As we mentioned, the difference between maximum takeoff weight and maximum landing weight for a large commercial aircraft can be more than 200,000 pounds. But let's say that soon after takeoff a passenger on board has a medical emergency that requires an early landing, or the captain becomes aware of a maintenance problem that requires attention. Neither of these situations are uncommon.
The pilot will have a few options. He or she may choose to fly around in order to burn fuel. In this case, the pilot can drop the gear or flaps to help a plane burn fuel faster (or "dirty up the airplane") [source: Getline]. However, this option is less than ideal for a medical emergency.
Another solution is to eject fuel. This can be as easy as the flip of a switch in the cockpit. The system consists of pumps and valves designed to let fuel jettison from nozzles on the wings of the aircraft. Such a system can eject thousands of pounds of fuel per minute, and it looks like a contrail when it's released [source: Scott].
Not all planes are equipped with this sophisticated system, however. FAA spokesperson Alison Duquette says that narrow-body planes, like the Airbus A320 or the Boeing 757, aren't built with fuel dumping abilities. But larger, wide-body planes, like the Boeing 777 and 747, which have added tanks, can dump fuel. In fact, the FAA requires certain planes to have fuel jettison systems [source: GPO].
Thankfully, most of the fuel will evaporate before it even hits the ground. This is more likely on warmer days and when an aircraft is high up in the atmosphere -- ideally above 5,000 feet (1,524 meters). Because fuel jettison isn't very common and is done in emergency-type situations, Duquette says the FAA doesn't have hard-and-fast regulations about it, and air traffic controllers will help pilots in the process, keeping them separated from other aircraft.
The last option in an emergency is to simply land overweight. In fact, Boeing maintains that neither landing overweight nor fuel dumping has led to any accidents [source: Colella]. In an emergency, when time is of the essence, landing overweight could be the best option. However, when you have time, damage to the plane (and the inspections and repairs that could entail) could be costlier than fuel dumping. What to do is largely a judgment call by the pilot in an individual situation.