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China - Aerial Refueling

The combination of long distances and the high vulnerability of forward air bases will require that most forward air defense (and strike) assets operate from bases further back in wartime. This, in turn, will create a high demand for aerial refueling capacities in order to keep sufficient aircraft on station. Chinese aerial refueling assets operating over the East China Sea would make employment of shorter-ranged tactical strike aircraft feasible. Chinas air refueling fleet, which now consists of one confirmed IL-78 tanker and 12 H-6U tankers, is still too small to support sustained, large-scale, long-distance air combat. China will address the size of its fleet with more acquisitions over the next decade.

Most of Chinas fighter and attack aircraft lack the combat range to conduct air operations in the Philippine Sea and the southern reaches of the South China Sea. Until the PLA Navys first carrier-based aviation wing becomes operational, China must use air refueling tankers to enable air operations at these distances from China.

Satellite imagery from mid-October confirms China has taken delivery of one Ilyushin IL-78/MIDAS air refueling tanker it ordered from Ukraine. China reportedly purchased three IL-78 tankers from Ukraine in 2011. The plane is the first modern addition to Chinas small and outdated fleet of air refueling aircraft, which consists of about 12 modified BADGER bombers designated H-6U. For comparison, the U.S. Air Force operates 473 tanker aircraft.2 The IL-78 carries two to three times as much transferable fuel as the H-6U tanker. Like the H-6U, the IL-78 features a probe-and-drogue refueling apparatus that allows it to refuel multiple small aircraft simultaneously.

Chinas paucity of air refueling tankers and their limited capacity to offload fuel could require Chinas bombers to conduct long-range strike missions without fighter escorts, potentially decreasing their effectiveness in some regional strike missions. China lacks adequate support infrastructure on the ground, and most of Chinas fighter aircraft cannot refuel in the air. The PLA Air Force will need to upgrade its infrastructure and modernize the rest of its aircraft to take advantage of its expanding air refueling fleet. China may build a large number of new tankers using the Y20 transport aircrafts airframe when it becomes available.

Two aerial refueling systems are in general use by air services of the world. The probe and drogue system is used by most air forces, including the U.S. Navy. The U.S. Air Force, which has the largest number of aerial tankers, uses an extendible boom system which is not readily adapted to probe and drogue techniques.

In a typical extendible boom aerial refueling system an aerial refueling tanker incorporates a boom which extends outwardly and downwardly from the rear of the tanker. The boom is controlled by an operator in the tanker who can guide a nozzle on the end of the boom into engagement with a receptacle in the receiver aircraft. Conventional methods for performing aerial refueling operations between a tanker aircraft and a receiver aircraft involve the use of a fuel line in the form of a boom extending downwardly and rearwardly from the tanker, means positioned on the boom, controllable from within the tanker, for maneuvering the boom into a position appropriate for coupling with a receiver aircraft, and a coupler at the end of the refueling boom for interconnection with a fuel receptacle on the receiver aircraft.

A receiver aircraft, prior to establishing a suitable position for receiving fuel from the tanker, must first locate the tanker by such means as radar or homing devices as are known in the art. Once the tanker is located, the receiver aircraft then must assume and maintain a position immediately behind and below the tanker within the so-called "refueling envelope", which is defined as that volume of space behind the tanker the allowable limits within which the refueling boom may be maneuvered horizontally and vertically and extended or retracted without imposing undesirable stress on the refueling boom. The receiver aircraft ordinarily must rely on visual contact with the tanker from a point several hundred yards (depending on lighting and weather conditions) from the tanker to the proper position within the refueling envelope. Of particular importance and difficulty are the delicate maneuvers, and the attendant careful coordination between boom operator and receiver aircraft pilot, required at the precise point of hookup and in maintaining the appropriate relative positions of tanker and receiver aircraft during refueling. This operation, although requiring a high degree of skill by the boom operator and receiver aircraft pilot, is relatively routine during daylight and favorable weather conditions, but with existing methods, aerial refueling operations performed at night or under other limited visibility conditions, is extremely difficult and hazardous.

After the boom operator has guided the boom nozzle into the receiver aircraft receptacle, latches within the receptacle are shifted by hydraulic actuators to lock the nozzle into the receptacle. Such actuators are operated automatically or by personnel in the receiver aircraft. Once the nozzle is engaged in the receptacle, then fuel can be transferred from the tanker to the receiver or, in some cases, when the tanker is low on fuel, fuel is transferred from the receiver to the tanker using receiver boost pumps.

