Instructions
Go to the helicopter on the "Wasteland" map. First jump on the machine gun at the nose of the chopper. From there to you can jump to the main fuselage and then to the rotors. You now have a superior vantage point.
Equip the Lightweight perk to get on the helicopter located on the "Highrise" map.
Climb the building to the roof.
Stand in front of the helicopter, aligning yourself with the blue landing light in the corner.
Sprint and jump so that you land on the barrels to the right of the helicopter. From there, it is an easy hop to the fuselage and rotor.