Aerial refueling, also called air refueling, in-flight refueling (IFR), air-to-air refueling (AAR) or tanking, is the process of transferring fuel from one aircraft (the tanker) to another (the receiver) during flight. Aerial refueling allows aircraft engines to receive fuel while in flight and today it is common for many large air forces. It is the equivalent of refueling your car by connecting it to a tanker truck while driving down the highway at high speed. In the rigid flying boom system of aerial refueling, the pilot of the receiving aircraft flies behind and below the tanker for refueling. To complete an aerial refueling, the tanker and receiver aircraft rendezvous, flying in formation. The receiver moves to a position behind the tanker, within safe limits of travel for the boom, aided by director lights or directions radioed by the boom operator. Once in position, the operator extends the boom to make contact with the receiver aircraft. Once in contact, fuel is pumped through the boom into the receiver aircraft. While in contact, the receiver pilot must continue to fly within the "air refueling envelope," the area in which contact with the boom is safe. Moving outside of this envelope can damage the boom or lead to mid-air collision. When the desired amount of fuel been transferred, the two aircraft disconnect, and the receiver aircraft departs the formation. While not in use, the boom is stowed flush with the bottom of the tanker's fuselage to minimize drag. Despite all the technological advances, commercial aircraft designers never adopted aerial refueling. They preferred to build aircraft with large internal fuel tanks because this was cheaper than operating a dedicated fleet of aircraft that simply served as flying gas tanks. Aerial refueling is now exclusively a military operation.