Aerial refueling

A C-17 Globemaster refuels through the boom of a KC-135 Stratotanker

Boom and receptacle: USAF KC-135R Stratotanker, two F-15s (twin fins) and two F-16s, on an aerial refueling training mission

Probe and drogue: USAF HC-130P HIFRs a HH-60 Pave Hawk

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. Applied to helicopters, it is known as HAR for Helicopter Aerial Refueling. The procedure allows the receiving aircraft to remain airborne longer and, more important, to extend its range and therefore those of its weapons or its deployment radius. A series of air refuelings can give range limited only by crew fatigue and engineering factors such as engine oil consumption. 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 and specially designed receiving receptacle.

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.

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 to extend the range of intercontinental strategic bombers, air refueling since the Vietnam War has been extensively used in large-scale military operations for many different aircraft and helicopters. 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

Pioneer experiments

Capt. Lowell H. Smith and Lt. John P. Richter receiving the first mid-air refueling on June 27, 1923, from a plane flown by 1st Lt. Virgil Hine and 1st Lt. Frank W. Seifert.

Some of the earliest experiments in aerial refueling took place in the 1920s; using a system designed by Anthony Greig, an Avro Aircraft engineer, 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 [1], staying aloft for more than 27 days.

A F-101A Voodoo (top right), B-66 Destroyer (top left) and F-100D Super Sabre refuel from a KB-50J tanker in the early 1960s

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 trialling refuelling-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. The aviation pioneer Alan Cobham bought a patent from David Nicolson and John Lord for £480 each and then developed the probe and drogue method and gave public demonstrations of the system. In 1934 he founded Flight Refuelling Ltd. (FRL), and by 1938 had used an automatic system to refuel aircraft as large as the Short Empire flying boat Cambria from an Armstrong Whitworth AW.23. Handley Page Harrows were used to refuel the Empire flying boats for regular transatlantic crossings. FRL's post-war trials used a Lancaster tanker trailing the hose and drogue from an HDU, and the modified Meteor III, EE397, fitted with a nose-mounted probe. On 7th August 1949, the Meteor flown by FRL test pilot Pat Hornidge took-off from Tarrant Rushton and, refulled ten times by the Lancaster tanker, remained airborne for 12 hours and 3 minutes, receiving 2,352 gallons of fuel from the tanker in ten tanker contacts and flying an overall distance of 3,600 miles, 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 Hunter's design, even at the higher speeds modern jet aircraft typically need to remain airborne.

Operational air refueling

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 Cobham's refueling equipment, which had been in practical use with 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 43rd ARS at Davis-Monthan Air Force Base, Arizona, and the 509th ARS at Walker Air Force Base, New Mexico. The first ARS aircraft used a 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 43rd ARS. Before the mission, crews of the 43rd 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 43rd and 509th ARS became fully operational.

The first use of aerial refueling in combat took place during the Korean War. Though the KC-135 was envisaged for refueling strategic bombers, it saw extensive use refueling smaller tactical aircraft in the Vietnam War supporting large strike packages. It also was used for in Desert Storm and current US air warfare doctrine. The HC-130 and KC-130 Hercules is used by the US Marine Corps and Navy to refuel jets, and by the US Air Force to refuel helicopters.

Aerial refueling systems

The two most common approaches for making the union between the two aircraft are the boom and receptacle system (sometimes called flying boom) and the probe and drogue system. There is also a combination “boom drogue adaptor” that combines the first two methods. Much less popular was the wing-to-wing system, which is no longer used.

Boom and receptacle

USAF C-5 approaches a KC-135R

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USAF AWACS approaches a KC-135R

Often called the Boeing "flying boom”, this system uses a rigid, telescoping tube controlled by two small wings that an operator on the tanker aircraft extends and inserts into a receptacle on the receiving aircraft. In practice, the pilot of the aircraft being refueled must position the aircraft so that the boom can be engaged, using visual references, including lights used during night refueling. All boom-equipped tankers (i.e. KC-135 Stratotanker, KC-10 Extender), have a single boom, and can refuel one aircraft at a time with this mechanism.

History of Flying Boom

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. Boeing engineers came up with the concept of the “Flying Boom” system. The B-29 was the first to employ the flying boom system, 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 Model 717, known to the Air Force as the KC-135 of which a total of 732 were built. Please note, this aiframe is not a version of the Boeing 707. It has a smaller wing and smaller fuselage in both length and diameter. Interestingly, the fact that Boeing listed the KC-135 as a 717 was forgotten by Boeing, when then released the MD designed MD-95 as the B-717 after the purchase of McDonnel Douglas.

