Overview

Aerial refueling is the process of transferring fuel from one aircraft to another aircraft during flight. It is also known as air-to-air refueling (AAR), in-flight refueling (IFR), tanking and air refueling. Two Airco DH-4B biplanes of the US Army Air Service were refueled for the first time in mid-air on June 27, 1923.

In August 1923, three DH-4Bs (two tankers and one receiver) set a record, in which the receiver aircraft remained in the air for more than 37 hours to transfer 38 gallons of engine oil and 687 gallons of aviation gasoline. The brothers Walter, Albert, Kenneth and John Hunter set a new record of 553 hours 40 minutes using two Stinson SM-1 Detroiters over Chicago, in July 1930. In July 1935, the brothers Fred and AI key broke a previous record of Hunter brothers, and set a new record of 647.5 hours (nearly 27 days in air) in the Curtiss Robin monoplane Greater St. Louis, (whose) refueling nozzle was designed by the A. D. Hunter.

Systems

There are two different methods used for aerial refueling, the probe and drogue system and the flying boom system (also known as boom and receptacle).

1. Probe and Drogue

Probe and Drogue system uses a flexible hose trailing from the tanker aircraft. The drogue is fitting; resembling a shuttlecock (or windsock) that is attached to its narrow end with a valve (present at the end of the probe) to a flexible hose. It provides a funnel to assist insertion of the receiver aircraft probe into the hose as well as stabilizes the hose in flight. The receiver has a probe that is located on the aircraft fuselage or nose. This probe is often pulled back when not in use, mostly on high speed aircraft.

All the US Marine and Navy Corps aircraft and US Air Force helicopters uses a probe-and-drogue system for refueling. This system was first used in service on KB-29M Superfortress model. On May 29, 1952, it was used for the first time in combat to refuel twelve F-84s aircraft during a mission from North Korea, Itazuke and from Japan to Sariwon.

The hose connects to a HDU (Hose Drum Unit), and if it is not in use then, the drogue/hose is rolled completely into it. Valve is closed until it mates with the drogue, after mating it opens and allows transferring of fuel from a tanker to a receiver. Valves were originally developed by the UK Flight Refueling Limited company and were of NATO (National Atlantic Treaty Organization) standard, deployed in the late 1940s and 1950s.

Airbus A330 MRTT, Airbus A310 MRTT, Airbus A440M, Boeing 707, KB-29M, KB-50, Lockheed L-1011 Tristar, A-4 Skyhawk, Tu-16N, Su-33, MiG-29K etc. are the (a) few examples of tanker aircrafts using probe and drogue system.

Functioning

• The tanker plane flies straight and extends the drogue/hose that trailed below and behind the tanker plane (in usual aerodynamic forces).
• The pilot of the receiver plane extends his probe (only if required) and uses normal flight controls to insert the refueling probe directly into the drogue.

Advantages

• Tanker design is cheaper and simpler.
• It can be used for refueling properly equipped helicopters, for instance, MH-53E Sea Dragon.
• Any aircraft that is not originally designed as a tanker can be converted to tanker aircraft by attaching a refueling pod.
• No boom operator is needed for the refueling as the drogue can be operated by the pilot of the tanker.
• Tankers can be equipped with multipoint hose-and-drogue systems allowing two or more aircraft(s) to refuel simultaneously from the same tanker.

Disadvantages

• The lower flow rate of 1,500–2,000 lbs/min is available from the lower pressure.
• Hose diameter is limited that result in longer refueling times.
• Drogue is subject to aerodynamic forces and turbulence of the receiver aircraft.
• Drogue may get damaged due to poor receiver technique.

2. Flying Boom

Flying Boom is a refueling system that could transfer fuel at a higher rate than the earlier systems using flexible hoses. It is named so because small movable airfoils and flight control surfaces are used to move the boom by creating aerodynamic forces. They are actuated hydraulically and controlled by the system operator using a control stick. Boeing developed the world’s first aerial tanker KC-97 Stratotanker with a flying boom system and extra jet fuel (kerosene) tanks feeding the boom. After development of KC-97, Boeing received the contract to build jet tankers based on the Boeing 367-80 aircrafts from USAF (United States Air Force), which resulted in Boeing KC-135 Stratotanker.

Mechanical limits of the booms are based on the length of the extendable inner fuel tube, the maximum deflection of the flexible nozzle and the structure of the trunnion and yoke system mounting the boom to the tanker. If these limits are exceeded then, it can break structural parts mounting the boom to the tanker.

Flying boom system is used by the US Air Force, Turkey (ex-USAF KC-135R), Israel (modified Boeing 707) and Netherlands (KDC-10). The Islamic Republic of Iran Air Force (IRIAF) 747’s equipped with three drogues and a single boom is the largest tanker aircraft. Both Italy and Japan have signed a deal with Boeing for tankers aircraft based on the B767. KC-135 Stratotanker, KC-10 Extender, KC-767, KB-50, KB-29P etc are the (a) few examples of tankers aircraft using the flying boom system.

Functioning of flying boom

• It is attached to the rear of the tanker aircraft, which allows the boom to move with the receiver aircraft that contains a rigid pipe to transfer fuel.
• Nozzle with a flexible ball joint situated at the end of fuel pipe, mates to the receptacle of receiver aircraft during fuel transfer. • A poppet valve prevents fuel from exiting the tube until the nozzle properly mates with the receptacle of the receiver. Once properly mated (,) toggles locked the nozzle during fuel transfer.

Advantages

• Higher fuel flow rates up to 1000 US gallons/6,500 lb per minute for the KC-135 tanker.
• Large diameter of the pipe in the flying boom that results in less refueling time.
• It eliminates the requirement for the receiver aircraft pilot to precisely fly a probe into a drogue.
• Tanker aircraft with a flying boom can be outfitted with an adapter for aircraft with probe system.

Disadvantages

• Tanker design is complex.
• Cannot be used for refueling the helicopters.
• Cost to train the boom operator is high.
• Only one aircraft can be refueled at a time.

Movies

• The movie “The Final Countdown” has a genuine scene of a probe and drogue operation between F-14 Tomcats and US Navy KA-6 Intruder from USS Nimitz (CVN-68).
• The movie “Dr. Strangelove” has an authentic scene of a flying boom operation between B-52 Stratofortress and USAF KC-135 Stratotanker in turbulent air.
• The movie “The Sum of All Fears” has a true-life scene of a mid-air refueling sequence with the E-4B NEACP.
• The movie “The Perfect Storm” has real-life scene where an HH-60 helicopter tries to refuel during bad weather, but was unable to make it.
• The movie “Stealth” has genuine scene of destruction of refueling station.