Air-to-air refuelling stands as one of the key force multipliers which allows projecting military power far beyond their national boundaries. Because the receiver aircraft can be topped up with extra fuel in the air, air refuelling 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 refuelling once airborne.
For today’s short combat radius platforms in particular, the growing frequency of sorties flown at ever-longer distances away from carriers or ground bases has increasingly made refuelling an essential mission component. The requirement for air-to-air refuelling is likely to increase in the future as there is a greater requirement for persistence. Intelligence, Surveillance, and Reconnaissance (ISR) and Command and Control (C2) assets, for example, are increasingly likely to demand air-to-air refuelling services to enhance situational awareness. The Falklands conflict of 1982 highlighted the necessity for air-to-air refuelling, particularly for the successful prosecution of an air war at long range.
Of all air power force multipliers, air-to-air refuelling is amongst the most significant. It provides an essential capability that increases the range, endurance, payload and flexibility of all capable receiver aircraft, and is especially important when forward deployment is limited or unavailable, or air base operations limitations impose constraints. “To support overseas deployments anywhere in the world at short notice air-to-air refuelling is only becoming more important,” says James Kemmitt, product management director at Cobham Mission Systems – one of the world’s leading refuelling technology suppliers.
Two approaches are currently used for aerial refuelling: the flying boom, where a retractable long thin tube extends from the tanker into a receptacle on the receiving aircraft. The boom is what will run the fuel from the tanker to the receiver.
A slightly easier aerial refuelling method is the drogue probe, a drogue fire hose with a parachute on the end comes out of the tanker, while the receiver extends a thin probe into the parachute, like a bullseye. The probe and drogue is simpler to adapt to existing aircraft, and the flying boom, which offers faster fuel transfer but requires a dedicated boom operator station.
A colourful history
The first airborne refuelling occurred in 1921 between two biplanes. With a five-gallon fuel canister and at an altitude of some 1,000 feet, Wesley May worked his way down the right wing of a plane flown by Frank Hawks; he then climbed onto the left wing of another plane and eventually poured the fuel into its gas tank. Although an impressive stunt, it did not obviously represent a practical way to refuel while airborne.
On June 27, 1923, above Rockwell Field in San Diego, the US Army Air Service used two Airco DH-4B biplanes to try out a less extreme approach. The refuelling aircraft a DH-4B carrying Lieutenants Virgil Hine and Frank Seifert passed gasoline through a hose to another DH-4B flying beneath it carrying Lieutenants Lowell Smith and John Richter.
A compelling practical reason to carry out aerial refuelling was slow to emerge, and early demonstrations became little more than record setting attempts.
On January 1, 1929, a six-man crew commanded by US Army Air Corps Major Carl Spaatz took off in a modified Fokker C-2A called Question Mark. The airplane bore the symbol as the team’s answer to the question: “How long do you plan to stay up?”. On January 7 the answer proved to be 150 hr. 40 min.
Alan Cobham, the British pioneer of long-distance flying, spent much of the 1930s developing systems intended to permit long-distance nonstop commercial flights. He would go on to establish Flight Refuelling Ltd (FRL) as a company in 1934. His early experiments were conducted using the Airspeed AS.5 Courier, before he developed the “looped-hose” gravity-feed method in the mid-1930s and created a system of connectors that increased safety and made refuelling a more easily repeatable process.
However, the loop-hose system proved unwieldy and difficult, particularly in bad weather. With a two-and-a-half-inch diameter refuelling hose, the FRL developed system transferred fuel at a rate of only 110 gallons per minute. With new high-speed, high-altitude jet bombers coming online, capable of operating at night and in bad weather, it quickly became apparent something better was needed.
Interestingly enough, in the 1940s Boeing already had a better system in mind. The company developed a “flying boom,” which featured a telescoping pipe with fins at the nozzle end. The fins were termed “ruddervators” because they functioned as both rudders and elevators. The boom operator, sitting in the B-29’s converted tail turret, literally flew the boom into a receptacle on the upper fuselage of the receiver aircraft. This design allowed more positive control of the air-to-air refuelling operation and with the boom’s four-inch diameter, it offered much faster fuel transfer.
Cobham developed a new system, which went into trials with the Royal Air Force, and on August 7, 1949, was used during the setting of a new endurance record for a jet aircraft when a Meteor F.3 was refuelled from an Avro Lancaster. The connectors used the new “probe and drogue” system.
FRL’s refuelling equipment was modified by Boeing for the US Strategic Air Command, which saw the flexible hose replaced by a boom with rudders that could be “flown” by the operator. Initially developed to permit refuelling at altitude, with operators in pressurized cabins, the system also increased the fuel transfer rate. It was used on the Boeing KC-97 Stratofreighter, the first production tanker aircraft and the concept remains in use by the US Air Force, RAAF and the air forces of Iran, Israel, the Netherlands and Turkey.
A typical air-to-air refuelling mission starts with a pre-mission brief where crews are updated on the mission profile and what is expected from the sortie.
From there, crews conduct individual crew briefs, complete the flight authorisation process and then ‘step’ to the jet for departure.
