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In the classification of systems and components on the aircraft - is an APU classified under the Airframe or the Powerplant?

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I was a tech writer in another life. Under ATA100/2100, the Air Transport Association system of categorization of systems for maintenance manuals, the APU is Chapter 49 (Airborne Auxiliary Power).

Power Plant chapters are 70 and up. (Some OEMs also use a equivalent numbering convention for engineering drawings to align them to ATA100/2100, where, say, if Flight Controls are Chapter 27, flight controls related drawings would have "27" at the start of the drawing number with the applicable sub-system numbering as appropriate - makes finding drawings in a database quick and easy).

Airframe systems are Chapters 20 thru 50. So from that standpoint, its Chapter 49 designation makes the APU an airframe system, not a Powerplant (which only refers to propulsion engines per ATA), as far as the industry is concerned, and I think you are pretty safe to go on that.

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In the U.S., auxiliary power units are categorized under Powerplant for the purposes of maintenance. The FAA Aviation Maintenance Technician Handbook - Powerplant, Volume 1 lists auxiliary power units in the category of turboshaft engines on page 1-38.

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It is a "power plant", so it would be classified as such. An APU isn't much different than a small jet engine.

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    $\begingroup$ Or it's part of the electrical system. The classification can go several ways. $\endgroup$ – Peter Kämpf Jul 17 '20 at 20:19
  • $\begingroup$ @PeterKämpf I'd argue that the engine part is a power-plant, the generator part is electrical, either way the tech that has to work on it will be a power plant person, not an airframe one. $\endgroup$ – Ron Beyer Jul 17 '20 at 20:27
  • $\begingroup$ From both the design and maintenance point of view it is a power plant (see the answer about US maintenance standards). APUs from big jets have occasionally even been used as engines in some light aircraft. $\endgroup$ – Guy Inchbald Jul 18 '20 at 11:52

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