 tons to 150 tons and can be facility-powered or diesel-driven. Smaller, electric 20-ton sizes can be side-mounted on bridges and the largest 150-ton units stand-mounted. Almost anything in between is possible.

All of these DX units come now with the following refrigerant: 410A, 407C and 134A. All three are used at JBT AeroTech because each refrigerant has pros and cons depending on what you are trying to accomplish.

In sizing the units for application, several factors have to be kept in mind. To name just a few:

  • Ambient conditions.
  • Aircraft to be serviced.
  • Type of power if electric-driven.
  • Desired cabin conditions.

We use an excellent calculation program that takes all of these variables and adds them up with the aircraft curves and provides recommended sizing. Even with these results, however, we still have to look harder at other factors. To name just a few:

  • Type and length of delivery hose.
  • The number of service doors that could be open at one time.
  • How often the worst case ambient conditions will exist.

With all of this information, basic performance requirements needed to do the job can be narrowed down. Common salient characteristics are a good thing to keep in mind at this point. To name just a few:

  • Tonnage.
  • Airflow.
  • Static pressure.
  • Discharge temperature.
  • Operating limitations.
  • Blower horsepower.

One quick note on tonnage: Many PC Air manufacturers are starting to give names to different sizes instead of tonnage descriptions because manufacturers know best what their units are capable of cooling. Tonnage is a difficult measurement anyway because compressor tonnage and actual tonnage performance make a big difference.

In some cases, when using 134A powered by a 50 Hz power supply, the compressor tonnage can be 90 tons, but the actual tonnage can be 50 tons. Derates for refrigeration and input power can be that significant with some manufacturers. This is why you really have to know what you are buying.


Extreme Conditions: For extreme hot locations, JBT AeroTech successfully developed a DX boost unit by taking a DX unit and adding a glycol coil – like a typical AHU unit would have – to supplement cooling capacity.

Until this hybrid design between a DX and an AHU, most operators had to run the aircraft’s APU to keep the aircraft cool.

But extreme heat and humidity are two things a good designed PC Air system can overcome. All DX and AHU use 100 percent outside air that is dehumidified in the process of cooling the air. If you’ve ever stood under a PC Air unit on a boarding bridge that doesn’t have the condensate hose connected, you stand a very good chance of getting soaked.

Central Systems: Central air is the other very popular option to DX POU units. Central systems are a good option for some since they offer a lower cost of operation and a better return on investment on projects with more than five parking positions.

A central system typically will have a greater initial capital investment than a POU, but has less overall compressor circuits, refrigeration components and has a diversity factor to help lower electrical requirements to provide as good or better cooling and heating temperatures than DX units.

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