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 HIFR for Helicopter IFR, pronounced "hi fur". 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.
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 balancing the larger payload with carriage of less fuel. Alternatively, a shorter take-off roll can be achieved because take-off can be at a lighter weight before refueling once airborne (as with the US SR-71 Blackbird reconnaissance aircraft).
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. In large-scale military operations, air refueling is extensively used. For instance, in the 1991 conflict with Iraq over its invasion of Kuwait and the 2003 war against Iraq, all coalition air sorties were air-refueled except for a few short-range ground attack sorties in the Kuwait area.
Some of the earliest experiments in aerial refueling took place in the 1920s, when it was as simple as two slow-flying aircraft flying 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 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 the first 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.
In the UK, Alan Cobham bought the patent from David Nicolson and John Lord for £480 each and then pioneered research on 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 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.
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 3 aerial refuelings from 4 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.
The “flying boom” is a rigid, telescoping tube that an operator on the tanker aircraft extends and inserts into a receptacle on the receiving aircraft. All boom-equipped tankers (i.e. KC-135, KC-10), have a single boom, and can refuel one aircraft at a time with this mechanism.
To complete an aerial refueling, the receiver aircraft's pilot begins by flying formation directly below and approximately behind the boom. When cleared, the receiver aircraft moves forward to the contact position aided with either voice commands (using radio) or visual commands (using lights on the bottom of the tanker) from the crew member operating the boom, called a "boom operator" or "boomer" -- an enlisted crew member in the USAF. On KC-135 tanker aircraft the boomer lies prone on a couch or pallet; on the KC-10 the boomer sits. On either tanker, the boomer's position is in the back of the aircraft facing aft. Once the receiver aircraft reaches the contact position its pilot attempts to hold in place with as little relative motion between the two aircraft as possible. Using the ruddevator control stick and the extension/retraction lever, the boomer precisely positions the boom's nozzle into the receiver's receptacle. Following toggle engagement (locking the boom in place) pumps operated by the tanker's pilot force fuel through the boom into the receiver.
While in contact, pilot director indicators (two rows of lights on the bottom of the tanker's fuselage change in relation to the nozzle's up/down and fore/aft movement) aid the receiver pilot in remaining within the air refueling envelope. The air refueling envelope -- a roughly cube-shaped area within which the nozzle and receptacle must remain during contact -- is slightly different for each receiver. Its boundaries are based either on boom movement limitations, or 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. The boom's mechanical limits stem from the both the structural limitations of yoke and trunnion system mounting the boom to the tanker, and the maximum deflection of the flexible nozzle. Should receiver movement left, right, up or down exceed the nozzle's deflection limits the nozzle could become mechanically bound in the receptacle (like trying to remove a key from a lock while pulling sideways instead of pulling straight back) preventing nozzle/receptacle disengagement. The boomer follows the receiver aircraft's movement with the ruddevator control stick to maintain alignment between the inner fuel tube and the outer structural portion of the boom. He or she also monitors the receiver's position, via three boom position indicators, and commands the toggles in the receptacle to disengage the nozzle -- a disconnect -- before the receiver aircraft exceeds any published air refueling envelope limit.
When fueling is complete, the boomer or receiver pilot (typically the boomer) commands a disconnect. Nozzle/receptacle disengagement might also occur following an automatic pressure disconnect when the receiver's fuel system has been 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) up to 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.
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.
As its name implies, 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 Refueling Limited. 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. Where similar types of fighter aircraft are pod-equipped (compared to large transport aircraft) this is known as "buddy tanking" or "buddy-buddy" refueling, and the HDU is known as a “buddy pod” or “buddy store”.
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 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 push 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.
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 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. Its home base was at Beale AFB in central California, but to make actual reconnaissance missions over enemy territory, it was necessary to deploy the craft to forward bases in Okinawa or in Europe. Hence, there were lots of trans-Pacific and trans-Atlantic flights. Also, for safe takeoff performance, it was necessary for the SR-71 to take off with less-than-full fuel tanks. The SR-71 would then rendezvous with a specially modified KC-135 to top up its tanks. Then the SR-71 was capable of flying for many hours on its own. This tanker variant was necessary because the SR-71 used a special fuel, JP-7, with a very high flash point (needed to withstand the high skin temperatures of Mach 3+ cruising flight) which could not be used in other aircraft engines and the KC-135Q was equipped with a separate internal bladder system to carry and deliver this non-standard fuel.
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.
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 "Operation Black Buck" sorties which involved 14 Victor tankers refueling single Avro Vulcan bombers 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 re-enforcement 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.
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 Iraq-Saudi Arabia 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 EA-3 ELINT 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.
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.
Note: 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. Therefore a less ambiguous meaning for the abbreviation HIFR would be HOVER In-Flight Refueling.
The key commercial benefits are:
On long haul routes where an aircraft cannot carry enough fuel to make the entire journey in one leg, the aircraft must make a 'technical stop' en route, to take on more fuel. Ironically, the fuel is required to descend into an airport, make an approach, and then climb back to cruising height once refuelled is a significant cost in this process, amounting to some 20% of the total fuel used during the flight. At cruising height, relatively little fuel is used per nautical mile. A single air refueling transfer at cruising height saves the fuel used in making a technical stop and the range of the aircraft is extended by several thousand miles. As an example, the London-Sydney route would no longer need a stop in Singapore to refuel.
The total capacity of an aircraft at take-off is a compromise between the weight of the payload and the weight of the fuel required. Often, capacity is further limited by weight and power considerations in order to make a safe take-off on shorter runways or in hot weather. Aerial refuelling allows an aircraft to take off safely with a full payload. Once at cruising height, the aircraft can be refuelled to give it the required range, now carrying a full payload.
Stopping to refuel on a long route not only uses significant fuel, it takes a considerable time too. A typical turn round can last 3-4 hours, especially where passengers are involved. There are significant savings to be made in reducing crew changes, fewer pre-positioned crews, reduced delays and cancellations due to crew duty time, not to mention the benefits to be gained from delivering your payload more quickly. A long-haul flight currently lasting 23 hours (including a 4-hour 'technical stop') could be reduced to just 19 hours, a saving of 18% of the route time.
Technical stops to upload more fuel do not just cost fuel and time. There are a host of associated cost savings to be made. Landing fees, ground handling, and passenger handling could all be negated. Plus, aircraft have a habit of developing faults on turn-rounds.
Fewer refuelling stops en route frees up slots to be used for other purposes, and free up slots currently used for through traffic that could be used for passengers and freight that are intended to terminate at that airport. Short-haul freight aircraft could utilise these slots with a further significant benefit. The actual amounts of fuel required to extend the range of a long-haul aircraft that remains at cruising height is relatively little, and could easily be dispensed from a short-haul dual-role transport aircraft before it continued to carry out its delivery task.
Commercial Air Refuelling could also significantly reduce through-life costs. Airframe life is measured in a number of ways, including number of pressurisations, number of landings and time spent in turbulence at lower altitudes. By removing the need for refuelling stops, the number of pressurisations and landings is halved, and the time in turbulent air is significantly reduced.
Aerial refuelling also contributes to reducing the environmental impact of aviation - less fuel is burnt, fewer overall airframes are needed to achieve a task, and with fewer approaches, there is less noise.
There is no doubt that there would be significant prestige and PR to be gained from flying the first genuinely commercial non-stop route. The first company to achieve this will have a significant advantage over competitors for several months, and if that company also owned the tanker aircraft, could then sell airborne fuel to their competitors.