Flying boom operation

The flying boom is attached to the rear of the tanker aircraft. The attachment is flexible, allowing boom movement up to 25 degrees left or right, and from flush with the bottom of the aircraft up to 50 degrees below horizontal. Mounted within the outer structural portion of the boom is a rigid tube through which the fuel passes. The tip end of the fuel tube has a nozzle attached on 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.

Mounted on the outer structural portion of the boom at the opposite end from where the boom attaches to the tanker (the end where the inner fuel tube exits) are a pair of aerodynamic control surfaces allowing precise two dimensional positioning (elevation and azimuth) of the boom. These control surfaces, called "ruddevators," provide the same functionality as the conventional, tail-mounted rudder and elevator control surfaces of a typical airplane. They are actuated hydraulically and controlled by the system operator using a control stick. Additionally, the boom operator hydraulically extends and retracts the inner fuel tube using the extension/retraction lever, thereby providing for positioning of the boom nozzle in a third dimension (telescoping length).

To complete an aerial refueling, the pilot of the receiver aircraft begins by flying formation in the "precontact" position, a position directly below and approximately 50 feet (15 m) behind the boom nozzle. The boom is flown in the "trail" position at 30 degrees below horizontal, on the tanker's centerline with the nozzle extended 10 feet. The receiver may be slightly left or right of centerline depending on operational guidance or receptacle installation (the refueling receptacle is typically fitted on an aircraft's centerline, but design considerations may require other locations). When cleared, the receiver pilot moves forward – "closes" – aided with either voice (using radio) or visual commands (using lights on the bottom of the tanker) from the enlisted crew member operating the tanker boom, the "boom operator" or "boomer."

On KC-135 tanker aircraft, the boomer lies in his stomach with a chin rest to help hold his head up; on the KC-10 the boomer sits upright. On either tanker, the boomer's position is in the back of the aircraft, facing aft, allowing an unobstructed view of the receiver aircraft and the boom through an aft-facing window. Once the receiver aircraft reaches a position in which the nozzle may be safely mated with the receptacle – the "contact position" – its pilot attempts to hold in that position, allowing as little relative motion between the two aircraft as possible. While the receiver holds in the contact position, the boomer flies the boom to precisely position the boom's nozzle into the receiver's receptacle. Following toggle engagement – or "contact" – pumps operated by the tanker's pilot force fuel through the inner fuel tube of the boom into the receiver.

USAF KC-135R boom operator view

While in contact, two rows of lights on the bottom of the tanker's fuselage – called pilot director indicators – change in relation to the nozzle's up/down and fore/aft movement to aid the receiver pilot in maintaining position within the air refueling envelope. The "air refueling envelope" is an area in relation to the tanker within which the nozzle and receptacle must remain during contact. Its boundaries are based on mechanical boom movement limitations, and to prevent the receiver from moving into a position where any portion of the boom might touch the receiver outside the receptacle while in contact. Exceeding an air refueling envelope limit can result in structural damage to the tanker and/or the receiver precluding further refueling operations for either or both.

The boom's mechanical limits are based on the structure of the yoke and trunnion system mounting the boom to the tanker, the length of the extendable inner fuel tube and the maximum deflection of the flexible nozzle. Exceeding mechanical limits can physically break structural parts mounting the boom to the tanker. Exceeding the nozzle's deflection limits can cause mechanical binding between the nozzle and receptacle (try removing a key from a lock by pulling sideways instead of straight back) and possible separation of the nozzle from the boom. Should the receiver move too far forward and push the inner fuel tube to its retraction limit, the outer structural portion of the boom could contact the receiver aircraft and damage both the boom and receiver; too far aft, and the nozzle locked in the receptacle could be physically broken from the boom when the inner fuel tube reaches its extension limit.

Air refueling envelope limits may also be receiver based. For example, a receiver with a receptacle mounted on the fuselage behind a canopy might have to limit upward movement while in contact to prevent the boom from hitting the canopy. Additionally, large receivers can adversely affect boom control resulting in envelope limitations.

As the crewmember with the best view of the operation, the boomer is primarily responsible for preventing damage to either aircraft during air refueling. While in contact, the boomer positions the ruddevator control stick to maintain boom/receptacle alignment in response to receiver aircraft left/right and up/down movement, thereby minimizing side forces on the mated nozzle/receptacle. A "freewheeling" extension/retraction motor allows the inner fuel tube to extend and retract as the receiver moves fore and aft. The boomer must monitor the receiver's position, via three boom position indicators, and command a disengagement of the nozzle/receptacle locking toggles – a "disconnect" – before the receiver aircraft exceeds any published air refueling envelope limit. If the boomer disconnects before the receiver get the full fuel offload, the entire operation – precontact, closure and contact – repeats.

