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I have heard that some aircraft that do not have an APU can run an engine in "Hotel Mode".

What is hotel mode? Are there any advantages to having it instead of an APU?

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up vote 12 down vote accepted

Hotel mode is a feature on ATR 42/72 turboprops replacing an APU by locking the right hand #2 engine (PW100) with the help of a 'propeller brake' whilst allowing the turbine to run and therefore also the generator, providing power and bleed air.

ATR describes this as the following:

The propeller brake is fitted on a countershaft on the RH engine reduction gearbox in order to stop the propeller (and the power turbine)

  • HP (High Pressure) spool drives the DC generator.

  • Bleed pressure is available downstream the HP compressor and supplies both packs.

This picture should give you sort-of an idea of how it works: hotel-mode

If I gather correctly, the propeller drive shaft is locked, and the (HP) compressor shaft is free to run.

Here's another pic apparently of an PW100 engine:

engine

Hotel Mode seems to be generally disliked and unpopular since:

  • It's very loud, especially if your lifting baggage next to it.
  • Fuel consuming since you're running a full engine.
  • Generally dangerous should it happen that the propeller brake suddenly would loose grip or is in bad condition.

This sort of configuration is rare if not unique. The rival Dash-8 has a proper APU, as does for instance the Saab 2000. It was optional feature on the Fokker 50. The smaller Jetstream 41 only has a battery.

Source: SmartCockpit ATR-72 documents.

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Older S-76 helicopters also had hotel mode. (Not sure if that's what they called it, but they were able to run a main engine with no main rotor rotation). –  Skip Miller Feb 18 at 20:17
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