Introduction To Airborne Refuelling (AP3456 1982)
1. One of the major problems that has always faced designers of aircraft is how to increase range without sacrificing other aspects of performance. If provision is to be made for the carriage of additional fuel then there must be a weight penalty in the basic structure which in turn affects either the height speed. manoeuvrability or take-off and climb performance that can be achieved. One solution to the problem is the use of Air-to-Air Refuelling (AAR). Its adoption by the RAF over recent years has been so rapid that almost all aircraft introduced into service today are given a flight refuelling facility.
2. The primary task of the Tanker Force is the support of aircraft directly committed to the NATO alliance. Priority is given to aircraft operating in th e Air Defence role. For geographical reasons, short range Interception is less important to Great Britain than long range combat air patrols (CAP). clearly AAR provides the endurance required for this concept.
3. Although long range strategic deployments attract publicity. and would seem to epitomise AAR. in fact the priority, in descending order is:
a. Support of Air Defence Aircraft.
b. Support of Interdictor Strike Aircraft.
c. Tactical support of Maritime Operations.
d. Overseas support. deployments and exercises.
However. situations may still occur where the swift deployment of aircraft to counter a distant threat could be a vital factor in military operations. Without AAR the speed of response would be seriously affected.
4. There are two main methods of transferring fuel from the tanker to the receiver aircraft; the Boom, and the Probe and Drogue.
5. The Boom Method.
The boom method was developed by the USAF, mainly for refuelling their bombers, and became operational in 1949. The boom is a telescopic metal tube fitted underneath the tanker fuselage and equipped with flying control surfaces. To make contact the tanker boom operator “flies” the end of the boom into the refuelling receptacle of the receiver formating behind and below the tanker. This method is not in use by the RAF.
6. The Probe and Drogue Method.
The probe and drogue method is a British development and is now in operational service with many air forces, including the RAF. From a power-driven drum the tanker trails a hose at the end of which is the drogue, a ribbed light alloy and nylon collapsible conical basket, the ribbing and shape of the drogue giving it aerodynamic stability and the nylon material keeping the mouth of the drogue open. When fully extended the hose is automatically “balanced” by varying the torque of the motor until it exactly counteracts the drag of the drogue in the slip-stream. The receiver has a probe mounted on the nose of the aircraft or leading edge of the wing and connected to the normal fuel system. The
probe may be fixed or retractable.
7. To make contact the receiver pilot flies his probe into the drogue while overtaking at a speed of 2 to 5 kt. When the probe makes contact with the reception coupling in the drogue, the probe nozzle is gripped by spring-operated toggles which make a fueltight joint and automatically opens the fuel passage between the two aircraft. As the receiver continues to overtake the tanker, the hose is wound onto the drum and prevents it becoming slack and forming a loop. When a few feet of hose have been wound in, the main fuel valve in the tanker opens and fuel is pumped to the receiver. At the same time the lock between the probe nozzle and reception coupling is reinforced hydrostatically
by the flow of fuel. On completion of fuel transfer the receiver pilot breaks contact by closing his throttles slightly dropping back very gently until the hose is fully extended and the probe is pulled out of the reception coupling.
8. Adjacent to the refuelling unit and facing aft, the tanker has a panel of coloured lights (red, amber and green). Amber indicates to the receiver pilot when it is clear to make contact, green indicates that the fuel valve is open and the red light indicates that contact is to be broken immediately or that no attempt is to be made to make contact. If required, refuellings can be made in radio silence solely by reference to these signal lights. Lighting quipment on the tanker, and probe lighting on some receivers, enable the refuelling to be done at night.
9. To give greater flexibility of operation, the size of the probe nozzle and reception coupling have been standardized throughout NATO countries. In addition, the boom method (used mainly by the USAF) can be adapted for use by receivers equipped with probes by attaching a short length of hose and a drogue to the end of the boom.
