A plane operated by JetBlue made an emergency landing in Los Angeles Wednesday night, after air-traffic controllers noticed that its landing gear was twisted to the side. The pilots first circled for three hours, to burn off fuel and lighten the aircraft. If they'd been flying a different kind of plane, they might have been able to dump the fuel instead. How do planes dump fuel?
It's as easy as flipping a switch. Many commercial airplanes (and almost all military aircraft) have a built-in fuel-dumping system that uses pumps and valves to release fuel from the wings and sometimes the tail. These systems, controlled from the cockpit, typically allow a plane to eject several thousand pounds of fuel per minute; a standard fuel-dumping operation could take around 10 minutes to complete.
Why would you need to dump fuel? A plane that lands too heavy risks structural damage (particularly to its landing gear), so manufacturers assign each model a maximum landing weight based on the sturdiness of its parts. Since planes burn fuel and get lighter as they fly, most are designed to carry much less weight at landing than take-off. A 747 that's fully gassed up for a long trip might burn through hundreds of thousands of pounds of jet fuel before it landed.
If the 747 were forced to land well ahead of schedule, it would be far too heavy for a safe touchdown. An overweight plane probably won't break apart when it hits the ground, but it may become unsafe for future flights. Airline mechanics would have to give it a thorough inspection, perhaps even dismantling parts of the plane, before allowing it to fly again. These hassles can be avoided, though, if the pilot just flips on the dumping system.
Dumped fuel flows out behind the plane like a contrail, and then most of it evaporates before it reaches the ground. Exactly how much of the fuel plume evaporates depends on several factors, including altitude, air temperature, and dumping pressure. In general, at least half of the fuel—and sometimes more than 99 percent of it—will dissipate. Fuel dumped from a high altitude in warm weather disperses best.
The Federal Aviation Administration's dumping policy prescribes a minimum altitude for dumping, and a five-mile separation from other aircraft. Air traffic controllers try to direct dumping planes away from populated areas and toward large bodies of water. (Experts guess that more than 15 million pounds of jettisoned fuel rained down into the oceans from civilian and military aircraft during the 1990s.)
Explainer thanks Al Dickinson of the University of Southern California and Cass Howell of Embry-Riddle Aeronautical University.