Air time is determined when your tires are not touching the ground. Turn off "Vehicle Roll Helper" in the "Gameplay Settings" menu. Then, drive over rocks or other terrain before driving off a cliff to get some air. Your vehicle should land on its roof. Your vehicle will flip back over after a short amount of time, but it may be long enough to get the "Maximum Air" achievement. Go to the volcano near the Proving Grounds camp in Cadavanaugh and use it as a ramp. You will receive some damage -- so attempt it with a fully repaired car. Some nitros will also be needed to climb the slope.