Equipment Currently in Use in the RAF
10. The Victor tanker fleet has been supplemented by tanker conversions of the VC 10, Tri-Star and Hercules aircraft. Buccaneer squadrons can operate a ‘buddy’ system of AAR. This concept consists of two aircraft, one carrying the military payload, the other the refuelling pod. At about two thirds of the distance to the target, fuel is transferred to the strike aircraft, the tanker returns to base.
11. The actual refuelling unit s themselves are of two types, the hose drum unit (HDU) and the underwing pod. The HDU is mounted at the rear of the fuselage and has a higher rate of flow than the pod. The HDU is primarily intended for use by tanker or transport aircraft but can be used by any type.
AIRCRAFT HDU POD
Buccaneer – One Mk 20
Hercules one Mk 17B –
VC 10 K2/3 one Mk 17B two Mk 32
Victor one Mk 17B two Mk 20
Tri-Star twin Mk 17T two Mk 32
12. The Mk 20B Underwing Pods (Fig 1).
The Mk 20B underwing pod is a self-contained, hydraulically-operated unit requiring only electricity, fuel and air supplied from the tanker. The entire power is provided by a featherable ram air turbine in the nose of the unit which dri ves a two-stage fuel pump and a hydraulic pump. In the centre section is a fuel tank which is replenished from the tanker’s main fuel system as fuel is transferred through the hose. The rear section of the pod contains a hydraulically-powered hose drum unit. A detachable fairing encloses the hose reel and provides storage for the collapsible drogue which folds when the hose is retracted. The hose drum unit carries 51 ft of 1t in bore hose and the unit
can deliver up to 1,250 lb of fuel per minute at a pressure of 50 psi. The complete unit is remotely controlled by the operator from a small panel in the tanker cabin.
13. The Mk 32 Underwing Pod.
This is a development of the Mk 20, requiring only access to fuel and a 28v dc electrical supply. The maximum delivery rate is 2800 lb/min. It has no hydraulic system and is fitted with a quick-response drum take-up, which
eliminates whip, following a fast contact.
14. The Mk 17 Hose Drum Unit (Fig 2).
The Mk 17 hose drum unit is fitted at the rear of the fuselage . The hose drum is powered by an electric motor, th e torque output being controlled by a variable scoop fitted in the fluid flywheel assembly. Fuel is supplied from the tanker’s high capacity booster pumps; the pressure normally being increased by a pump coupled to a bleed-air turbine supplied from the main engine compressors . The hose drum unit carries 80 ft of 3 in bore hose which can deliver up to 4,800 lb of fuel per minute at a pressure of 50 psi. The unit is fully automatic and is remotely controlled by th e operator from a panel in the tanker cabin.
15. While a receiver is approaching the drogue the tanker aircraft should be flown as smoothly as possible because any movement of the aircraft will cause the drogue to oscillate and reduce the chances of a successful contact. It is incumbent upon the tanker captain, therefore, to find the best possible flying conditions conducive to a receiver making contact, ie air space relatively free from cloud or clear air turbulence. When a receiver is in contact, it is usually possible to hold position in all but the most turbulent conditions. At all times however, the tanker should be as stable as possible; turns may be made but they should be made smoothly and at such a rate as to allow the receiver to remain in contact easily.
16. Once a receiver is in contact the tanker captain assumes executive control of both aircraft. On large formations during accompanied flights the lead tanker captain is responsible for the safety aspects of the formation. In the event of a receiver emergency the course of action to be taken by the receivers is determined by the receiver leader.
17. The techniques for successful receiver flying are not difficult to acquire and are well within the capability of the average pilot. Techniques vary slightly but accurate and smooth flying is the basis for success for all types of receiver aircraft. The drogue is approached from behind and slightly below at an overtaking speed of 2 to 5 kt, care being taken in the final stage of the approach not to overcorrect on the controls . If the receiver makes contact too slowly the probe will not engage correctly in the coupling; this is termed a “soft” contact. If an approach is made too fast the drum will not be able to take up the hose quickly enough, the hose will bow and the resulting whip will probably break off the probe nozzle. Terminology used includes a ‘spokes contact’, defined as the receiver probe striking the ribs or canopy of the drogue, possibly causing damage which may cause the drogue to lose aero dynamic
stability. This would render further attempts to make contact difficult or impossible. A ‘rim contact’ is made when the probe makes a hard contact on the rim of the drogue.
