Years ago, while working on the manufacture of an in-flight refuelling aircraft, someone made a comment that has stuck with me ever since: “The two most difficult actions that can be performed with two aircraft in flight are: on the one hand, crawling out of the window of one aircraft, jumping onto the wing of the other aircraft, and crawling in through one of its windows; on the other hand, refuelling in flight.” This comparison was made as a joke, but it still reflects the huge technical and operational complexity involved in transferring fuel while in the air. Ironically, and as incredible as it may seem, the first in-flight refuelling event actually combined both situations. On 12th November 1921, Wesley May, flying on a Lincoln Standard biplane piloted by Frank Hawks, grabbed a fuel can, climbed onto a Curtiss JN-4 that was flying above him piloted by Earl Daugherty, and managed to pour 18 litres of fuel into its tank. It was the fascinating era of aviation pioneers, and that achievement was just one of many bold acrobatics that astonished the general public. The first in-flight refuelling with real operational interest was carried out shortly afterwards by the US army, on 23rd June 1923, using a gravity-feed fuel hose system.
In-flight refuelling is one of the most difficult and risky aeronautical operations, both technically and operationally.
In general, all aircraft need fuel for propulsion. This is usually loaded and then almost fully consumed over the course of the journey. At the departure airport, fuel tanks are usually filled with as little fuel as possible in order to reach the destination safely, since the lighter the aircraft, the lower the operating costs. The amount of fuel required is calculated by multiplying the journey length by the average consumption per km, and adding a small percentage as a safety margin for contingencies. This is the procedure on most civilian flights, as both the destination and consumption levels are usually known beforehand. However, in the military field there are other types of requirements, such as patrol flights, where the aircraft must remain in flight for as long as possible without returning to base, or missions in which the aircraft’s range has to be extended beyond the capacity of its fuel tanks. This type of operation requires the design of an in-flight refuelling system.
Refuelling procedures always involve two roles: the aircraft that provides the fuel, which is called a tanker, and the aircraft that receives it. The second aircraft is usually smaller in size; however, this is not always the case, as it may also be a tanker, or even a larger aircraft like a bomber.
Generally, aircraft carry fuel inside their wings. This helps to keep the centre of mass as balanced as possible to ensure good airworthiness. In addition, as it is a fluid, in order to control load distribution, the fuel must be distributed across several interconnected but independent tanks. The fuel transferred by the tankers can either come from the same fuel tanks they themselves are using, or the tanker can incorporate additional fuel tanks, which are usually located in the cargo hold.
There are two different methods for dispensing fuel:
– Pod, hose and basket: this system involves aerodynamic containers on the outside of the aircraft called pods. When activated by the crew, they release flexible hoses that hang freely, trailing behind the aircraft. The hoses end in baskets that provide aerodynamic stability and serve as a funnel that helps the process of coupling with a probe or boom on the receiver aircraft. For aerodynamic reasons, this probe can be retractable, and only deployed for the refuelling process.
There are traffic lights (i.e. amber, red and green lights) on the pods that communicate the refuelling phase. There is usually a pod on each wing, allowing the refuelling of two aircraft simultaneously, and there can even be a third hose hanging from the central fuselage. The length of the hose can vary between 15 and 30 metres, and the fuel flows at a rate of between 1500 and 2000 lbs per minute. Longer hoses mean less aerodynamic impact suffered by the receiving vessel; however, the rate of transfer is also reduced.
This refuelling system is common for small aircraft such as fighters and helicopters, which are also the most frequent recipients of in-flight refuelling.
– Boom: a rigid, telescopic tube attached to the rear of the central fuselage. In order to begin operating, it is extended downwards, like a compass opening up. In this position, it can also be extended or retracted. Its larger size means that it suffers from aerodynamic forces to the extent that it needs control surfaces in the form of fins (either fixed or mobile), almost like a mini-aeroplane. Its greater complexity means that it requires a specific operator dedicated to controlling its movement and action, which increases operational costs.
The telescopic part of the boom is inserted into a hole called a UARRSI (Universal Air Refuelling Receptacle Slipway Installation) in the receiving vessel. The coupling is a very delicate operation due to the danger of damaging the rigid components of both aircraft.
Only one boom is available per aircraft, which means only one aircraft can be refuelled at a time. The boom is shorter than the hoses, between 10 and 15 metres long, and wider, which allows faster transfer rates, about 6000 lbs per minute.
This system is commonly used for large aircraft such as passenger aircraft, patrol aircraft, bombers, etc. It is also used for fighters, although they cannot take full advantage of it, as they can only receive fuel at a rate of around 3000 lbs per minute. However, this system cannot be used for helicopters.
Now that both systems have been explained, it should be noted that they are compatible, and they are very often combined.
In terms of their shape and other systems, tankers do not differ much from normal medium-sized aircraft. In fact, existing programmes are often modified, to a greater or lesser extent, in order to provide the aircraft with a refuelling function. Sometimes, tankers are even made by modifying previously built aircraft.
Here are a few examples of some of the most famous tankers:
– The first tanker to carry a boom system was the Boeing KC-135 Stratotanker, created from the Boeing 367-80 (similar in appearance to the 707) for the US Army in the 1950s. It was so successful that it is still in use today.
– In the 1980s, McDonnel Douglas created the KC-10, which was based on the DC 10-30. Sixty aircraft were manufactured, which are also still in service.
– Recent stand-outs include the MRTT (Multi Role Tanker Transport), created by Airbus and based on the A330-200 passenger aircraft. It is now being exported to Europe, Australia and the United Arab Emirates.
– The latest tanker to arrive on the scene – with the first unit being delivered in January 2019 – was the KC-46A model. Boeing designed it based on the 767, and almost 200 units have already been contracted, which the USAF intends to use to renew its fleet of tankers. There is therefore now competition on the current market between the MRTT and the KC-46A. Let’s hope that this competition encourages new developments and innovations, as is so often the case in aeronautics.