Midnight Launch 20
Got significant air in the Warthog at midnight in mission 2.
Campaign Mission 2 'Requiem' specific. Can be obtained on Easy+.
Use 'Mission Select' after initial Campaign playthrough if missed.
Be the driver/passenger/gunner in a Warthog and achieve significant air when your console time is at 12:00 am.
Only Driver gets it, just did this with 3 people and only I got the achievement, gunner and the passenger did not.