- Aerial refueling
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.
The procedure allows the receiving aircraft to remain airborne longer, extending its range or loiter time on station. A series of air refuelings can give range limited only by crew fatigue and engineering factors such as engine oil consumption. Because the receiver aircraft can be topped up with extra fuel in the air, air refueling can allow a take-off with a greater payload which could be weapons, cargo or personnel: the maximum take-off weight is maintained by carrying less fuel and topping up once airborne. Alternatively, a shorter take-off roll can be achieved because take-off can be at a lighter weight before refueling once airborne. Aerial refueling has also been considered as a means to reduce fuel consumption on long distance flights greater than 3000 nautical miles. Potential fuel savings in the range of 35-40% have been estimated for long haul flights (including the fuel used during the tanker missions).
The two main refueling systems are probe-and-drogue, which is simpler to adapt to existing aircraft, and the flying boom, which offers greater fuel transfer capacity, but requires a dedicated operator station.
Usually, the aircraft providing the fuel is specially designed for the task, although refueling pods can be fitted to existing aircraft designs if the "probe-and-drogue" system is to be used (see later). The cost of the refueling equipment on both tanker and receiver aircraft and the specialized aircraft handling of the aircraft to be refueled (very close "line astern" formation flying) has resulted in the activity only being used in military operations. There is no known regular civilian in-flight refueling activity. Originally employed shortly before World War Two on a very limited scale to extend the range of British civilian trans-Atlantic flying boats, and then after World War Two on a large scale to extend the range of strategic bombers, aerial refueling since the Vietnam War has been extensively used in large-scale military operations for many different military aircraft operations. For instance, in the Gulf War and the Iraqi invasion of Kuwait and the Iraq War, all coalition air sorties were air-refueled except for a few short-range ground attack sorties in the Kuwait area.
History and development
Some of the earliest experiments in aerial refueling took place in the 1920s; 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 refueling 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 airplane remained aloft for more than 37 hours using nine mid-air refuelings to transfer 687 gallons of aviation gasoline and 38 gallons 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 Canadian border to Tijuana, Mexico, landing in San Diego, using mid-air refuelings at Eugene, Oregon, and Sacramento, California.
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 refueling remained a very dangerous process until 1935 when brothers Fred and Al Key demonstrated a spill-free refueling 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 Trans-Atlantic flight for the 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 refueling after take off as a possible solution.
There were parallel experiments conducted in Europe; at Le Bourget 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 trialing refueling-in-mid-air, 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 refueling between two Vickers Virginias, with fuel flow controlled by an automatic valve on the hose which would cut off if contact was lost.
First Practical Aerial Refueling Systems
The aviation pioneer Alan Cobham bought a patent from David Nicolson and John Lord for £480 each and then started development of the Grappled-line looped-hose air to air refueling system, which while complex in operation, was the world's first practical air-to-air refueling system and gave public demonstrations of the system. In the system the receiver aircraft, at one time an Airspeed Courier, trailed a steel cable which was then grappled by a line shot from the tanker, a Handley Page Type W10. The line was then drawn back into the tanker where the receiver's cable was connected to the refueling hose. The receiver could then haul back in its cable bringing the hose to it. Once the hose was connected, the tanker climbed sufficiently above the receiver aircraft to allow the fuel to flow under gravity. (see reference #8 Gas Station in the Sky for detailed drawing of this type of operation.)
While today aerial refueling is used exclusively by military aircraft, when Cobham was developing his system, he saw the need as purely for long range trans-ocean commercial aircraft flights.
In 1934 Cobham founded Flight Refuelling Ltd. (FRL), and by 1938 had used the 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 aerial refuel 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 refueling system. After the sixteen crossings further trials were suspended due to the war.
During the closing months of World War II it had been intended that Tiger Force's Lancaster and Lincoln bombers would be in-flight refueled 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. This was the air-to-air refueling system that the Lucky Lady II used to make its famous first non-stop around the world flight in 1949.
