Why do planes dump fuel on emergency landings?

In scenarios when a plane must land prior to reaching its final destination — such as the weekend flight out of Houston making an emergency landing in Chicago to remove an unruly passenger — the crew must sometimes make a decision to dump extra fuel to achieve an appropriate landing weight.



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Airlines find that fuel dumping can actually be cheaper than not dumping in certain circumstances. But it's not something pilots do on a routine basis. We spoke with Alison Duquette, spokesperson for the Federal Aviation Administration (FAA), who assured us that it doesn't happen very often.

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If the dumping occurs at least 5,000 feet above the ground in freezing temperatures, 98 percent of the fuel will evaporate before reaching the ground, according to the FAA. The fuel remaining in droplet form could cause local rain, but it would be spread over a wide area, minimizing the effects at the ground level.

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The maximum landing weight (MLW) is the maximum aircraft gross weight due to design or operational limitations at which an aircraft is permitted to land. The MLW is set in order to ensure safe landings; if an aircraft weighs too heavy during touchdown, it may suffer structural damage or even break apart upon landing.

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If the aircraft tried to land without dumping the fuel first, it is considered an overweight landing attempt and could place immense stress on the airframe. It could also increase the risk of fire and fuel leaking onto the tarmac.

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The Gimli Glider incident occurred 40 years ago when an Air Canada Boeing 767 ran out of fuel at 41,000 feet, but the pilots successfully glided the plane to a former airfield turned race track for a safe landing.

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Typically in the case of an emergency landing, the plane will be landing somewhere well short of its destination. If it has not been flying long enough to burn off enough fuel to get below the maximum landing weight, they need to dump fuel in order to land safely.

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In most cases, airlines only dump air in midair immediately before landing. They don't do it before or during takeoff, nor do they do it halfway through their flight. Rather, airlines may dump some of their airplane's excess fuel immediately before landing so that it lowers the weight of their aircraft.

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It would seem strange that pilots would purposefully dump fuel from their aircraft at a time when fuel prices have skyrocketed. What's more bizarre is that they do it in the air while flying. Tossing fuel into the air is a safe procedure for a good reason. Additionally, it is not as wasteful as it seems.

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When 150 or more passengers are loaded onto the plane, they produce more heat and humidity. Preboarding, aircraft will often use external units to pump air preconditioned to a temperature considered acceptable to passengers, usually around 70 degrees.

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Over a certain altitude it is always considered safe to dump fuel. The high speed of the airplane means the fuel is spread very thin and will evaporate before it hits the ground. Accidents do of course happen where it is not possible to safely dump fuel and they end up dumping over populated areas.

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Can a wide body airliner land with a full fuel tank? Yes! But it is safer to land an aircraft below its published MLW.

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Among other things, this means descending to a lower altitude and, potentially, reducing the airplane's speed. If all of an airplane's engines fail simultaneously, the pilot will perform an emergency landing.

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The North American X-15 may be the fastest plane in the world, with speeds at 4,520 mph and Mach 5.93. It's an experimental aircraft used and powered by NASA and USAF.

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