Why do planes dump fuel?

Every airliner has two weight limits that matter here. Maximum take-off weight is what the wing and engines can lift; maximum landing weight is what the landing gear, wing structure and brakes can absorb on touchdown, and on long-haul jets it can be far lower. A Boeing 777-300ER can take off at around 350 tonnes but is limited to about 251 tonnes on landing; the difference is fuel meant to be burned over ten or twelve hours. If something forces the aircraft back within the first hour, it is still 60 or 80 tonnes above its landing limit.

The crew then has three options: circle to burn the fuel, which can take hours; land overweight, which is permitted in an emergency but requires a structural inspection afterwards and longer stopping distances; or jettison fuel to come down to the limit within twenty or thirty minutes. Which one they choose depends on how urgent the landing is. A passenger with a heart attack means an overweight landing now; a failed engine or hydraulic system on an otherwise stable aircraft usually means a dump.

Aircraft equipped for it have jettison pumps and valves feeding nozzles near the wingtips or the trailing edge, well clear of the engines. Dump rates are typically one to two tonnes a minute per side. Certification rules require that the fuel leaves the aircraft without any risk of fire, that the crew can stop the dump at any point and that enough remains for a safe approach. Air traffic control clears a dedicated area, usually over the sea or unpopulated land and at height, normally well above 600 m (2,000 ft) over terrain, where the kerosene atomises and largely evaporates before reaching the ground. Airbus and Boeing set out the procedure in each aircraft's operating manual.

Certification only demands a jettison system when an aircraft cannot meet climb requirements at its maximum take-off weight with an engine out, or when the gap between take-off and landing weight makes an overweight return impractical. Wide-body long-haul types such as the A330, A350, A380, 747, 777 and 787 have it. Narrow-body aircraft like the A320 family and the 737 do not: their landing weight is close to their take-off weight, so they land overweight instead and are inspected. This is why a short-haul flight returning to the airport never dumps fuel, however dramatic the situation looks.

Fuel dumps are uncommon and most pass unnoticed on the ground. The exceptions make news. In January 2020 a Boeing 777 returning to Los Angeles with an engine problem dumped fuel at low altitude over several schools, and the incident led to enforcement action because the crew had not coordinated the dump with air traffic control. Properly executed, a jettison leaves no measurable trace at ground level.