FRL realized 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 refueling system and today is one of the two systems chosen by air forces for air-to-air refueling, 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 Meteor III, 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 2,352 gallons of fuel in ten refuelings from a Lancaster tanker. Hornidge flew an overall distance of 3,600 miles (5,800 km), achieving a new jet endurance record. FRL still exists as part of Cobham plc.
Modern specialized tanker aircraft have equipment specially designed for the task of offloading fuel to the receiver aircraft, based on drogue and probe, even at the higher speeds modern jet aircraft typically need to remain airborne.
First operational use
In January 1948, General Carl Spaatz, then the first Chief of Staff of the new United States Air Force made aerial refueling a top priority of the service. In March 1948 USAF purchased two sets of FRL's looped-hose in-flight refueling 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 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-50's and subsequent bombers with receiving equipment. Two dedicated Air Refueling units were formed on June 30, 1948: the 43d Air Refueling Squadron at Davis-Monthan Air Force Base, Arizona, and the 509th Air Refueling Squadron at Walker Air Force Base, New Mexico. The first ARS aircraft used FRL's looped-hose refueling system, but testing with a boom system followed quickly in the autumn of 1948. In 1949 from February 26 to March 3 an American B-50 Superfortress Lucky Lady II of the 43rd Bomb Wing flew non-stop around the World in 94 hours, 1 min., a feat made possible by three aerial refuelings 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 refueling contact. The flight started and ended at Carswell Air Force Base in Fort Worth, Texas with the refuelings accomplished over 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 refueling, 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.
The first use of aerial refueling in combat took place during the Korean War, involving F-84 fighter-bombers flying missions from Japanese airfields (i.e. due to Chinese-North Korean forces over-running many of the bases for jet aircraft in South Korea) refueling from converted B-29s using the drogue-and-probe in-flight refueling system with the probe located in one of the F-84's wing tip fuel tanks.
Two different methods are used to connect tanker to receiver: the flying boom system (sometimes called boom and receptacle), and the probe-and-drogue system. The less popular wing-to-wing system is no longer used.
The flying boom is a rigid, telescoping tube with movable flight control surfaces that an operator on the tanker aircraft extends and inserts into a receptacle on the receiving aircraft. All boom-equipped tankers (e.g. KC-135 Stratotanker, KC-10 Extender), have a single boom, and can refuel one aircraft at a time with this mechanism.
In the late 1940s, General Curtis LeMay, commander of the Strategic Air Command (SAC), asked Boeing to develop a refueling 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 Stratotanker, a piston-engined Boeing Stratocruiser (USAF designation C-97 Stratofreighter) with a Boeing-developed flying boom and extra kerosene (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. 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 (Dash-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 refueling 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 airfoils, are used to move the boom by creating aerodynamic forces. They are actuated hydraulically and controlled by the system operator using a control stick. The operator also telescopes the boom to make the connection with the receiver's receptacle.
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 (see 1966 Palomares B-52 crash). Approaching the 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 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.
Systems in service
USAF fixed-wing aircraft use the flying boom system. In addition to the US Air Force, the boom system is used by the Netherlands (KDC-10), Israel (modified Boeing 707), Turkey (ex-USAF KC-135R), and Iran (Boeing 747). New tankers are under development; see KC-X.
- Higher fuel flow rates (up to 1000 US gallons/6,500 lb 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 refueling operations than probe-and-drogue systems. Fighter aircraft cannot accept fuel at the boom’s maximum flow rate, requiring a reduction in refueling pressure when servicing these aircraft, reducing (but not eliminating) the flying boom's advantage over the drogue system when refueling fighter aircraft.
- The boom method eliminates the requirement for the receiver aircraft pilot to precisely fly a probe into a drogue, something that is easily performed by fighter-sized aircraft, but is more difficult for larger and less-maneuverable aircraft.
- A tanker with a flying boom can be equipped with an adapter for probe-equipped aircraft.
- The cost to train and employ the boom operator.
- Complexity of tanker design.
- Only one receiver aircraft can refuel at a time.
- Cannot be used to refuel most helicopters.