When the desired amount of fuel has passed from tanker to receiver, the boomer or receiver pilot, typically the boomer, commands a disconnect. Disconnect might also occur automatically (a sensor monitors fuel pressure during refueling to prevent a rupture in the fuel lines or tanks) should the receiver's fuel system be filled to capacity. Following disconnect, the boomer retracts the fuel tube from the receptacle and flies the boom clear of the receiver. While not in use, the boom is flown or hoisted (via a cable and hydraulic pump) to the stowed position, flush with the bottom of the tanker, and latched in position to minimize drag. The receiver backs away and clears the tanker, then continues on its mission.

Flying boom systems in service

USAF fixed wing aircraft use the flying boom system exclusively. In addition to the US Air Force, the boom system is in use by the Netherlands (KDC-10), Israel (modified Boeing 707), and Turkey (ex-USAF KC-135R). The largest tanker aircraft that ever entered service are the Islamic Republic of Iran Air Force (IRIAF) 747's, Iran took delivery of Boeing 747 tankers equipped with a single boom and three drogues in early 1976. Both Japan and Italy have contracted with Boeing for tankers based on the B767.

The European EADS group has developed a boom refueling system using "fly-by-wire" controls that is compatible with other boom systems. This is offered on modified European Airbus type aircraft that are configured as tankers.

Advantages of boom and receptacle

• Higher fuel flow rates (up to 1000 US gallons/6,000 lbs per minute for the KC-135 tanker) can be achieved with the large diameter of the pipe in the flying boom, resulting in much less time required to refuel compared to the smaller diameter required of a flexible hose system. Unlike bombers and other large aircraft, however, fighter aircraft can only ‘’accept’’ fuel at 1000–3000 lbs per minute and cannot use the boom’s maximum flow rate. However, even though the offload rate through the boom is reduced to smaller fighter type aircraft, it is still much faster than the probe and drogue system that the navy uses. Offload rates of 1200-2000 lbs/min are possible to Air Force fighters, while Navy fighters can only accept about 800 lbs/min.
• The boom method eliminates the requirement for the (often very large and less maneuverable) receiver aircraft pilot to precisely fly a probe into a drogue, something that is easily performed by fighter-sized aircraft, but would be extremely challenging even for the best pilot in a larger aircraft.
• A tanker with a flying boom can be converted in the field to accommodate probe-equipped aircraft, if necessary.

Disadvantages of boom and receptacle

• The cost to train and employ the "boomer"—1990s estimates place the cost to train a boom operator at nearly $1,000,000.
• Incompatibility with probe and drogue systems, which are prevalent on US Navy, and most non-US aircraft.
• Complexity of tanker design.
• Only one receiver aircraft can refuel at a time.
• Cannot be used to refuel most helicopters.
• Cannot be installed on carrier based aircraft.

Probe and drogue

Tornado GR4 refueling from the drogue of an RAF VC10 tanker over Iraq.

The nose probe on an RAF maritime patrol Nimrod MR2

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.

Buddy store

F/A-18 buddy refueling

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.

Probe and Drogue Operation

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.

S-3 Viking buddy tanker with drogue deployed

Probe-and-drogue 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 hose-and drogue. The probe-and-drogue system was first used in service on late models of the KB-29M Superfortress. Its first use in combat occurred on May 29, 1952 when twelve F-84s were refueled during a mission from Itazuke, Japan to Sariwon, North Korea. Also in the 1950s, the Royal Air Force converted two squadrons of Valiant bombers to the tanker role by mounting a Hose Drum Unit (HDU) in the bomb bay. These were No. 214 Squadron RAF at Marham, operational in 1959, and later No. 90 Squadron RAF at Honington. In the 1960s the Valiant was replaced by Victor tankers that had up to three refueling points, one under the fuselage and a pod under each wing.

Advantages of 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.

Disadvantages of probe and drogue

• The lower flow rates (1,500–2,000 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.
• Precise placement of the probe into the drogue by the receiver aircraft pilot precludes large receiver aircraft installation.
• 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/droge parts can be ingested into the plane's engines.

Boom drogue adapter units

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Cockpit view of KC-135 boom in the US Navy "Iron Maiden" configuration.

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Air National Guard KC-135 tanks a US Navy Hornet

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 utilize an aluminum basket, grimly known as the “wrecking ball” 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 become disengaged ceasing 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.

