More about the prop brake
The term hotel mode is actually a nautical term. When cruise ships would pull into port, they would disengage the screws from the powerplants so that they could still get electricity to keep the ship powered without turning the screws.
On the ATR, hotel mode refers to the operation of the right engine with the prop brake engaged. The ATR series, having Pratt and Whitney 100-series free-turbine engines, has a multi-disk propeller brake installed on the right propeller's reduction gear box. When it is engaged, the crew can run the right engine without turning the propeller. This supplies DC power and bleed air for the air conditioning. It is hydraulically actuated, electrically controlled, and mechanically locked. It can be engaged with the engine shut down, or with the right propeller spinning in feather. Its logic is designed so that it cannot be engaged in flight. ATR did design it as a substitute for an APU to save weight, complexity, and to minimize DOC. Since every aircraft design decision is a compromise, the prop brake solution does have its limitations.
Hotel mode cannot be used full-time at the gate like a conventional APU. Typically, the ATR cannot be fueled while in hotel mode. Also, most rampers do not care to load bags through the aft service door with a noisy, hot engine running 30 feet or so from them. Even though the prop brake can be a very reliable system, there is always the danger of the brake letting go and the propeller rotating with ground personnel in its vicinity. As someone else said, hotel mode can be tricky in a tailwind. The nacelles of the ATR are primarily composite. It is easy to overheat the right engine nacelle when running in hotel mode with a tailwind, since there is no propeller spinning to move air over the engine nacelle. The right engine nacelle is monitored for overheat. Naturally, this somewhat reduces the flexibility of hotel mode. Also, the engine running in hotel mode (typically at 68%-72% Nh) does not provide very high pressure air to the packs, so it does not cool the aircraft as effectively as an APU that runs around 100% of its rated RPM. In a warm climate, pre-cooling a heat-soaked ATR without passengers becomes neccesary. Airlines that choose to use the prop brake will typically start the right engine with the prop brake on a few minutes prior to departure, disengage it to taxi, and then re-engage the prop brake arriving at the destination and then operate in hotel mode until a GPU is plugged in.
The main maintenance problem with the prop brake is slippage. If the brake disk gets sufficiently worn, the prop can slip and rotate while in hotel mode. Although there is no cockpit indication, it is noticeable from the cockpit. Naturally, this is a dangerous condition for anyone near the right engine. If the prop brake fails, it must either be removed from the aircraft or repaired. Some airlines have had bad luck with this system and have removed the system from their aircraft. The airline I work for now has had good luck, and I believe this is due to the fact that we realize its limitations. We carry very few prop brake deferrals and have very few failures.
I know this was a windy post, but the prop brake is definitely a unique and defining feature of the ATR series. Therefore, I enter my scant knowledge of it into the record.