Occasionally it is necessary to make an emergency disconnection between the boom nozzle and the receptacle in the receiver aircraft. One reason for making such a disconnection is when there is a component failure within either the receptacle or within the tanker equipment. A second reason for making such an emergency disconnection is when the receiver aircraft wanders outside of the safe operating range causing either the nozzle to be broken off from the boom, or completely breaking the boom from the tanker aircraft with corresponding hazards occurring.

At the present time, there are three methods for effecting a disconnection between the boom nozzle and the receiver aircraft receptacle. The normal method consists of personnel in the receiver aircraft actuating a control valve therein that causes the hydraulic actuator to rotate the latches in the receptacle so that they no longer engage the boom nozzle. In the second method, the receiver latch release mechanism is triggered by the boom operator in the tanker aircraft by means of an electrical signal transmitted through communications coils at the nozzle/receptacle interface. The third method is called "Brute Force" which requires decreasilng the speed of the receiver aircraft sufficiently relative to the refueling tanker either by reducing its power or by extending flaps or other drag inducing devices to put sufficient force on the nozzle and, in turn, on the latches of the receptacle to overpower the hydraulic pressure which controls the latches. The first two methods have proved to be not sufficiently reliable and the third method is somewhat dangerous and can cause aircraft damage.

In a hose and drogue system, the drogue is attached to the outlet end of a hoses The inlet end of the hose is attached to a reel onto which the hose is wound. The reel is typically mounted either within the tanker aircraft's fuselage or on a refueling pod or module which is attached to the bottom of the tanker aircraft's fuselage. When the hose is deployed, the outlet end of the hose, with its attached drogue, extends behind the tanker aircraft. Depending upon the combinations of tanker and receiver aircraft and the specifications of the particular refueling system used, the total length of the hose may be 50 feet or more, and the drogue is in a preferred refueling range when it is extending about 30 feet from the reel.

When the hose and drogue are in the fully extended position (with several turns of hose still remaining on the reel), the pilot of the aircraft to be refueled maneuvers his or her aircraft into a position such that the refueling probe of the receiver aircraft enters into and engages with the drogue. The pilot continues to urge the receiver aircraft forward relative to the tanker aircraft until the drogue is in the refueling range. As the receiver aircraft is moving forward, the hose is retracted onto the reel to take up the slack in the hose. A refueling range marker is disposed on a predetermined portion of the hose. When the pilot of the receiver aircraft sees the refueling range marker reenter the tanker aircraft's fuselage or refueling pod, the receiver aircraft's pilot knows that the drogue, engaged with the receiver aircraft's probe, is in the refueling range. When the engaged drogue and probe are in the refueling range, fuel is pumped from the tanker aircraft to the receiver aircraft. After refueling is completed, the pilot of the receiver aircraft reduces its speed relative to the tanker aircraft.

The development of drogues for inflight refueling of aircraft has been primarily directed by the need to stabilize a refueling hose trailing from a tanker aircraft in a generally horizontal attitude. The drogue provides drag for the refueling coupling, which must resist the forward movement of a probe from a refueling aircraft. Cone shaped drogues have proved to be suitable for low speed inflight refueling; however, it has been found that such drogues can become highly unstable at higher speeds, causing the refueling coupling device at the end of the refueling hose to become an oscillating, bobbing target, and occasionally causing the structure of the cone shaped drogue to collapse.

The high amount of drag provided by a drogue for low speed refueling also can cause the trail angle of the refueling hose and coupling to become flattened. It is desirable that the refueling aircraft not follow directly in the wake of the tanker aircraft, so that the configuration of a drogue which may be appropriate for low air speed refueling can be inappropriate for high speed refueling, even if the motion of the refueling coupling can be stabilized by aerodynamic designs.

It has therefore been a common practice to install either a low or high speed configuration drogue on the refueling coupling of a tanker aircraft on the ground, depending upon whether the aerial refueling is to take place at low or high speed, and to land to change the drogue to one of another speed configuration when an aircraft needs to be refueled at a different speed. Another approach has been to fabricate the ring shaped parachute canopy of the drogue of elastic material with openings which permit pressure modulation to provide for a constant amount of drag over varying speeds. However, such elastic openings have been found to be subject to deterioration, which can cause the amount of drag provided by the drogue to decrease over time, and can also cause an asymmetrical drag configuration to develop.



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Page last modified: 01-03-2020 18:46:56 ZULU