Multiple refueling systems

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, including large aircraft that are probe-equipped and do not have the maneuverability to take fuel from an off-centerline wing pod. 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).

Wing-to-wing refueling

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).

Grappling systems

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, and also the first ever non-stop around-the-world flight by Strategic Air Command's B-50 nuclear-capable bomber nicknamed the Lucky Lady II in 1949.

Compatibility issues

The probe-and drogue system is not compatible with flying boom equipment, creating a problem for military planners where mixed forces are involved. For this reason (as well as other advantages of probe/drogue systems), the USAF has considered converting boom systems to probe-and-drogue.

Strategic and tactical implications

An F-15E Strike Eagle disengages from a KC-10 Extender, using a boom with an He 162 style of control surface

Strategic 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-speeed, 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 unusable 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.

A KC-10 Extender from Travis Air Force Base, California, refuels an F-22 Raptor

Tactical uses and considerations

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 life – the faster your aircraft is moving, the greater the tactical advantage for you, and the greater the tactical difficulty for your foe. But speed requires fuel, so the prudent pilot must balance the need for speed with the need to get home. Pilots flying aircraft with aerial refueling capability all but eliminate low-fuel concerns and can maintain life-saving speed as long as necessary (for all practical purposes) to meet mission and tactical needs. This psychological advantage cannot be over-emphasized.

Vietnam War

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.

Falklands War/South Atlantic War

During the Falklands War, aerial refueling played a vital role in all of the Argentine successful 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 8,000nm/15,000km "Operation Black Buck" sorties which needed 14 Victor tankers to refuel a single Avro Vulcan bomber (with both the Vulcan, and many of the Victors needing refueling), to attack the Argentine-captured airfield at Port Stanley on the Falkland Islands. They attempted to knock out the Port Stanley runway, blocking the Argentine C-130 Hercules reinforcement operations. 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.

Libya

During Operation El Dorado Canyon, several F-111 Aardvark fighter-bombers stationed in the United Kingdom utilized 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

F-14 Tomcats from the Red Sea and Persian Gulf await their turn refueling from a KC-10A over Iraq during Desert Storm

During the time of Operation Desert Shield, the military build up to the Persian Gulf War, US Air Force 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 tankers thereby providing an additional tanking point. RAF Handley Page Victor and VC-10 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.

Kosovo War

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 VC-10s), French Air Force and Turkish Air Force KC-135s, 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.

Aerial rearming

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.

HIFR (Helicopter In-Flight Refueling)

Raising a hose from ship to an HH-65

Hose connect and refueling of an HH-65

A variation of aerial refueling is 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.

Developments

• 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.

Tanker aircraft by refueling system

Boom and receiver

A B-2 Spirit prepares to refuel from a KC-135R

• KB-29P (No longer in service)
  • adapted from the B-29 Superfortress bomber
• KC-97 Stratotanker (No longer in service)
  • derived from the C-97 Stratofreighter
• KC-135 Stratotanker
  • related to the Boeing 707 airliner
  • also used by the French Air Force, Colombian Air Force, Republic of Singapore Air Force and Turkish Air Force
  • boom can be fitted preflight with a drogue adapter. French Air Force KC-135 Stratotankers use the probe and drogue system
  • MPRS models will have two drogue hose reels (pods) at the wingtips
• KC-10 Extender
  • adapted from the McDonnell Douglas DC-10 airliner
  • also has a retractable hose and drogue that can be selected in-flight
  • can be fitted with two underwing pods(similar to the KC-135's MPRS) capable of simultaneously refueling two receiver aircraft (Wing Air Refueling Pods or WARPs)
  • Boom operator is in a rear-facing seat with a downward facing window with fly by wire controls.
  • The Royal Netherlands Air Force operates two KDC-10s – former civil aircraft modified to a similar standard to the KC-10
• KC-767
  • adapted from the Boeing 767 airliner
  • to be used by the Italian Air Force and Japan Air Self Defense Force (Entered service on March 26, 2009)
• Airbus A330 MRTT
  • development of Airbus A330-200 airliner. Australian aircraft will be equipped with both a flying boom and probe-and-drogue units to accommodate both RAAF F-111s as well as F/A-18s and will be known as KC-30B. UK aircraft will be probe-and-drogue only. The advanced refueling boom system (ARBS) places the boom operator and mission planning station in the cockpit. It includes remote controls with an enhanced 2D/3D vision system rather than a rear facing seat in the tail.

de Havilland Sea Vixens probe-and-drogue refuelling at a 1960s Farnborough Air Show

