An investigation has been launched into how a Delta Air Lines passenger aircraft dumped a mist of jet fuel over a number of schools in Los Angeles before landing at the city’s airport.

The incident on Tuesday led to a number of children being treated for minor skin and lung irritations.

The Boeing 777 - which had taken off from Los Angeles International minutes earlier bound for Shanghai - was forced to turn around to make an emergency landing after the aircraft developed an engine issue, the airline said.

But why did the plane release what might have been gallons of jet fuel over the crowded neighbourhoods of Los Angeles?

Broadly speaking, because - full with fuel ahead of its 14-hour flight - it was too heavy to land. And the only way to reduce its load was, short of chucking away luggage, to lose fuel.

Children were treated for minor irritations Credit: ap

The maximum landing weight for a 777-200, the model involved in Flight 89, is 201,840kg, while its maximum take-off weight is 247,200kg. While it is not known how heavy the 777 was on Tuesday, it is that discrepancy that means aircraft are required to jettison fuel before landing in emergency situations.

The Federal Aviation Administration (FAA), the US body in charge of American skies, said in a statement there are “special fuel-dumping procedures for aircraft operating into and out of any major US airport”. The FAA pointed to two elements of the regulations - to avoid populated areas and for dumping to take place at higher altitudes “so the fuel atomises and disperses”; contact with jet fuel has been linked with the skin condition, dermatitis. Data from FlightRadar24 showed the aircraft reached an altitude just short of 8,000 feet before descending.

Instances of fuel dumping are, of course, common enough that they rarely make the news but some still attract attention.

In 2014, shortly after Flight VS43 took off from London Gatwick, 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 a malfunctioning landing gear.

“The maximum weight for takeoff is often considerably greater than the maximum weight for landing,” explained Patrick Smith, airline pilot and author of Cockpit Confidential.

Flight VS34 landed safety after jettisoning fuel over southern England Credit: GETTY

“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 breathe.”

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 lose all that kerosine: flight VS43 was in the air for several hours before it could land at Gatwick. In an emergency, however (as was the case with Flight 89), there might not be 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.”

Inspiration for your inbox

Sign up to Telegraph Travel's new weekly newsletter for the latest features, advice, competitions, exclusive deals and comment.

You can also follow us on Twitter, Facebook and Instagram.