How Airplanes Refuel in the Air?

When your car runs out of fuel, the first thought that comes to your mind is to rush to the nearest gas station. But if you’re an F-22 Raptor, who’s running low on fuel at 40,000ft, what do you do? Obviously, you would not nose dive the jet 90 degrees and try to glide it to the nearest airport and refuel it. You would refuel it mid-air, yes, mid-air! By saying ‘mid-air’ refueling, it does not mean that there will be a floating gas station somewhere in the clouds. To put it simply, aircraft refuel themselves with the help of other aircraft. Let’s get into the details.

A330 MRTT refueling a F-35 (Photo Credits: NATO)

Details of aerial refueling

Aerial refueling, also referred to as tanking, is the process of transferring jet fuel from one military aircraft to another during flight. The one providing the fuel is called the tanker while the one refueling is called the receiver.

There are mainly two refueling systems that are commonly used:

The first one is the probe-and-drogue, which is simpler to adapt to existing aircraft, and the flying boom, which offers faster fuel transfer, but requires a dedicated boom operator station. Aerial refueling allows the aircraft to remain airborne longer, extending its range time on station or while at duty. Looking at the probe-and-drogue procedure, the engineer unrolls a long hose from a wingtip or below the fuselage. There is a basket or a drogue at the end of the hose that looks like a windsock. Once the hose has reached the maximum extension, the receiver pilot must insert a retractable probe into the basket or drogue. The retractable probe is mounted on the plane’s nose. The engineer and the receiver pilot must smoothly maneuver the probe so that it will latch into the basket. This maneuver requires years of practice to perfect because the slightest movement could cause the probe to tear which would result in a leakage of precious jet fuel.

The other procedure is the flying boom where there is a dedicated operator who sits at the back of the tank and navigates a telescope tube into a receptacle, which is located near the front of the receiver plane. A signal is sent to the tanker to begin pumping fuel as soon as the boom latches on. Some tankers can hold up to 29,000 gallons of gas and can pump the fuel faster as much as almost 900 gallons per minute. This is efficient for large airplanes which have much larger fuel tanks such as the C-5 or C-17. Also, the flying boom procedure is more natural to connect because it is flown to the receiver aircraft and the pilot holds the hose in position. The pilot in the receiver needs to steer the boom in place and latch itself.

Probe-and-drogue procedure
Photo Credits: Stack Exchange
Flying Boom procedure
Photo Credits: Stack Exchange

Advantages

Aerial refueling is extremely advantageous since it enables the pilot to fly further without landing. Military aircraft cannot afford to land and refuel, hence aerial refueling is extremely beneficial. Both procedures operate over a range of altitudes and flight speeds. Each type of receiver has an altitude and a speed range at which to refuel that the tanker must match by either speeding up or slowing down. The receiver’s maximum altitude can force the tanker to fly at a lower elevation. The speed and weight are factors that can influence each aircraft. The result of altitude, weight, and speed can cause numerous variations in the field that surrounds the tanker. The refueling systems for the tanker are complicated. They must be designed to address the altitude, speed, and weight. Aerial refueling also helps during combat missions by rescuing the pilots out of dangerous situations.

KC-10 Extender refueling an F-16 with an F-15 and another F-16 to the side
Photo Credits: Wikipedia

Fun fact of the SR-71

When the SR-71 Blackbird would refuel mid-air, the pilots were forced to keep one afterburner on. This was due to the fact the jet required so much fuel that its gross weight would increase and the aircraft was forced to increase thrust to stay in line with the tanker. As the SR-71 pilots activated one after-burner, the asymmetric thrust made the plane fly slightly sideways. Refueling the SR-71 was a very complex procedure but what a sight it was, seeing a spaceship lookalike refueling itself and then disappearing on the horizon.

An SR-71 refueling
Photo Credits: The Aviation Geek Club

List of tankers

Below are some of the most widely tankers used and their procedure of refueling.

  1. Airbus A310 MRTT – Procedure: Probe and Drogue.
  2. Boeing 707 – Procedure: Probe and Drogue.
  3. Boeing KC-46 Pegasus – Procedure: Flying Boom.
  4. Boeing KC-135 Stratotanker – Procedure: Flying Boom.
  5. Boeing KC-767 – Procedure: Flying Boom.
  6. McDonnell Douglas KC-10 Extender – Procedure: Flying Boom.
USAF Refueling Fleet
Photo Credits: Aerotime

Sources