- Requires modification to support USN/USMC or NATO tactical jet aircraft.
This refueling 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 windsock or 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 arm placed on the aircraft's nose or fuselage. This probe is often 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, 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 refueling valve to the end of the probe. This is so that if a large side-load or up-and-down 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" (actually a broken fuel valve, as described above) 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 refueling 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 HDU. 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 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 (if required) and uses normal flight controls to "fly" the refueling probe directly into the basket. This requires a closure rate of approximately two knots (walking speed) in order to establish solid probe/drogue couple and pushing the hose several feet into the HDU. Too little closure will cause an incomplete connection and no fuel flow (or occasionally leaking fuel). 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 (not level with) 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 is pushed forward by the receiver a certain distance (typically, a few feet), and the hose is reeled slowly back onto its drum in the HDU. This opens the tanker's main refueling 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 cutoff switch will inhibit fuel flow, which is typically accompanied by an amber light. Disengagement is commanded by the tanker pilot with a red light.
Systems in service
USAF helicopters, and all US Navy and Marine Corps aircraft refuel using the “hose-and-drogue.” NATO countries and other western allies also refuel with the probe-and-drogue method. The USSR also reverse engineered the NATO hose and drogue system, which is called UPAZ , so all Russian aircraft are also equipped with probe and drogue.
- Simpler/cheaper tanker design.
- The probe-and-drogue method allows aircraft not originally designed as tankers to be converted by attaching a refueling pod.
- Tankers can be equipped with multipoint hose-and-drogue systems allowing two (or more) aircraft to refuel simultaneously from the same tanker, reducing time spent by as much as 75% for a four aircraft strike package. Multiple refueling points also offers redundancy over the single boom system.
- Can be used to refuel properly equipped helicopters, such as the MH-53E Sea Dragon.
- No boom operator is needed for the refueling as the drogue can be operated by the pilot of the tanker.
- The lower flow rates (1,500–4,500 lbs/min)  available from the lower pressure and limited diameter of the hose used in the probe-and-drogue system result in longer refueling times compared to the flying boom for larger aircraft.
- Drogue subject to turbulence and aerodynamic forces (bow wave) of approaching aircraft.
- Drogue subject to damage by poor receiver technique, making further refueling difficult or impossible.
- Drogue only equipped tankers cannot be easily fitted with boom systems.
- The probe is generally located in the front of the refueling plane, presenting several problems: sensitive avionics equipment (pitot static and angle of attack probes, etc.), can easily be damaged by the drogue, and FOD, including fuel or probe/drogue parts can be ingested into the plane's engines.
Boom drogue adapter units
USAF KC-135 and French Air Force KC-135FR refueling-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 used on 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 center, wherein the probe is guided into the hose receptacle by the canvas drogue. The metal drogue, when contacted even slightly off center, 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 fueling. 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 fueling is complete, the receiver carefully backs off until the probe refueling valve disconnects from the valve in the basket. Off center 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. Where these are attached to the wings, the system is known as the Multi-Point Refueling System or MPRS. 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 centerline 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 refueled in a single mission, without landing to install an adapter. Many KC-135 and some KC-10s are also equipped with dual under-wing hose-and-drogue attachments known as Wing Air Refueling Pods (WARPs).
In this method, similar to the probe-and-drogue method but more complicated, the tanker aircraft released a flexible hose from its wingtip. An aircraft, flying beside it, had to catch the hose with a special lock under its wingtip. After the hose was locked, and the connection was established, the fuel was pumped. It was used on a small number of Soviet Tu-4 and Tu-16 only (the tanker variant was Tu-16Z).
Some historic systems used for pioneering aerial refueling used the grappling method, where the tanker aircraft unreeled the fuel hose and the receiver aircraft would grapple the hose midair, reel it in and connect it so that fuel can be transferred either with the assistance of pumps or simply by gravity feed. This was the method used on the Question Mark endurance flight in 1929.
