The Airfield Drops event begins with a cargo plane circling the airfield. Eventually it will head down to the airfield, where it will drop supply crates on the runway. Make sure you eliminate the airfield guards before trying to get those supply crates or you could have a bad day.
This event works hand-in-hand with the Airplane Crash event. While heading to the airfield, or while circling it, the cargo plane might crash.