Aerial refuelling, also referred to as air refuelling, in-flight refuelling, air-to-air refuelling (AAR), and tanking, is the process of transferring aviation fuel from one military aircraft (the tanker) to another (the receiver) during flight.
Some of the earliest experiments in aerial refuelling took place in the 1920’s; two slow-flying aircraft flew in formation, with a hose run down from a hand-held fuel tank on one aircraft and placed into the usual fuel filler of the other. The first mid-air refuelling, based on the development of Alexander P. de Seversky, between two planes occurred on June 27, 1923, between two Airco DH-4B biplanes of the United States Army Air Service. An endurance record was set by three DH-4Bs (a receiver and two tankers) on August 27–28, 1923, in which the receiver aircraft remained aloft for more than 37 hours using nine mid-air refuelling to transfer 2,600 L of aviation gasoline and 140L of engine oil. The same crews demonstrated the utility of the technique on October 25, 1923, when a DH-4 flew from Sumas, Washington, on the Canada–United States border, to Tijuana, Mexico, landing in San Diego, using mid-air refuelling at Eugene, Oregon and Sacramento, California.
Similar trial demonstrations of mid-air refuelling technique took place at the Royal Aircraft Establishment in England and by the Armée de l'Air in France in the same year, but these early experiments were not yet regarded as a practical proposition and were generally dismissed as stunts.
As the 1920s progressed, greater numbers of aviation enthusiasts vied to set new aerial long-distance records, using inflight air refuelling. One such enthusiast, who would revolutionise aerial refuelling was Sir Alan Cobham, member of the Royal Flying Corps in World War I, and a pioneer of long-distance aviation. During the 1920’s, he made long-distance flights to places as far afield as Africa and Australia and he began experimenting with the possibilities of in-flight refuelling to extend the range of flight.
Cobham was one of the founding directors of Airspeed Limited, an aircraft manufacturing company which went on to produce a specially adapted Airspeed Courier that Cobham used for his early experiments with in-flight refuelling. This craft was eventually modified by Airspeed to Cobham's specification, for a non-stop flight from London to India, using in-flight refuelling to extend the plane's flight duration.
Meanwhile, in 1929, a group of U.S. Army Air Corps fliers, led by then Major Carl Spaatz, set an endurance record of over 150 hours with the Question Mark over Los Angeles. Between June 11 and July 4, 1930, the brothers John, Kenneth, Albert, and Walter Hunter set a new record of 553 hours 40 minutes over Chicago using two Stinson SM-1 Detroiters as refueler and receiver. Aerial refuelling remained a very dangerous process until 1935, when brothers Fred and Al Key demonstrated a spill-free refuelling nozzle, designed by A. D. Hunter. They exceeded the Hunters' record by nearly 100 hours in a Curtiss Robin monoplane, staying aloft for more than 27 days.
The US was mainly concerned about transatlantic flights for faster postal service between Europe and America. In 1931 W. Irving Glover, the second assistant postmaster, wrote an extensive article for Popular Mechanics concerning the challenges and the need for such a regular service. In his article he even mentioned the use of aerial refuelling after take-off as a possible solution.
At Le Bourget Airport near Paris, the Aéro-Club de France and the 34th Aviation Regiment of the French Air Force were able to demonstrate passing fuel between machines at the annual aviation fete at Vincennes in 1928. The UK's Royal Aircraft Establishment was also running mid-air refuelling trials, with the aim to use this technique to extend the range of the long-distance flying boats that serviced the British Empire. By 1931 they had demonstrated refuelling between two Vickers Virginias, with fuel flow controlled by an automatic valve on the hose which would cut off if contact was lost.
Royal Air Force officer Richard Atcherley had observed the dangerous aerial-refuelling techniques in use at barnstorming events in the US and determined to create a workable system. While posted to the Middle East he developed and patented his 'crossover' system in 1934, in which the tanker trailed a large hooked line that would reel in a similar dropped line from the receiver, allowing the refuelling to commence. In 1934, Cobham sold off the airline Cobham Air Routes Ltd to Olley Air Service and turned to the development of in-flight refuelling, founding the company Flight Refuelling Ltd. Atcherly's system was bought up by Cobham's company, and with some refinement and continuous improvement through the late '30s, it became the first practical refuelling system.
In 1934, Cobham had founded Flight Refuelling Ltd and by 1938 had used FRL's looped-hose system to refuel aircraft as large as the Short Empire flying boat Cambria from an Armstrong Whitworth AW.23. Handley Page Harrows were used in the 1939 trials to perform aerial refuelling of the Empire flying boats for regular transatlantic crossings. From August 5 to October 1, 1939, sixteen crossings of the Atlantic were made by Empire flying boats, with fifteen crossings using FRL's aerial refuelling system. After the sixteen crossings further trials were suspended due to the outbreak of World War II.
