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Race cars can be refueled while the engine is running. I assume this is a dangerous task done by highly specialised technicians.

If ground support is not available, an airliner may keep the APU running to be able to provide power to restart the engines.

If ground support is not available and refuel is needed, can an airliner refuel while the APU is running (technically and legally)?

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    $\begingroup$ Why do you think it couldn't be? Most cars, including the one in your driveway can be safely refueled while they are running, even by Joe Sixpack. The fueling locations are a significant distance from the APU, and APU operation shouldn't be affected by refueling. $\endgroup$ – Ron Beyer Feb 9 '16 at 3:56
  • $\begingroup$ In Ontario anyway, a car's ignition must be off for Joe Sixpack to legally refuel it at a gas station... $\endgroup$ – DJohnM Feb 9 '16 at 5:34
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    $\begingroup$ Things may have changed since I retired in 1999, but in the 1990s 747s were routinely fueled with the APU running at both of the 747 carriers I flew for. In fact, you had to have electrical power of some sort to use the normal pressurized underwing fueling. Without electrical power, one would have to use the overwing fuel caps and gravity flow. In 10 years on the 747, I never saw that done. $\endgroup$ – Terry Feb 9 '16 at 6:10
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    $\begingroup$ There are cautionary signs at every gas station in America warning you to turn off the ignition while refueling your car. I don't know if it is law or just a recommendation. There are also signs about not using your cell phone while refueling. While that's proven to be just another myth about cell phones, the signs remain. I think the "highly specialized technicians" involved in pit-stops are so because they need to be able to quickly insert the fueling nozzle into a still moving vehicle in order to minimize stop time (in many, but not all race categories). $\endgroup$ – FreeMan Feb 9 '16 at 15:26
  • $\begingroup$ There needs to be power to the systems before fueling as it is needed to operate the pumps; so either ground support or APU; where APU being the most commonly used simply as its quicker to get up and going. $\endgroup$ – Burhan Khalid Feb 10 '16 at 4:48
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Refueling an airliner with APU running is a perfectly normal procedure and it is done daily all around the world. The APU is located at the tail cone partially to reduce the risk of fire spreading to the fuel tanks, which are normally in the wings and center fuselage.

Some operators and authorities (not all) restrict starting and/or stopping the APU while fueling.

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    $\begingroup$ Not all aircraft have APU in the tail cone. In particular the three-engined jets (B727, DC-10/MD-11) have it somewhere around the wing box as they don't have much space left in their tail cones. $\endgroup$ – Jan Hudec Feb 22 '20 at 22:27

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