Everytime that I start a flight, the engines are automatically started.
You don't really need to create a new flight every time you want to fly. When you're done with a flight after taxing to the gate, shut everything down and save the flight. Next time you want to fly, load the flight where you last left off

That's what I do, and I rarely have to create a new flight. I just continue where I've left off and do this for weeks on end.