18. When at the end of an approach and the receiver is sure of making contact, a small amount of power is applied to counteract the drag from the drogue and to maintain the correct closing speed. Immediately a successful contact has been made and the probe is observed to be positively locked in the drogue, a definite reduction in closing speed should be made by slightly closing the throttles before moving up the refuelling position. The normal refuelling position, and one giving the largest refuelling flight envelope, is when the hose is lying at its normal trail angle with the forward edge of the orange band marked on the hose just entering the mouth of the pod, or the serving carriage of the HDU. In all types of receiver aircraft it is essential that pilots are capable of close formation flying for periods of ten minutes of more .
19. To break contact the throttles are closed slightly and the receiver allowed to drop gently back along the line of the hose with the aim of breaking contact with the drogue in the normal full trail position. In an emergency the throttles are fully closed and,because of the hose drum braking system,contact will be broken almost immediately
with the hose partially wound on the drum.
20. Apart from requiring greater concentration, the techniques for night refuelling are identical to those used by day. Any additional difficulties are due mainly to the inability to judge final closing speeds and the distance of the probe from the drogue owing to lack of outside references. To assist the receiver pilot, the serving carriages are lit and the drogue canopy has beta lights around the rim. The underside of the tanker fuselage and wings are illuminated to provide a horizontal reference. Most receivers also carry lights to illuminate the probe so that it can be seen clearly.
OPERATIONAL USE OF AAR
Methods of Employment
21. Tactical Applications. As stated in para 2, extending the radius of action of aircraft on operational sorties is possibly the most important application of AAR. The developing range of operational requirements means a continuing process in evolving the tactical procedures necessary to make the best use of the available effort. For instance, the role of the air defence tanker (ADT), in support of the fighter on a CAP has increased in importance due to changing
fighter tactics and the need to provide air defence over maritime areas. Method of close support of strike aircraft, such as the Tornado, would change with each particular attack sortie due to the variety and location of targets. Inevitably, such wide variations in operational requirements make flexibility paramount for an efficient Tanker Force. Similarly, detailed tanker Standard Operating Procedures (SOPs) are essential to cover every likely operating condition.
22. Overseas Reinforcement.
There are two ways in which AAR can be used for aircraft deployment: accompanied and unaccompanied flights.
a. Accompanied Flight. On accompanied flights the receivers make a rendezvous with the tanker close to the receivers’ air field of departure. Thereafter the receivers remain in close proximity to the tanker, taking fuel as planned, until they reach their terminal airfield. In normal peacetime operations the refuellings en route are planned so that, if for some reason the receivers are unable to take on fuel, they have sufficient fuel remaining to either
return to their departure airfield or divert to a suitable airfield near track. Accompanied flight is usually employed when the receiver aircraft have poor navigation or communications facilities and/or a short ferry range, eg fighter-type aircraft. After the last refuelling the receivers may leave the tanker and continue to their destination independently. In this way, by flying at their own optimum speeds and heights rather than those of the formation, the receivers’ range may be increased by 50 to 100 nm. This ‘breakaway’ by the receivers may also be used when the tanker is required to land at an airfield other than the receivers’ destination.
b. Unaccompanied Flight.
On unaccompanied flights the receiver makes a rendezvous with the tanker at a convenient point along its track to the destination airfield.For maximum benefit the refuelling should take place at a point as far as possible from the airfield of departure commensurate with aircraft safety. After refuelling, the receiver proceeds alone to its destination. Although this method may be used with long range fighters, it is usually restricted to aircraft with good
navigation and communications facilities and /or a large internal fuel capacity, eg bomber or transport aircraft.
