Bring Trevor to the airstrip and ride the biplane. Keep following the cargo plane, making sure you don't go higher than 50 feet off the ground. When the airspace clears, stay directly behind the rear door of the cargo plane, then force your way in once the door opened up.
Continue ahead the leftside area of the plane while going from cover to cover. Watch out for the jeeps as you make your way through. Kill the pilot located on the upper deck. Take the plane's controls and fly to the central area of the city and climb up as much as you can. Don't mind the fighters until they shoot at you. Go to the cargo bay, jump out, then finally pull out the ripcord.