An Australian Boeing 707 refueling a US Navy F/A-18 in 2002

German Luftwaffe Airbus A310 MRTT ready for refueling, shown at the Paris Air Show 2007

Probe and drogue

• Airbus A330 MRTT
  • RAF Future Strategic Tanker Aircraft, due in service around 2008.
  • RAAF due in service beginning 2009.
• Airbus A310 MRTT
  • 4 for Luftwaffe
  • 2 for Canadian Forces (as CC-150 Polaris)
• Airbus A400M, future strategic transport and tanker aircraft due in service around 2009.
• Avro Lancaster (retired)
  • Refueled Gloster Meteors in 1940's
• Avro Lincoln (retired)
  • Refueled Gloster Meteors in 1940's
• Avro Vulcan (No longer in service)
  • specially modified for operations during the Falklands War
• Blackburn Buccaneer (No longer in service)
  • Equipped for buddy tanking
• Boeing 707
  • Used by the Royal Australian Air Force, Brazilian Air Force, Canadian Forces Air Command, South African Air Force and others
  • French Air Force KC-135 Stratotankers use the probe and drogue system.
  • RCAF (later as Canadian Forces) used to use CC-137 Husky (Boeing 707's) for refueling. Both were retired in the late '90's.
• KB-29M (No longer in service)
  • adapted from the B-29 Superfortress; earlier versions used a "grappling hose" system; later models used a true probe-and-drogue.
• KB-50 (No longer in service)
  • adapted from the B-50 Superfortress, an improved model of the B-29 Superfortress.
• HC-130 Hercules and KC-130 Hercules, especially in United States Marine Corps service as well as KC-130s in Argentine Air Force, Brazilian Air Force, Israel Air Force, Republic of Singapore Air Force, Royal Canadian Air Force and Spanish Air Force service.
  • variants of the C-130 Hercules
• Lockheed L-1011 Tristar
  • K1 and KC1 variants deployed by the Royal Air Force
• Vickers Valiant (No longer in service)
• Vickers VC-10
• Handley Page Victor (No longer in service)
• KA-3B Skywarrior (No longer in service)
  • Dedicated tanker variant of the Skywarrior. Primary US Navy tanker from the Vietnam War until retirement prior to Desert Storm.
• A-4 Skyhawk
  • Equipped for buddy tanking with several nations.
• KA-6D Intruder (No longer in service)
  • Dedicated tanker variant of the Intruder. Attack variants of the A-6 also capable of buddy tanking
• A-7 Corsair II (No longer in US service)
  • Equipped for buddy tanking in US Navy and Greek service
• S-3 Viking (No longer in service)
  • The current primary carrier-based tanker, equipped for buddy tanking
• F/A-18E/F Super Hornet
  • Equipped for buddy refueling as "Strike tankers". Taking over tanking duties as the S-3 is retired from service.
• Ilyushin Il-78 Midas
  • Standard Russian tanker, adapted from Il-76 cargo/transport aircraft
  • Variants of this tanker aircraft also deployed by the air forces of Algeria, mainland China, India and Pakistan.
• Myasishchev M-4-2
  • adapted from the M-4 bomber
• Myasishchev 3MS-2
  • adapted from the 3M bomber
• Tu-16N (and Tu-16Z with wing-to-wing system)
• Su-24M
  • equipped for buddy refueling with the UPAZ container as "Strike tankers"
• Su-33
  • equipped for buddy refueling
• MiG-29K
  • equipped for buddy refueling

Popular culture

• The film The Starfighters has a great deal of footage of F-104 Starfighter jet fighters refueling. This was extensively mocked when The Starfighters showed up on Mystery Science Theater 3000 as episode #612.
• 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 OVA series Macross Zero shows a VF-0 variable fighter refueling from a KS-3 tanker aircraft, while SV-51 variable fighters are seen using the buddy pack system.
• 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 shows a CG sequence of a US Air Force KC-10 Extender refueling the hijacked Presidential aircraft, "Air Force One", a specially converted Boeing 747.
• In the 2005 film Stealth the futuristic F/A-37 Talons refuel at an automated aerial refueling station.

Media

NASA's Dryden Flight Research Center are evaluating the capability of an F/A-18A aircraft as an in-flight refueling tanker to develop analytical models for an automated aerial refueling system for unmanned air vehicles (UAVs)

See also

• Docking maneuver for spacecraft

External links

• Photo aerial tanking 1929
• Aerial tanking history
• Airborne rearming – A comprehensive article including a video.
• Aerial Refueling on APA
• Official KC-45 Tanker website