The Mile High Club

How aerial refueling works.

F-16 CJ Fighting Falcon

The Pentagon may have to delay the purchase of aerial refueling tankers as a cost-cutting measure, CQ Daily reported on Monday. Military leaders have been waiting years to replace its aging fleet of tankers, a process that could cost up to $100 billion over the next two decades. How does aerial refueling work, anyway?

It’s like siphoning gas from a car traveling at 350 mph. The purpose of air refueling is to extend the natural range of an aircraft. Instead of wasting time by landing to refuel on the ground, a military pilot can arrange to rendezvous with a tanker plane along the way. The receiver aircraft—the one that needs gas—approaches the tanker from behind and, once it’s within 100 feet or so, slows down so that both planes are flying at the same speed. (For fighter planes, that’s usually around 300 knots; for larger planes, it’s slower.)

At that point, there are two main refueling techniques: “flying boom” or “probe and drogue.” In the former method, a boom operator sitting in the back of a tanker navigates a giant telescoping tube into a receptacle located near the front of the receiver plane. After the boom latches in, it sends a signal to the tanker to start pumping gas. In the latter system, tanker engineers unspool a long hose from either below the fuselage or a wing tip. At the end of the hose is a basket, or drogue, that looks like a giant windsock. Once the hose is fully extended, the receiver pilot maneuvers a retractable probe mounted on the plane’s nose into the drogue. If the pilot maneuvers too gently, the probe won’t latch into the basket. If he pushes too hard, he’ll stab the drogue, which can cause the hose to bunch up and fly away. (Here’s the right way to do it. Here’s the wrong way.) The tanker starts pumping only when the probe fits snugly into the basket, forming a seal.

Each method has its advantages. Booms can pump gas faster, at around 6,000 pounds per minute, while a probe and drogue pumps at less than half that rate. (The most common American tanker, the KC-135, can hold up to 200,000 pounds of gas, or 29,000 gallons.) But tankers equipped with probe-and-drogue systems can refuel as many as three planes at once (although two is usually the maximum). In general, the Air Force uses flying booms, while the Navy and Marines use probe and drogue—a situation that caused compatibility issues during the Gulf War and which the two military branches are still trying to resolve.

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Explainer thanks John B. Sams Jr. of the Aerial Refueling Systems Advisory Group and Dave Sloan of Boeing.