Auxiliary Power Units and You!

If you look closely at commercial aircraft, you might notice something that looks a bit like an extra engine. Consider the CRJ-900 for example. It clearly has only two engines, but take a look at the tail. It has an additional jetpipe which surely resembles a third engine.

Learn all about how the APU provides redundancy in my guest article at

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About the author
Pat Flannigan is a professional pilot and aviation blogger. He has been flying for fifteen years and is currently working as an airline pilot in the United States.

3 Replies to Auxiliary Power Units and You!

  1. Jeffrey Synk says:

    Nice article Patrick.

    Here is little more info on the APU for the CRJ:

    1. Its Primary purpose is electrical and its secondary purpose is air conditioning, though if it’s hot out you would think that its primary purpose in air conditioning.

    2. During approaches, when there is icing, you have to start and use the CRJ200’s APU since the engines are not strong enough to both provide thrust and bleed air for anti-ice and de-icing purposes.

    3. In the CRJ200, the crew has to transfer the bleeds from the engine to the APU. The CRJ700 and CRJ900 does it automatically

    4. The overall operating characteristics of the APU on the CRJ700 and CRJ900 are far superior to the APU on the CRJ200.

    Thanks for the post!


    Jeffrey Synk

    • Thanks for the added info Jeffrey. You’re kidding, the 700 and 900 automatically transfer the bleeds!? I swear those four switchlights are the most confusing buttons on the 200.

  2. Thinh Bui says:

    The CRJ900 APU mount on the tail so it gives clearance and reduce cabin noise. You’ll need to turn on bleed air switch in the center of overhead panel to run air conditioning.

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