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Turbojets idle very inefficiently, so jetliners can save a lot of fuel if they can use an electric drive wheel powered by the APU to push back from the terminal and to taxi out to the runway, only starting their turbofans once they get close to the end of the taxiway prior to takeoff. This is especially true if the airport if busy and they are having to sit in line on the taxiway with a bunch of other jetliners waiting for their turn to takeoff.
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