Why Do Commercial Planes Sometimes Dump Fuel?

When aviation fuel can (usually) cost so much, why do commercial airliners sometimes need to dump or jettison the precious resource?  The answer to this simple riddle is landing weight and safety. Aircraft are designed to land at or below set weight limits. These weight limits are less than the aircraft’s take-off weight. The heavier the plane is, the more likely it is to hit the ground hard and suffer structural damage.

Why Do Commercial Planes Sometimes Dump Fuel?
All Airbus A380s have a fuel dumping system. Photo: Getty Images

Before a commercial airliner ever leaves the ground, a flight planner calculates how much fuel is needed for a particular flight. Unlike your car, you don’t just fill up the tank and fly until that needle moves down to ‘Empty’.

Fuel put on aircraft is not measured in gallons or liters but pounds and kilos. This is because when it comes to taking off and landing, weight is everything. The amount of fuel on a plane is calculated to have been burned off with a little to spare when the aircraft reaches its destination.

Dumping fuel is only done when  no other options are available

Unless a flight has a severe problem or a medical emergency that requires the plane to land immediately, the pilots would rather keep flying in circles to burn off fuel rather than dumping it. If no choice is available, modern fuel jettisoning systems can dump thousands of pounds of fuel in seconds, allowing the aircraft to get to an allowable landing weight in less than 15 minutes. By using a combination of pumps and valves, fuel is diverted to the wingtips where it is then expelled into the air. There is also a safety measure to ensure that the pilots don’t dump all the fuel but just enough so that it is safe to land.

Pilots try to avoid populated areas when dumping fuel

When a pilot decides to dump fuel, the Federal Aviation Administration dictates that the aircraft must be 2,000 feet above the highest obstacle on its route, five miles from any other aircraft, and away from populated areas or water bodies.

Delta 777
A Delta 777 dumped fuel on a playground by mistake. Photo: Getty Images

While the jettisoned fuel is immediately vaporized, it is still in the atmosphere and will eventually hit the ground. Incidents involving the need to dump fuel are quite rare and is why environmental groups have not taken as much issue with it as one might expect.

Some planes don’t have fuel jettisoning systems

Depending upon the severity of the problem, pilots have three options:

  • Fly around and burn off excess fuel
  • Jettison/dump fuel
  • Land the aircraft heavy

Of the three choices, the first two are self-explanatory, while the third choice of landing the plane heavy presents options. As we move away from four-engine aircraft like the Boeing 747, Airbus A340 and Airbus A380, many smaller twin-engine aircraft have no fuel jettisoning systems installed.  This leaves them with two options (burn or land heavy), rather than three.

Icelandair 757
The Boeing 757 has a similar takeoff and landing weight. Photo: Getty Images

If a heavy landing is required, planes are inspected for damage before being allowed to fly again. One aircraft you never have to worry about with heavy landings is the Boeing 757, which has a similar takeoff and landing rating.

Fuel dumping is an inelegant solution, even if the damage to the environment is marginal. However, it is sometimes an unfortunate necessity in order to perform a safe landing.

Have you ever been on a flight that had to jettison fuel? Let us know in the comments.