Shortly after flight VS43 took off from Gatwick Airport in December 2014, the pilots realised there was a problem: the landing gear hadn't retracted properly. One set of wheels had become jammed. They would have to turn back.
Aside from having less rubber to touch down with, the pilots had another problem: the fuel-laden plane was too heavy to land safely. Too heavy to land under normal circumstances, let alone with malfunctioning landing gear.
"The maximum weight for takeoff is often considerably greater than the maximum weight for landing," explains Patrick Smith, an airline pilot and author of Cockpit Confidential.
"This is the case for a few reasons, the obvious ones being that touching down puts higher stresses on an airframe than taking off, and heavy-weight landings require a very high touchdown speed, which makes stopping more problematic."
The solution, then? Dump fuel, which is exactly what the pilots of VS43 did, releasing thousands of gallons of kerosine over southern England before returning to Gatwick, where they landed safely.
"Generally dumping happens at a high enough altitude for it to dissipate – it doesn't reach the ground in liquid form or come raining down on people," says Smith. "It sounds terrible but one way or another that fuel is going into the atmosphere."
Fuel is stored in the wings of a plane and is jettisoned from small nozzles also located in the wings. The pilot typically goes through a three or four step process to engage the plumbing and start dumping fuel.
Smith himself has been in a situation where he had to jettison fuel.
"About 20 years ago, when I was flying a cargo plane, the crew oxygen system failed," he recalls. "It wasn't an emergency, but it meant we had no supplemental oxygen, so if there had been a depressurisation we wouldn't have been able to breath."
Smith was forced to land at the nearest airport – Bangor in Maine, US – but not before dumping several thousand gallons of fuel.
It can take a while to jettison all that kerosine: flight VS43 was in the air for several hours before it could land at Gatwick. In an emergency, however, there simply isn't enough time to dump fuel, meaning the pilot will have to land overweight. This also happened to Smith.
"We took off from New York once and we had a medical issue on board that was becoming more serious by the minute," he said.
"We decided to go back and because the situation was becoming urgent we opted to land overweight – we didn't take time to do the dumping.
Smith liaised with the airline's technical team to ensure the safest possible landing. After touching down the plane was inspected and given the all clear to fly. "We were in the air again within a few hours," said Smith.
Not all planes are capable of dumping fuel. Boeing's 747 and 777 both have the ability to jettison kerosine, as has the Airbus A380 and an A330. However, regional jets such as a Boeing 737 and Airbus A320 can not.
"These smaller jets must circle or, if need be, land overweight," says Smith. "For some planes, landing and takeoff limits are the same, in which case it doesn't matter."
The Telegraph, London
See also: Why planes fly at 35,000 feet
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