In-flight refueling was one way to set airborne endurance records. The Atlantic-Fokker X-2A “Question Mark” stayed aloft for seven days in January of 1929, refueling 42 times in the air. Because radio communication was unreliable at this time, the pilots in the two planes communicated by notes dropped to the ground and by hand signals, flashlight signals, ground panels, and messages written on blackboards carried in the planes.
A Boeing KB-29P refueling in flight using the flying boom system. This is the most common method for in-flight refueling.
In the rigid flying boom system of aerial refueling, the pilot of the receiving aircraft flies behind and below the tanker for refueling.
A B-47 Stratojet taking on a load of fuel. The B-47 Stratojet was the first jet-to-jet refueling tanker.
A Boeing B-52H being refueled by a Boeing KC-135A.
The C-130 Hercules was originally designed during the 1950s as an assault transport but was adapted for a variety of missions including mid-air refueling of helicopters.
Aerial refueling expanded helicopter rescue capability. Here a U.S. Air Force CH-3C simulates refueling from a Marine Corps KC-130F tanker during a test on December 17, 1966.
Two Northrop B-2As and a McDonnell Douglas KC-10A Extender.
Aerial refueling allows aircraft engines to receive fuel while in flight and today 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 1917, a pilot in the Imperial Russian Navy, Alexander P. de Seversky, proposed increasing the range of combat aircraft by refueling them in flight. De Seversky soon emigrated to the United States and became an engineer in the War Department. He applied for and received the first patent for air-to-air refueling in 1921.
The first actual transfer of fuel from one aircraft to another was little more than a stunt. On November 12, 1921, wingwalker Wesley May climbed from a Lincoln Standard to a Curtiss JN-4 airplane with a can of fuel strapped to his back. When he reached the JN-4, he poured the fuel into its gas tank. Needless to say, this was not the most practical way of refueling an airplane in flight.
In 1923, the U.S. Army undertook tests at Rockwell Field, San Diego, California, to test a more practical way to lower a hose from one airplane to refuel another in flight. In its tests, a DH-4B biplane outfitted as a tanker and equipped with a 50-foot (15-meter) length of hose and a quick-acting shutoff valve would fly above the receiver and lower the hose. The person in the rear seat of the receiver aircraft would grab the hose and connect it to the aircraft. If the hose became detached, the valve would immediately cut off the flow, preventing it from spraying fuel over the receiving aircraft and its pilot.
The first flight was made on April 20, 1923. The aircraft remained attached for 40 minutes but intentionally passed no fuel. The equipment was tested over the next several months with numerous fuel transfers. On June 27, the pilots made an attempt on the aircraft flying endurance record. By August 27, using this technique, one of the DH-4Bs established 14 world records with a flight lasting more than 37 hours.
This achievement prompted many private pilots to attempt aerial (or in-flight) refueling, primarily to establish long duration flying records. By June 1930, the record surpassed 553 hours in flight (requiring 223 refueling contacts). In July, the record was 647.5 hours in the Curtiss Robin monoplane Greater St. Louis