The probe-and-drogue system is not compatible with flying boom equipment, creating a problem for military planners where mixed forces are involved. Incompatibility can also complicate the procurement of new systems - the Canadian Forces currently wish to purchase the F-35A, which can only refuel via the flying boom, but Canada only possess probe-and-drogue refuelers. The potential cost of converting Canadian F-35As to probe-and-drogue refueling (as will be used on U.S. Navy F-35Cs) has added to the political controversy which already surrounds F-35 procurement in Canada.
These concerns can be addressed by drogue adapters (see section "Boom drogue adapter units" above) that allow drogue aircraft to refuel from boom-equipped aircraft, and by refuelers which are equipped with both drogue and boom units and can thus refuel both types in the same flight, such as the KC-10, MPRS KC-135, or Airbus A330 MRTT.
Uses and considerations
The development of the KC-97 and KC-135 Stratotankers was pushed by the Cold War requirement of the United States to be able to keep fleets of nuclear-armed B-47 Stratojet and B-52 Stratofortress strategic bombers airborne around-the-clock either to threaten retaliation against a Soviet strike for mutual assured destruction, or to bomb the U.S.S.R. first had it been ordered to do so by the President of the United States. The bombers would fly orbits around their assigned positions from which they were to enter Soviet airspace if they received the order, and the tankers would refill the bombers' fuel tanks so that they could keep a force in the air 24 hours a day, and still have enough fuel to reach their targets in the Soviet Union. This also ensured that a first strike against the bombers' airfields could not obliterate the U.S.'s ability to retaliate by bomber. A noted example of refueling used in this manner in the movies can be seen in the opening credits of Dr. Strangelove.
In the UK, in 1958 Valiant tankers were developed with one HDU mounted in the bomb-bay. Valiant tankers were used to demonstrate radius of action by refueling a Valiant bomber non-stop from UK to Singapore in 1960 and a Vulcan bomber to Australia in 1961. Other UK exercises involving refueling aircraft from Valiant tankers included Javelin and Lightning fighters, also Vulcan and Victor bombers. For instance, in 1962 a squadron of Javelin air defense aircraft was refueled in stages from the UK to India and back (exercise "Shiksha"). After the retirement of the Valiant in 1965, the Handley Page Victor took over the UK refueling role and had three hoses (HDUs). These were a fuselage-mounted HDU and a refueling pod on each wing. The center hose could refuel any probe-equipped aircraft, the wing pods could refuel the more maneuverable fighter/ground attack types.
A byproduct of this development effort and the building of large numbers of tankers was that these tankers were also available to refuel cargo aircraft, fighter aircraft, and ground attack aircraft, in addition to bombers, for ferrying to distant theaters of operations. This was much used during the Vietnam War, when many aircraft could not have covered the transoceanic distances without aerial refueling, even with intermediate bases in Hawaii and Okinawa. In addition to allowing the transport of the aircraft themselves, the cargo aircraft could also carry matériel, supplies, and personnel to Vietnam without landing to refuel. KC-135s were also frequently used for refueling of air combat missions from air bases in Thailand.
The USAF SR-71 Blackbird strategic reconnaissance aircraft made frequent use of air-to-air refueling. Indeed, design considerations of the aircraft made its mission impossible without aerial refueling. Based at Beale AFB in central California, SR-71s had to be forward deployed to Europe and Japan prior to flying actual reconnaissance missions. These trans-Pacific and trans-Atlantic flights during deployment were impossible without aerial refueling. The SR-71's designers traded takeoff performance for better high-speed, high-altitude performance, necessitating takeoff with less-than-full fuel tanks from even the longest runways. Once airborne, the Blackbird would accelerate to supersonic speed using afterburners to facilitate structural heating and expansion. The magnitude of temperature changes experienced by the SR-71, from parked to its maximum speed, resulted in significant expansion of its structural parts in cruise flight. To allow for the expansion, the Blackbird's parts had to fit loosely when cold, so loosely, in fact, that the Blackbird constantly leaked fuel before heating expanded the airframe enough to seal its fuel tanks. Following the supersonic dash, and to stop the fuel leaks, the SR-71 would then rendezvous with a tanker to fill its now nearly empty tanks before proceeding on its mission. At its most efficient altitude and speed, the Blackbird was capable of flying for many hours without refueling. The SR-71 used a special fuel, JP-7, with a very high flash point to withstand the extreme skin temperatures generated during Mach 3+ cruise flight. While JP-7 could be used by other aircraft, its burn characteristics posed problems in certain situations (such as high-altitude, emergency engine starts) that made it less than optimal for aircraft other than the SR-71.
