There is a flight simulator in the Nellis air force base, in the mess hall/hanger, but there are no codes to be found. Is there a code?

No,it cannot be used,under any circumstances.
(besides mods)

No, it can't be used.
There's speech checks for asking for a code for it, though.

If it'd been there, they'd have had to make a pilot mini game.

Or they could have had a textbox describing the experience, and reward some XP or a perk for it.