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IN-FLIGHT REFUELLING TECHNOLOGY

In-flight refuelling is very much a necessity during extended patrols and long-distance ferry flights or on military missions over hostile territory which cannot be approached via a direct route due to the enemy presence. The need to refuel applies not only to strategic bombers and transporters, including tanker aircraft themselves, but also to tactical fighter aircraft and helicopters.

The history of air-to-air refuelling has seen a variety of technical solutions, two of which are predominantly used today: the flexible drogue trailed from a tanker into which the receiving aircraft thrusts a probe ("probe and drogue") and the rigid boom. A third variant, a wing-to-wing hose refuelling system, has only been used by Russian strategic bomber forces and will therefore not be discussed further in this article.

The probe and drogue system consists essentially of a hose with a metal basket at the end that is connected to the tanker aircraft's transferable fuel tanks. The rim of the basket is made from a strong fabric and is designed to inflate in the air stream, forming a wide cone. At the centre of the drogue is a hose coupling with a quick-acting locking mechanism, into which the approaching pilot of the aircraft to be refuelled thrusts the tip of his probe. Once contact has been made, depending on the particular system, between 700 and 2,600 kg of fuel flow into his tanks per minute. In this way, it normally takes only a few minutes to refuel the aircraft.

If any problems occur during refuelling, it is naturally possible to disconnect the system rapidly and without complications. Otherwise, the heavy hose and drogue trail behind the tanker in a relatively stable manner, so that after making visual contact the pilot of the following aircraft can fly up to the tanker, adjust his speed to that of the tanker and lock on to it, all in a matter of seconds. The use of special positioning lights makes the procedure a lot easier.

The drogue system is based on an invention by the Englishman Alan Cobham who back in the 1930s had developed a system known as the "cross-over" system whereby the approaching pilot had to catch a trailing fuel hose and draw it towards him using a kind of harpoon. The apparatus, which seems somewhat complicated by today's standards, was viewed with interest by the American forces, but due to the outbreak of war it was not implemented. Only after the Second World War did the US Air Force introduce the system, converting a hundred B-29 bombers fitted with the system into KB-29M tankers. However, the need for entire crews of tankers and bombers made the cross-over method unsuitable for smaller aircraft.

Cobham now went on to develop the probe and drogue system, in which the receiving aircraft locked its probe, which was initially attached either to the nose of the fuselage or on the wingtip, onto the drogue. Around 300 kg of fuel could be transferred per minute at that time, and the introduction of this system extended the range of American tactical fighter aircraft by an order of magnitude overnight.

Two different versions of this technique continue to be employed today. Under the first variant, one or more hoses fitted with a drogue are located together with the associated drive, switching mechanisms and pumps in the aft section of the tanker fuselage and are operated and controlled there by an operator. In the early days, the operator lay face down on a kind of stretcher and controlled the system with levers while reading off, for example, the fuel flow rate from indicators within his field of view. With today's more modern systems the operators sit in comfortable seats and can monitor the operation, sometimes even with the aid of a remote-controlled camera.

The second variant makes use of special pods which look very similar to auxiliary tanks and are generally suspended beneath the wings of the tanker. The system originally invented by Douglas and known as D-704 was refined by the American Sargent Fletcher company, which today is part of the Flight Refuelling Division of Cobham plc. These pods are available in various designs, but they all work on the same principle. A small propeller at the tip drives the fuel pump, and the fuel is pumped out of fuel lines running from the wings or the fuselage directly into the hose, which in turn is accommodated in the pod along with the drogue and hose reel. After refuelling, the hose is wound up again using a motor, and the drogue is drawn back into the pod. These relatively small, light and compact containers even permit smaller aircraft to refuel each other, as, for example, has been practised on German Tornados during prolonged flights.

Whereas it was only a few years ago that fuel-pump nozzles with rapid, automatic cut-off first appeared at filling stations, this technology has been in use ever since probe and drogue in-flight refuelling was first introduced. This is essential since neither the receiving aircraft nor the tanker can tell on the basis of the instruments available to them exactly when the tank is really full so that refuelling needs to be terminated. Sensors in the system are therefore used to report when the tank is full and automatically shut the valve, following which the operator issues the disconnect command to the receiving pilot.

However, due to its relatively low flow rate, the hose system also has a major disadvantage: larger aircraft, such as strategic transporters or bombers, would need to spend too long connected up to the tanker if they were to use this method. For this reason, Boeing developed a special rigid boom, which can be steered via two V-shaped stabilising control surfaces and has a telescopic, sprung pipe fitted with a fuel nozzle at the end. With this system, which is only in service with the US Air Force and was used for the first time with KC-97 tankers, up to three-and-a-half tonnes of fuel can be transferred per minute, reducing the length of the operation considerably.

With this refuelling variant the role of the boom operator becomes quite critical, as the boom moves out of the pilot's field of view during the final stages of aligning his aircraft. From this point onwards all further adjustment of the aircraft's position must be executed relying on commands issued by the boom operator until the latter can insert the "sting" into the special receptacle on the receiving aircraft, which is normally on the upper side of the fuselage.

These manoeuvres are always a little risky and have to be rehearsed by the crew at length and extremely thoroughly on a simulator before performing them for real. There have been incidents in which probes have been damaged due to clumsy manoeuvring on the part of the pilot or under the influence of adverse weather conditions, and this of course can be a major problem for other aircraft waiting to be refuelled. There have even been occasional crashes caused by aircraft colliding with the boom. Nevertheless, there is no sign of any alternative to the system currently employed emerging in the foreseeable future.

From page 80 of FLUG REVUE 11/2000