During the closing months of World War II, it had been intended that Tiger Force's Lancaster and Lincoln bombers would be in-flight refuelled by converted Halifax tanker aircraft, fitted with the FRL's looped-hose units, in operations against the Japanese homelands, but the war ended before the aircraft could be deployed.
After the war ended, the USAF bought a small number of FRL looped-hose units and fitted a number of B-29s as tankers to refuel specially equipped B-29s and later B-50s. The USAF made only one major change between the system used by the RAF. The USAF version had auto-coupling of the refuelling nozzle, where the leader line with the refuelling hose is pulled to the receiver aircraft and a refuelling receptacle on the belly of the aircraft, allowing high-altitude air-to-air refuelling and doing away with the aircraft having to fly to a lower altitude to be depressurised so a crew member could manually do the coupling.
From February 26 to March 3, 1949, an American B-50 Superfortress Lucky Lady II of the 43rd Bomb Wing flew non-stop around the world in 94 hours and 1 minute, a feat made possible by four aerial refuelling from four pairs of KB-29M tankers of the 43d ARS. Before the mission, crews of the 43d had experienced only a single operational air refuelling contact. The flight started and ended at Carswell Air Force Base in Fort Worth, Texas with the refuelling accomplished over the Azores, West Africa, the Pacific Ocean near Guam, and between Hawaii and the West Coast.
This first non-stop circumnavigation of the globe proved that, because of aerial refuelling, vast distances and geographical barriers were no longer an obstacle to military air power. In 1949, four additional ARS units were organized by the USAF and both the 43d and 509th ARS became fully operational.
Cobham's company FRL soon realised that their looped-hose system left a lot to be desired and began work on an improved system that is now commonly called the probe-and-drogue air-to-air refuelling system and today is one of the two systems chosen by air forces for air-to-air refuelling, the other being the flying-boom system. In post-war trials the RAF used a modified Lancaster tanker employing the much improved probe-and-drogue system, with a modified Gloster Meteor F.3 jet fighter, serial EE397, fitted with a nose-mounted probe. On 7 August 1949, the Meteor flown by FRL test pilot Pat Hornidge took off from Tarrant Rushton and remained airborne for 12 hours and 3 minutes, receiving 10,690 L of fuel in ten refuelling’s from a Lancaster tanker. Hornidge flew an overall distance of 5,800 km, achieving a new jet endurance record.
In January 1948, General Carl Spaatz, then the first Chief of Staff of the new United States Air Force, made aerial refuelling a top priority of the service. In March 1948, the USAF purchased two sets of FRL's looped-hose in-flight refuelling equipment, which had been in practical use with British Overseas Airways Corporation (BOAC) since 1946, and manufacturing rights to the system. FRL also provided a year of technical assistance. The sets were immediately installed in two Boeing B-29 Superfortresses, with plans to equip 80 B-29s.
Flight testing began in May 1948 at Wright-Patterson Air Force Base, Ohio, and was so successful that in June orders went out to equip all new B-50s and subsequent bombers with receiving equipment. Two dedicated air refuelling units were formed on June 30, 1948: the 43d Air Refuelling Squadron at Davis-Monthan Air Force Base, Arizona, and the 509th Air Refuelling Squadron at Walker Air Force Base, New Mexico. The first ARS aircraft used FRL's looped-hose refuelling system but testing with a boom system followed quickly in the autumn of 1948.
The first use of aerial refuelling in combat took place during the Korean War, involving F-84 fighter-bombers flying missions from Japanese airfields, due to Chinese-North Korean forces overrunning many of the bases for jet aircraft in South Korea, refuelling from converted B-29s using the drogue-and-probe in-flight refuelling system with the probe located in one of the F-84's wing-tip fuel tanks.
The flying boom is a rigid, telescoping tube with movable flight control surfaces that a boom operator on the tanker aircraft extends and inserts into a receptacle on the receiving aircraft. All boom-equipped tankers have a single boom and can refuel one aircraft at a time with this mechanism.
