Once you complete enough of Trevors story you will gain the ability to purchase properties, and McKenzie Field Hangar was the property that I was outside when this happened so I suspect you will be in a similar position! If not, McKenzie Field Hangar will be marked on your map as a Property for purchase so just set a waypoint and head there.
You will need $150,000 to buy the field and once you have done so, there's a cutscene where you meet the Control Tower operator Carlos.
Carlos will now inform you that there is an ongoing arms trade 'war' going on, so you will be required to either use the dune buggy provided to carry out weapons drop collections (travel to a set location, sometimes racing against rival gunrunners and sometimes staying offroad to avoid the police) or using the plane provided to either drop weapons or bombs on customers (or rivals). Once you have completed 5 of each type of mission, the achievement will unlock.
Each time you complete a plane or dunebuggy mission, you may need to drive away from the area for a short while so the next job(s) can 'spawn' but it's possible to do 5 of each without having to do any side missions or main missions in between.
*EDIT* Capt Stanners has pointed out in the comments that, to save time, instead of driving away to get the jobs to respawn, you can simply QuickSave from your in-game phone, the reload the save and the jobs will have respawned. Not tried it myself, but once saved you won't have lost any progress so nothing to lose! :)
*EDIT pt.2* I mistakenly mentioned that the 5 flights and 5 dune-buggy missions count towards your 100% stat in the comments - as pointed out by a few, that's NOT the case, but Rockstar Social Club site will actually track how many of the 5 required that you've done.
Secondly, in order to keep this solution spoiler-free, I won't go into too much detail but Kawwaah Kawwaah has pointed out in the comments that the last mission of the single-player campaign may affect your ability to complete this achievement so best complete the requirements for this achievement BEFORE you begin the big (UD) final heist. Just sayin'.