Got significant air in the Warthog at midnight in mission 2.
This can only be unlocked in mission 2 and is time-specific. Take note that you should be the driver of the Warthog and you have to make the jump as soon as your console\'s time has reached 12AM. If its past 12AM (which is most likely the case), you have to manually set your console\'s time around 23:50 then load Mission 2.
There are no enemies at the start of the stage so just run until you find a warthog. Drive your warthog and follow the path until you reach a destroyed metal tunnel. Wait for 12AM to strike then rush through it until you make a jump. Doing it correctly should unlock the achievement. If not, repeat the whole process again. This video is from our unofficial game guide. You can view the full guide here
Added 20th Nov 2012, ID #16358