In late 1948, General Curtis LeMay, commander of the Strategic Air Command (SAC), asked Boeing to develop a refuelling system that could transfer fuel at a higher rate than had been possible with earlier systems using flexible hoses, resulting in the flying boom system. The B-29 was the first to employ the boom, and between 1950 and 1951, 116 original B-29s, designated KB-29Ps, were converted at the Boeing plant at Renton, Washington. Boeing went on to develop the world's first production aerial tanker, the KC-97 Stratofreighter, a piston-engine Boeing Stratocruiser with a Boeing-developed flying boom and extra jet fuel tanks feeding the boom. The Stratocruiser airliner itself was developed from the B-29 bomber after World War II. In the KC-97, the mixed gasoline/kerosene fuel system was clearly not desirable and it was obvious that a jet-powered tanker aircraft would be the next development, having a single type of fuel for both its own engines and for passing to receiver aircraft. The 230 mph cruise speed of the slower, piston-engine KC-97 was also a serious issue, as using it as an aerial tanker forced the newer jet-powered military aircraft to slow down to mate with the tanker's boom, a highly serious issue with the newer supersonic aircraft coming into service at that time, which could force such receiving aircraft in some situations to slow down enough to approach their stall speed during the approach to the tanker. It was no surprise that, after the KC-97, Boeing began receiving contracts from the USAF to build jet tankers based on the Boeing 367-80 airframe. The result was the Boeing KC-135 Stratotanker, of which 732 were built.
The flying boom is attached to the rear of the tanker aircraft. The attachment is gimballed, allowing the boom to move with the receiver aircraft. The boom contains a rigid pipe to transfer fuel. The fuel pipe ends in a nozzle with a flexible ball joint. The nozzle mates to the "receptacle" in the receiver aircraft during fuel transfer. A poppet valve in the end of the nozzle prevents fuel from exiting the tube until the nozzle properly mates with the receiver's refuelling receptacle. Once properly mated, toggles in the receptacle engage the nozzle, holding it locked during fuel transfer.
The "flying" boom is so named because flight control surfaces, small movable aerofoils that are often in a V-tail configuration, are used to move the boom by creating aerodynamic forces. They are actuated hydraulically and controlled by the boom operator using a control stick. The boom operator also telescopes the boom to make the connection with the receiver's receptacle.
To complete an aerial refuelling, 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 contact 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 refuelling 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, for example the 1966 Palomares B-52 crash. If the receiving aircraft approaches the outer limits of the envelope, the boom operator will command the receiver pilot to correct his position and disconnect the boom if necessary.
When the desired amount of fuel has been transferred, the two aircraft disconnect and the receiver aircraft departs the formation. When not in use, the boom is stored flush with the bottom of the tanker's fuselage to minimize drag.
US Air Force fixed-wing aircraft use the flying boom system. Typically, countries operating F-16 or F-15 variants have had a need for boom equipped tankers. Therefore, in addition to the USAF, the boom system is used by Australia (KC-30A), the Netherlands (KDC-10), Israel (modified Boeing 707), Turkey (surplus US KC-135Rs), and Iran (Boeing 747).
Higher fuel flow rates, up to 1,000 US gallons 2,900 kg per minute for the KC-135 tanker, can be achieved with the large diameter of the pipe in the flying boom, requiring less time to complete refuelling operations than probe-and-drogue systems.
Fighter aircraft cannot accept fuel at the boom's maximum flow rate, requiring a reduction in refuelling pressure when servicing these aircraft, reducing the flying boom's advantage over the drogue system when refuelling fighter aircraft.
The probe-and-drogue refuelling method employs a flexible hose that trails from the tanker aircraft. The drogue (or para-drogue), sometimes called a basket, is a fitting resembling a shuttlecock, attached at its narrow end with a valve to a flexible hose. The drogue stabilizes the hose in flight and provides a funnel to aid insertion of the receiver aircraft probe into the hose. The hose connects to a Hose Drum Unit (HDU). When not in use, the hose/drogue is reeled completely into the HDU. The receiver has a probe, which is a rigid, protruding or pivoted retractable arm placed on the aircraft's nose or fuselage to make the connection. Most modern versions of the probe are usually designed to be retractable, and are retracted when not in use, particularly on high speed aircraft.
At the end of the probe is a valve that is closed until it mates with the drogue's forward internal receptacle, after which it opens and allows fuel to pass from tanker to receiver. The valves in the probe and drogue that are most commonly used are to a NATO standard and were originally developed by the company Flight Refuelling Limited in the UK and deployed in the late 1940s and 1950s. This standardization allows drogue-equipped tanker aircraft from many nations the ability to refuel probe-equipped aircraft from other nations. The NATO standard probe system incorporates shear rivets that attach the refuelling valve to the end of the probe. This is so that if a large side or vertical load develops while in contact with the drogue, the rivets shear and the fuel valve breaks off, rather than the probe or receiver aircraft suffering structural damage. A so-called "broken probe" may happen if poor flying technique is used by the receiver pilot, or in turbulence. Sometimes the valve is retained in the tanker drogue and prevents further refuelling from that drogue until removed during ground maintenance.