Normally, all the fuel aboard a tanker aircraft may be either offloaded, or burned by the tanker as necessary. To make this possible, the KC-135 fuel system incorporated gravity draining and pumps to allow moving fuel from tank to tank depending on mission needs. Mixing JP-7 with JP-4 or Jet A, however, rendered it unsuitable for use by the SR-71, so the US Air Force commissioned a specially modified KC-135 variant, the KC-135Q, which included changes to the fuel system and operating procedures preventing inadvertent inflight mixing of fuel intended for offload with fuel intended for use by the tanker. SR-71 aircraft were refueled exclusively by KC-135Q tankers.
Tankers are considered "force multipliers," because they convey considerable tactical advantages. Primarily, aerial refueling adds to the combat radius of attack, fighter and bombers aircraft, and allows patrol aircraft to remain airborne longer, thereby reducing the numbers of aircraft necessary to accomplish a given mission. Aerial refueling can also mitigate basing issues which might otherwise place limitations on combat payload. Combat aircraft operating from airfields with shorter runways must limit their takeoff weight, which could mean a choice between range (fuel) and combat payload (munitions). Aerial refueling, however, eliminates many of these basing difficulties because a combat aircraft can takeoff with a full combat payload and refuel immediately.
Aside from these issues, the psychological advantage of full fuel tanks – and a tanker likely available nearby – gives a pilot a distinct edge in combat. In most combat situations, speed is a necessity for optimal completion of the mission at hand. As high speeds require fuel, pilots must always balance fuel and speed requirements. Pilots operating aircraft with aerial refueling capability eliminate low-fuel concerns.
During the Vietnam War, it was common for USAF fighter-bombers flying from Thailand to North Vietnam to refuel from KC-135s en-route to their target. Besides extending their range, this enabled the F-105s and F-4 Phantoms to carry more bombs and rockets. Tankers were also available for refueling on the way back if necessary. In addition to ferrying aircraft across the Pacific Ocean, aerial refueling made it possible for battle damaged fighters, with heavily leaking fuel tanks, to hook up to the tankers and let the tanker feed its engine(s) until the point where they could glide to the base and land. This saved numerous aircraft.
The US Navy frequently used carrier-based aerial tankers like the KA-3 Skywarrior to refuel Navy and Marine aircraft such as the F-4, A-4 Skyhawk, A-6 Intruder, and A-7 Corsair II. This was particularly useful when a pilot returning from an airstrike was having difficulty landing and was running low on jet fuel. This gave him fuel for more attempts at landing for a successful "trap" on an aircraft carrier. The KA-3 could also refuel fighters on extended Combat Air Patrol. USMC jets based in South Vietnam and Thailand also used USMC KC-130 Hercules transports for air-to-air refueling on missions.
During the 1980s war, the Iranian Air Force maintained at least one KC 707-3J9C aerial tanker, which the Islamic Republic had inherited from the Shah's government. This was used most effectively on April 4, 1981, refueling eight IRIAF F-4 Phantoms on long range sorties into Iraq to bomb the H-3 Al Walid airfield near the Jordanian border.
Israeli Tunisian Strike
In 1985, Israeli F-15s used heavily modified Boeing 707 aircraft to provide aerial refueling over the Mediterranean Sea in order to extend their range for the 2,000km flight during the Operation Wooden Leg air raid on the headquarters of the PLO near Tunis, Tunisia. 
