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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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    $\begingroup$ Well it saves a lot of weight and space that the APU would need. Given the fact that most of the airports in Europe are equipped with GPU, the Hotel Mode is hardly used. Most oftne just minutes before departure $\endgroup$
    – Ivo
    Commented Dec 24, 2016 at 21:28

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

engine

Hotel Mode seems to be generally disliked for a number of reasons:

  • It's very loud, especially if you're lifting baggage next to it
  • It consumes a lot of fuel, since you're running a full engine.
  • It could be dangerous if the propeller brake is in poor condition and suddenly loses its grip.

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

Source: SmartCockpit ATR-72 documents.

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    $\begingroup$ 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). $\endgroup$
    – Skip Miller
    Commented Feb 18, 2014 at 20:17
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    $\begingroup$ Does this cause a pressure build-up between the turbines or make it harder to start? $\endgroup$
    – Andy Joiner
    Commented Jul 7, 2016 at 0:24
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    $\begingroup$ And why is it called "hotel mode" ? $\endgroup$
    – summerrain
    Commented Jan 2, 2019 at 21:06
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    $\begingroup$ @summerrain hotel mode would be powering all non-propulsion electrical loads on an airplane. It comes from the maritime term, hotel load which is the electrical load for all non-propulsion systems on a ship. - en.wikipedia.org/wiki/Glossary_of_nautical_terms#H $\endgroup$
    – gwally
    Commented Jan 2, 2019 at 22:18
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The term, hotel mode would be powering all non-propulsion electrical loads on an airplane using the power generating capabilities of the airplane, such as an engine or an Auxiliary Power Unit (APU).

The term comes from the maritime industry from the expression, hotel load. This is the electrical load for all non-propulsion systems on a ship.

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