23. There are three main methods of effecting a rendezvous (RV):
a. Pure Navigation Technique .
By this method both the tanker and receiver use their own internal navigation systems to place themselves at a planned position and flight level at a pre-determined time: thereafter depending on visual sighting to make contact.
b. Air Transported Rendezvous Aids.
Information obtained from three types of airborne equipment is used to aid the rendezvous . From a display of range and bearing of the receiver from the tanker, the tanker crew can direct the receiver to within visual sighting range. The equipments used are:
(1) TACAN air-to-air mode in both tanker and receiver provides the range.
(2) Collins UHF D/F carried in the tanker provides relative bearing of the receiver from the tanker by monitoring the receiver’s UHF transmissions.
(3) AI equipment carried by fighter aircraft.
In the event of an equipment failure emergency procedures are available to assist the rendezvous.
c. Ground Controlled Rendezvous.
When the rendezvous position is within ground radar cover, the receiver can be directed onto the tanker using normal GCI techniques.
The basic principle to be observed in the selection of a rendezvous position is that the receiver should be diverted as little as possible from the direct route to its destination.
24. Weather has an important influence on the conduct of AAR operations. In addition to taking account of the weather conditions at the operating and diversion airfields the following factors have to be considered.
a. Rendezvous Weather.
Weather conditions in the actual area of the rendezvous can be critical. Although it is not impossible to make a rendezvous in thin cloud, it is much more difficult and hazardous than in clear conditions. A visual sighting can be made much more easily if the rendezvous is at a height at which contrails are found .
b. En Route Weather.
During accompanied flights IMC can make visual station keeping in large formations very difficult. Although the internal radar of the tankers and receivers can be used to assist station keeping in cloud, such conditions are avoided as far as possible by small deviations in route and /or height. Contacts may be made in cloud but flight in heavy cumulo-nimbus formations or clear air turbulence can result in drogue oscillation and difficulties in making contact.
25. Before a deployment may be undertaken, it must be established that it is feasible. The purpose of the feasibility study is two-fold: to establish that the receiver is capable of undertaking the deployment format and to establish the tanker/receiver ratio required. Many factors are taken into account: the relative performance of the tanker and receiver aircraft, the availability of diversion airfields along the selected routes, the tanker effort allocated to the deployment, the availability of parking space at the staging airfields, and the climatology of the route and the staging and terminal bases. Receivers can normally be classified as high performance aircraft and in many cases the endurance is, apart from fuel limited by the consumption of oil and oxygen. The endurance of the receiver frequently dictates the format of the deployment.
26. When the feasibility study has been completed, it is then possible to calculate the movement table which provides details of the daily movements of tankers and receivers . From this the personnel and equipment needed at en route bases for ground support can be determined.
The Refuelling Plan
27. When the tanker movements have been determined, the full refuelling plan is calculated;
this plan sets out:
a. The positions where fuel is to be transferred
to the receivers.
b. The quantity of fuel to be transferred.
c. The Mandatory Diversion Position (MDP)-this is a geographical position on a receiver’s track associated with a specific refuelling bracket. Should a receiver reach an MDP without the planned transfer, for the appropriate bracket, having commenced; diversion action must be taken. This will allow the receiver to arrive overhead the planned diversion with the minimum fuel reserve.
d. The nominated diversion airfields to be used if a planned fuel transfer fails for any reason or if diversion is necessary for other contingencies.
e. The fuel remaining in the tanker after each transfer, and overhead the planned destination airfield.
28. AAR was introduced into the RAF to assist in strategic deployment about the world. As the threat changes and our global commitments shrink, so the role of AAR changes. The term ‘force multiplyer’ has been applied to AAR and perhaps that sums up the modern concept of air-to-air refuelling. By this relatively simple technique the effectiveness of our limited forces can be increased considerably.