A "buddy store" or "buddy pod" is an external pod loaded on an aircraft hardpoint that contains a hose and drogue system. Buddy stores allow fighter / bomber aircraft to be reconfigured for "buddy tanking" other aircraft. This allows an air combat force without dedicated/specialized tanker support (for instance, a carrier air wing) to extend the range of its strike aircraft. In other cases, using the buddy store method allows a carrier-based aircraft to take-off with a heavier than usual load, the aircraft then being topped-up with fuel from a HDU-equipped "buddy" tanker, a method previously used by the Royal Navy in operating its Supermarine Scimitar, de Havilland Sea Vixen and Blackburn Buccaneers, in the Buccaneer's case using a bomb-bay-mounted tank and HDU.
The tanker aircraft flies straight and level and extends the hose/drogue which is allowed to trail out behind and below the tanker under normal aerodynamic forces. The pilot of the receiver aircraft extends his probe and uses normal flight controls to "fly" the refuelling probe directly into the basket. This requires a closure rate of approximately two knots (walking speed) in order to establish solid probe/drogue coupling and push the hose several feet into the HDU. Too little closure will cause an incomplete connection and no fuel flow. Too much closure is dangerous because it can trigger a strong transverse oscillation in the hose, severing the probe tip. Another significant danger is that the drogue may hit the recipient aircraft and damage it—instances have occurred in which the drogue has shattered the canopy of a fighter aircraft, causing great danger to its pilot.
The optimal approach is from behind and below the drogue. Because the drogue is relatively light (typically soft canvas webbing) and subject to aerodynamic forces, it can be pushed around by the bow wave of approaching aircraft, exacerbating engagement even in smooth air. After initial contact, the hose and drogue are pushed forward by the receiver a certain distance and the hose is reeled slowly back onto its drum in the HDU. This opens the tanker's main refuelling valve allowing fuel to flow to the drogue under the appropriate pressure (assuming the tanker crew has energized the pump). Tension on the hose is aerodynamically 'balanced' by a motor in the HDU so that as the receiver aircraft moves fore and aft, the hose retracts and extends, thus preventing bends in the hose that would cause undue side loads on the probe. Fuel flow is typically indicated by illumination of a green light near the HDU. If the hose is pushed in too far or not far enough, a cut-off switch will inhibit fuel flow, which is typically accompanied by an amber light. Disengagement is commanded by the tanker pilot with a red light.
US military helicopters, and all US Navy and Marine Corps aircraft (except the Boeing E-6 Mercury and Boeing P-8 Poseidon) refuel using the "hose-and-drogue." Western-European tactical aircraft manufacturers typically design with the probe-and-drogue method. The Soviet Union also reverse engineered the NATO hose and drogue system, which is called UPAZ, so all Russian aircraft are also equipped with probe and drogue. The Chinese PLAF has a fleet of Xian H-6 bombers modified for aerial refuelling as well as forthcoming Russian Ilyushin Il-78 aerial refuelling tankers.
USAF KC-135 and French Air Force KC-135FR refuelling-boom equipped tankers can be field converted to a probe-and-drogue system using a special adapter unit. In this configuration, the tanker retains its articulated boom, but has a hose/drogue at the end of it instead of the usual nozzle. The tanker boom operator holds the boom in a static position, while the receiver aircraft then flies the probe into the basket. Unlike the soft canvas basket used in most drogue systems, the adapter units use a steel basket, grimly known as the "iron maiden" by naval aviators because of its unforgiving nature. Soft drogues can be contacted slightly off centre, wherein the probe is guided into the hose receptacle by the canvas drogue. The metal drogue, when contacted even slightly off centre, will pivot out of place, potentially "slapping" the aircraft's fuselage and causing damage.
The other major difference with this system is that when contacted, the hose does not "retract" into an HDU. Instead, the hose bends depending on how far it is pushed toward the boom. If it is pushed too far, it can loop around the probe or nose of the aircraft, damage the windscreen, or cause contact with the rigid boom. If not pushed far enough, the probe will disengage, halting fuelling. Because of a much smaller position keeping tolerance, staying properly connected to a KC-135 adapter unit is considerably more difficult than staying in a traditional hose/drogue configuration. When fuelling is complete, the receiver carefully backs off until the probe refuelling valve disconnects from the valve in the basket. Off centre disengagements, like engagements, can cause the drogue to "prang" the probe and/or strike the aircraft's fuselage.
Some tankers have both a boom and one or more complete hose-and-drogue systems. The USAF KC-10 has both a flying boom and also a separate hose and drogue system manufactured by Cobham plc. Both are on the aircraft centreline at the tail of the aircraft, so only one system can be used at once. However, such a system allows all types of probe- and receptacle-equipped aircraft to be refuelled in a single mission, without landing to install an adapter. Many KC-135s and some KC-10s are also equipped with dual under-wing hose-and-drogue attachments known as Multi-point Refuelling System (MPRSs) or Wing Air Refuelling Pods (WARPs), respectively.