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Is there a big difference between what an Auxiliary Power Unit (APU) and an Air Turbine Starter (ATS) system actually does?

My understanding is that an APU provides power for the electrical systems of the aircraft and pneumatic pressure for air-conditioning and starting the main jet engines. An ATS provides starting power (compressed air) to a gas turbine engine.

  1. Is the only difference that the APU also generates electricity for on-board electrical systems?
  2. Would you ever have an APU and an ATS on the same aircraft? What are the use-cases for each product if they are mutually exclusive?
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The ATS is powered by the APU, so you typically need both. It's a bit different for some fighter jets though.

See: How is bleed air used to start a jet engine?

The battery is used to start the APU, the APU then provides electrical and pneumatic power.

The pneumatic power is used for air conditioning, engine starting, pressurizing the hydraulic reservoirs, and other things.

The ATS is a turbine that needs to spin, it is spun using this high pressure air from the APU. When the ATS spins, it spins a shaft connected to the engine, which then spins the engine just enough to compress air and start.

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An APU can be also be found on many piston aircraft and can itself be a piston engine. Whether the power provided is electrical, mechanical, hydraulic, pnuematic or a combination has no impact on the fact that it is an APU. Neither does the end use of the power matter, as long as the power is supplemental or non-essential in normal operations, rather than the sole source.

An ATS is just an air powered starter motor used to spin up the turbines.

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