E-3 AEW (Airborne Early Warning) aircraft require aerial refuel to extend their on-station time.
The following video shows a U.S. KC-135 Stratotanker with the 340th Expeditionary Air Refueling Squadron, refueling a RAF E-3D Sentry over Iraq.
Both aircraft were involved in missions in support of Operation Inherent Resolve and the fight against the terrorist group ISIL.
Even though E-3s perform AAR quite regularly, the size of both the refueler and the receiver make the operation a bit more difficult, at least according to Sentry pilots.
In that case, the AWACS came within a few meters from the KC-135 and performed a quite aggressive evasive maneuver pulling several negative g-forces to gain quick separation from the refueler.