During the Falklands War, aerial refueling played a vital role in all of the successful Argentine attacks against the Royal Navy. The Argentine Air Force had only two KC-130H Hercules available and they were used to refuel both Air Force and Navy A-4 Skyhawks and Navy Super Etendards in their Exocet strikes. The Hercules on several occasions approached the islands (where the Sea Harriers were in patrol) to search and guide the A-4s in their returning flights. On one of those flights (callsign Jaguar) one of the KC-130s went to rescue a damaged A-4 and delivered 39,000 lb (18,000 kg) of fuel while carrying it to its airfield at San Julian.
On the other hand, the Mirage IIIs and Daggers lack of air refueling capability prevented them from achieving better results. The Mirages were unable to reach the islands with a strike payload, and the Daggers could do so only for a five minute strike flight.
On the British side, air refueling was carried out by the Handley Page Victor K.2 and after the Argentine surrender by modified C-130 Hercules tankers. These aircraft aided deployments from the UK to the Ascension Island staging post in the Atlantic and further deployments south of bomber, transport and maritime patrol aircraft. The most famous refueling missions were the 8,000 nm/15,000 km "Operation Black Buck" sorties which used 14 Victor tankers to allow an Avro Vulcan bomber (with a flying reserve bomber) to attack the Argentine-captured airfield at Port Stanley on the Falkland Islands. With all the aircraft flying from Ascension, the tankers themselves needed refueling. The raids were the longest-range bombing raids in history until surpassed by the B-52 in the 1991 Gulf War and later B-2 flights.
The Victor tankers, retired in 1993, were replaced in RAF service by Lockheed L-1011 and Vickers VC10 transports which were bought second-hand and fitted as tankers. The L-1011s, converted by Marshall Aerospace, and VC10s, converted by British Aerospace, can refuel any aircraft fitted with the NATO standard probe system.
During Operation El Dorado Canyon, several F-111 Aardvark fighter-bombers stationed in the United Kingdom used aerial refueling to enable them to operate non-stop against targets in Libya. Since the aircraft were allowed to cross neither French nor Spanish airspace, they had to make a detour around the Iberian Peninsula and stay above International waters during all transit.
Persian Gulf War
During the time of Operation Desert Shield, the military build up to the Persian Gulf War, US Air Force Boeing KC-135s & McDonnell Douglas KC-10As, and USMC KC-130 Hercules aircraft were deployed to forward air bases in England, Diego Garcia, and Saudi Arabia. Aircraft stationed in Saudi Arabia normally maintained an orbit in the Saudi–Iraqi neutral zone, informally known as "Frisbee", and refueled Coalition Aircraft whenever necessary. Two side by side tracks over central Saudi Arabia called "Prune" and "Raisin" featured 2–4 basket equipped KC-135 tankers each and were used by Navy aircraft from the Red Sea Battle Force. Large Navy strike groups from the Red Sea would send A-6 tankers to the Prune and Raisin tracks ahead of the strike aircraft arriving to top off and take up station to the right of the Air Force tankers thereby providing an additional tanking point. RAF Handley Page Victor and Vickers VC10 tankers were also used to refuel British and coalition aircraft and were popular with the US Navy for their docile basket behavior and having three point refueling stations. An additional track was maintained close to the northwest border for the E-3 AWACS aircraft and any Navy aircraft needing emergency fuel. These 24-hour air-refueling zones helped make the intense air campaign during Operation Desert Storm possible. An additional 24/7 tanker presence was maintained over the Red Sea itself to refuel Navy F-14 Tomcats maintaining Combat Air Patrol tracks. During the last week of the conflict, KC-10 tankers moved inside Iraq to support barrier CAP missions set up to block Iraqi fighters from escaping to Iran.
On January 16–17, 1991, the first combat sortie of Desert Storm, and the longest combat sortie in history, at that time, was launched from Barksdale AFB, Louisiana. Seven B-52Gs flew a thirty-five hour mission to the Persian Gulf region, and back, to launch Boeing Air Launched Cruise Missiles (ALCMs) with the surprise use of conventional warheads. All of this was made possible by in-flight refueling, and by the secret switch away from nuclear warheads on the ALCMs.