Mission exercises like Cope North and Talisman Sabre are vital in the learning of air-to-air refuelling.
Flying Officer Delchau, an Air Refuelling Officer from Number 33 Squadron, said “Exercise Talisman Sabre has been a great learning experience.
“Assisting with mission planning, understanding the tactical air picture and communicating with other squadrons and their crew on different aircraft has been challenging, but rewarding.“The exercise taught me a lot about operating with foreign military aircraft, their expectations and how a combined air war would be coordinated with dislocated units.
“The role of air mobility officers extends far beyond operating the air-to-air refuelling systems.
FLGOFF Delchau, however, agreed the most heart-racing part of his day job was doing a refuel flight – the last few seconds before directing the boom into contact with a receiver aircraft.
“If you blink, all of a sudden there could be four jets on our wing and another four checking in to get gas and that takes a lot of coordination.”
Flight Lieutenant Dunn who was part of Exercise Cope North 21 says “The KC-30A is usually one of the first jets to enter the airspace and one of the last to leave,”.
“Fully integrated in the planning process, the tanker planner helps shape the mission profile by working out fuel requirements, tanker availability and any limitations, and then comes up with the airborne refuelling plan,” FLTLT Dunn said.
“All of our co-pilots cycled through the position – it was a big learning curve for them and a great opportunity for their professional development.
“They’ve begun to understand how we integrate with partner forces, how a typical planning cycle works for a large force employment mission, and what great capabilities the KC-30A brings to the table.”
“We also had two junior air refuelling operators who achieved their day fighter boom qualification and one aircrew member who achieved his B Category KC-30A captain categorisation.”
“We’d expect to be airborne for approximately four hours, refuel a combination of US F-35A Lightning IIs, F/A-18 Hornets, F-16 Fighting Falcons and EA-18G Growlers, and offload around 45,000 litres of fuel.
“We’d also expect to hold contingency fuel for any issues that the fighters may have during the sortie or on recovery to Andersen Air Force Base.
“Once all fighters and other air assets are safely on their way home, we’ll recover as well, and when we’re back on the ground, our maintainers refuel the aircraft, conduct any maintenance required and have the tanker ready to launch for the next wave.”
By the end of the exercise, callsign ‘Willowbank’ had flown a total of 17 missions and offloaded 318,000 litres of fuel to US fighter aircraft.
Although pilots and boom operators undergo rigorous refuelling training, it nevertheless remains stressful. To that end, both Airbus and Boeing have worked on automatising the boom operation. Peter Thomas, senior lecturer in Aerospace Engineering at the University of Hertfordshire in England, and a specialist on air-to-air refuelling of unmanned aerial vehicles, says that “if you want to automate the system you must outperform what a human can do, which is using his eyes to gage where the boom must go and couple up with the receiver.”
In April 2020 Airbus successfully completed the world’s first fully automatic air-to-air refuelling operation with 120 dry fuelling contacts with a Portuguese Air Force F-16 fighter jet which needed no modifications at all to use the system. (A dry contact means they don’t actually transfer fuel.)
The tech allows the boom operator to simply activate the system and then supervise while the computer takes control of the boom, maintaining the right angle and distance to fly it to within centimetres of the receiver aircraft. The boom’s telescope is then extended into the receiver, the fuel transferred, then automatically disconnected and the boom brought away.
More recently history was made when the U.S. Navy conducted its first ever air to air refuel between a manned aircraft and an unmanned tanker on June 4 2021, with a Boeing-owned MQ-25 Stingray test vehicle performing its first mid-air tanking mission with a Navy F/A-18E-F Super Hornet.
The test mission out of MidAmerica Airport in Mascoutah, Illinois, proved the unmanned tanker could successfully use the Navy’s standard probe-and-drogue aerial refuelling method.
During the initial part of the flight, the F/A-18 test pilot flew in close formation behind MQ-25 to ensure performance and stability prior to refuelling. The manoeuvre required as little as 20 feet of separation between the two aircraft, both flying at operationally relevant speeds and altitudes. When the MQ-25 drogue was extended, the F/A-18 pilot moved in to “plug” with the unmanned aircraft and receive the scheduled fuel offload.
Future of AAR
From remote vision systems to drones, aerial refuelling technology has made leaps in recent years. If perfected, it could have a lasting impact on the future of military aviation.
From Vietnam through to the Falklands and Iraq, aerial refuelling has been used by air forces in conflict for well over half a century.
Tankers continue to play an important role in serving bombers, fighter jets and, more recently, drones. According to market intelligence firm Stratview Research, the global aerial refuelling systems market could reach a value of $671.9m by 2024, owing to increasing demand and more funds being allocated to their development.
Recent developments around the world suggest that going unmanned is a primary goal for new refuelling technology. Flying tankers in perfect sync with a receptacle aircraft at hundreds of miles of an hour – not to mention hooking up pipelines with highly combustible fuels – requires vast reserves of precision and patience.
If perfected, unmanned or autonomous refuelling solutions could help take the pressure off pilots and human boom operators, as well as reducing risk to personnel involved.