I don't know about FSX,....
.... but in FS 2004, if the previous aircraft's engines were shut down by pulling the Mixture to weak/cut-off in Piston engines, or closing the Fuel valve in Turbo Props, Jets, etc,
and left there, pressing Ctrl + Shift + F4 for the new aircraft, will put the fuel in the maximum state to enable the engines to be started.
(Ctrl + Shift + F1 is fuel cut-off).
Its easy to forget how you shut down the previous aircraft!...

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Paul....

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Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.