An extremely useful aerial tanker in Desert Storm was the USAF KC-10A Extender. Besides being larger than the other tankers, the KC-10A is equipped with the USAF "boom" refueling and also the "hose-and-drogue" system. This makes it possible for the KC-10A to refuel USAF aircraft, and also USMC and US Navy jets that use the "probe-and-drogue" system, and also allied aircraft, such as those from the UK and Saudi Arabia. KC-135s may be equipped with a drogue depending on the mission profile.
The KC-10A was originally designed for the support of NATO in Europe by the USAF. In the case of armed conflict, with a full jet fuel load, the KC-10A is capable of flying from a base on the east coast of the US or Canada, flying nonstop to Europe, transferring a considerable amount of fuel in air-to-air refueling, and then returning to its home base, all without landing anywhere. This could have been very useful in the case when numerous European bases become disabled by Warsaw Pact strikes in Germany, the Netherlands, France, and Great Britain.
The USAF provided nearly 90 percent of the NATO tanker force, 112 active and 63 Reserve-component KC-135 and KC-10 tankers. Tankers were also provided from Britain’s RAF (TriStars and VC10s), French Air Force and Turkish Air Force KC-135s, Italian Air Force B.707 T/T, Spanish Air Force KC-130 Hercules and Royal Netherlands Air Force KDC-10s. Although some European nations provided air-refueling aircraft, the conflict highlighted the problem Europe has with a lack of such aircraft and dependence on the United States for tanker support during a major operation. Some European nations sought to address this lack of capability, such as the Italian Air Force purchase of the Boeing KC-767, but there is still a huge difference in air-refueling capability between the US and Europe.
In 2003 the U.S. Air Force and Far Technologies applied secretly for patents on mid-air rearming of aircraft. The technique proposed is similar in many respects to airborne refueling, with a number of notable modifications. The airborne rearming system comprises a rearming plane with an internal bomb storage area and loading device consisting of a large aft door and a modified remote-driven robotic arm (boom) equipped with a day-night camera as well as sensors. A special pylon to receive the arms from the boom is fitted onto the attack aircraft. At present financial and technological problems stand in the way of aerial rearming, mainly the need for an automatic system to perform the rearm currently under development for aerial refueling.
Helicopter In-Flight Refueling (HIFR) is a variation of aerial refueling when a naval helicopter approaches a warship (not necessarily suited for landing operations) and receives fuel through the cabin while hovering.
Alternatively, some helicopters equipped with a probe extending out the front can be refueled from a drogue-equipped tanker aircraft in a similar manner to fixed-wing aircraft by matching a high forward speed for a helicopter to a slow speed for the fixed-wing tanker.
- Commercial tankers are increasingly being used by military forces. The Omega Company  is contracted by the US Navy.
- Autonomous (hands off) refueling using probe/drogue systems is being investigated by NASA, potentially for use by Unmanned aerial vehicles.
- The film Dr. Strangelove from 1964 has genuine footage of a "boom and receptacle" operation between USAF KC-135 Stratotanker and B-52 Stratofortress in turbulent air.
- The film The Perfect Storm contains a scene where an HH-60 helicopter attempts to refuel during adverse weather, but is unable to succeed. This was based on true events that occurred during the 1991 Perfect Storm.
- The film The Final Countdown from 1980 has genuine footage of a "probe-and-drogue" operation between US Navy KA-6 Intruder and F-14 Tomcats from USS Nimitz (CVN-68).
- The film The Sum of All Fears features a mid-air refueling sequence with the E-4B NEACP. In the commentary, author Tom Clancy mocks mid-air refueling as "sex between two planes at 35,000 feet".
- The film Air Force One contains a sequence where a KC-10 tanker is ordered to carry out the mid-air refueling of Air Force One, but the hijacked plane becomes unstable. The hijacker runs Air Force One into the refueling boom causing a fire which then causes the KC-10 tanker to explode.
- In 2011 BMW launched a commercial featuring the 2011 BMW 5-Series named "refuel." In the advert the driver has his coffee cup refilled by an aerial refueling aircraft, similar to that of a KC-135 Stratotanker boom but a KC-10 nozzle.
