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.

Close to nine years ago, I was involved in a bid for more than 100 preconditioned air units for a low-cost U.S. airline. This airline was one of the first to completely outfit all its boarding bridges with both 400 Hz and preconditioned air.

This was also just prior to the last fuel cost spike and the industry’s move to be “more green.” Today fuel costs are back up and green airport initiatives are much more common. The difference today is there are so many more choices, but also so many misunderstandings about what preconditioned air units will best service a particular operation.

I am not sure if it is the industry’s engineers or creative marketers, but there are a lot of acronyms in the preconditioned air industry:

  • ACU (aircraft cooling unit).
  • PCA or PC Air (preconditioned air).
  • DX (direct expansion).
  • AHU (air handler unit).
  • CAS (central air system).
  • POU (point of use).
  • TMAC (trailer-mounted air conditioner).
  • DAC (diesel air conditioner).

With that information we can now discuss PC Air like the pros. PC Air’s main purpose is to save aircraft operators money. Yes, it cools and heats an aircraft, but the aircraft already can do that for itself with an APU. The APU is a very expensive little engine, however.


To understand how PC Air saves energy and fuel we should follow an aircraft coming into a boarding bridge or ramp parking position that does not have power and air available.

Either on approach or during the taxi in, the captain will turn on the APU. That way it will be ready to take over the aircraft’s power loads and cooling and heating needs before the captain shuts down both main engines.

From that point until the aircraft is disembarked, catered, cleaned, boarded and engines started again, the APU would be running the entire time. For years this seemed OK until some smart people started to calculate what the costs were to do this.

The first reaction was to make sure 400 Hz and 28V DC power were at either the boarding bridges or available at the ramp parking spaces. That way the power requirement of the aircraft could be transferred to an external power supply.

The problem with that was the captains still did not turn off the APUs. While air and heat are not required for aircraft operations to continue, the captains had a responsibility to make their customers comfortable and would keep the APUs running just to run conditioned air into the aircraft.

That would be like having a $1,000 electric bill to cool your house for a few hours. It is a big waste of money and energy. Besides burning a lot of fuel and making a lot of noise, many aircraft operators pay fixed maintenance costs based on cycles or run time.

After all, this also defeats the main purpose of having an APU – to provide a backup generator on board just in case of an engine generator problem and to start aircraft engines before flight.

I used to be an airline pilot for a legacy airline and one operational delay that was common would be a failed APU unit. The main reason for failure? We would be limited to a small quantity of available external air-start units to get the first engine started, which inevitably caused a delay on push back.

More importantly, if an APU is “MEL’d” (minimum equipment listed), you can still fly with that broken APU because the FAA has preapproved that failure to be acceptable for a short period of time – but not in bad weather.

Having a MEL’d APU at the airline I flew for meant that flying in icy conditions, expected turbulence or CAT III approaches – basically very low visibility – was prohibited. This FAA requirement would often cause delays or cancellations because the weather criteria could not be met.


PC Air units are a part of the solution to help lower these APU outages, save energy and keep the APU ready for use when it’s really needed.

By using the APU less and using external PC Air with power instead, the APU can remain off until 5 minutes before departure to be ready to start a main engine.

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