Tip: When you start the game set the "Machine" to "286 (8MHz)" to have the plane move through the sky at a realistic speed.

At the time I'm writing this the default setting was "286 (16MHz)" and the game ran way too fast, as if time compression was on.

I'm pretty sure 8MHz has it running at the realistic speed or very close to it. At 300 knots and 20 degrees nose up it should take you ~6 seconds to gain 1000 feet of altitude, and that's what I counted (just counting in my head) when I timed it in-game.