- Docking maneuver for spacecraft
- In-space refueling
- Underway replenishment – the transfer of refuel and stores at sea
- List of firsts in aviation
- Military logistics
- ^ Note AAR also stands for "After Action Review" (de-briefing) and in aviation, IFR also stands for "Instrument Flight Rules".
- ^ Nangia, R.K (November 2006). "Operations and aircraft design towards greener civil aviation using air-to-air refuelling". The Aeronautical Journal Paper No. 3088. (November): 705–721. http://www.greenerbydesign.org.uk/_FILES/publications/3088.pdf. Retrieved 20 October 2011.
- ^ There is no indication that any relationship exists between A.D. Hunter and the Hunter brothers.
- ^ a b History of Aviation, Part 19, 1938
- ^ "Wings Across The Atlantic", March 1931, Popular Mechanics page 190 second paragraph
- ^ Flight Magazine archive, January 10, 1929
- ^ Flight Magazine archive, July 3, 1931, p623
- ^ "Gas Station In The Sky" , January 1947, Popular Science
- ^ "Refuelling In Flight" , Flight Magazine, November 22, 1945 close-up drawing of receiver pawl grapnel and tanker haul line projectile
- ^ "Refueling In Flight" , Flight magazine, 1947
- ^ Note - on one flight there was a high Westerly wind and no need for aerial refueling
- ^ "Refuelling In Flight" , Flight magazine, August 25, 1940
- ^ "A Girdle Around The Earth" , Flight magazine, March 10, 1949
- ^ "B-50s In Great Britain" , Flight magazine, September 1, 1949
- ^ http://www.cobham75.com/cobham-the-company-1934-1985/air-to-air-refuelling-takes-off.aspx
- ^ "Jet Refuels Like Humming Bird" , January 1949, Popular Science
- ^ http://www.the-best-of-british.com/PlaneCrazyHeritage/airpictorial/1999/RefuellingFDC.htm
- ^ KC-135 Aerial Refueling Manual T.O. 1-1C-1-3.
- ^ Bolkcom, Christopher (2005-05-11). "Air Force Aerial Refueling Methods: Flying Boom versus Hose-and-Drogue". CRS order code RL32910. US Congressional Research Service via CRSWeb. http://digital.library.unt.edu/govdocs/crs/permalink/meta-crs-6702:1. Retrieved 2009-04-01.
- ^ a b A1-F18AC-NFM-000 Naval Aviation Training and Operating Procedures Standardization (NATOPS) Manual. United States Department of the Navy. 1 August 2006. pp. 364.
- ^ http://www.zvezda-npp.ru/english/06.htm
- ^ Maj. Marck R. Cobb, “Aerial Refueling: The Need for a Multipoint, Dual-System Capability,” AU-ARI-CP-87-3, Air University Press, July 1987.
- ^ Air Force Aerial Refueling Methods: Flying Boom versus Hose-and-Drogue
- ^ note - even today there is much confusion about how this system actually worked
- ^ a b http://digital.library.unt.edu/govdocs/crs/permalink/meta-crs-6702:1
- ^ Air Forces Monthly Special F-4 Phantom ISBN 0-946219-46-X article by Farzin Nadimi p77
- ^ http://www.globalsecurity.org/military/world/israel/iaf.htm
- ^ Kosovo and Theater Air Mobility
- ^ ABRAcadabra – AirBorne ReArming – TFOT
- ^ Omega Air Refueling
- ^ Ares Homepage
- ^ HSF Final Report: Seeking a Human Spaceflight Program Worthy of a Great Nation, October 2009, Review of U.S. Human Spaceflight Plans Committee, p. 65-66.
- Photo aerial tanking 1929
- Aerial tanking history
- Airborne rearming – A comprehensive article including a video.
- Aerial Refueling on APA
- Twenty-one Years of Flight Refuelling a 1955 Flight article on the development of Flight Refuelling Ltd's method
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