A Delta plane dumped jet fuel over an elementary school, causing minor injuries to children and adults on Tuesday.

Planes dump fuel when they end up making a shorter flight than expected and the plane is too heavy for a safe landing.

They are supposed to do so over unpopulated areas and at a high enough altitude for the fuel to vaporize before reaching the ground.

It is not yet clear why the plane could not fly higher while dumping the fuel, but Delta said its plane had to return to LAX “quickly” because of an “engine failure.”

Visit Business Insider’s homepage for more stories.

A Delta plane making an emergency landing in Los Angeles on Tuesday dumped jet fuel as it flew over an elementary school, injuring parents and children who were in the playground, authorities said.

Delta flight 89, which was travelling from LAX to Shanghai, China, suffered an “engine issue requiring the aircraft to return quickly to LAX,” releasing fuel on the way, the airline said.

17 children and nine adults at Park Avenue Elementary School got minor injuries, Reuters reported, though as many as 67 people were treated by paramedics at the scene.

The US Federal Aviation Administration said it was “thoroughly investigating” what had happened, and suggested that the airline had not followed proper procedures in dumping the fuel in the way it did.

Here’s why airlines dump fuel, and what is typically done to avoid it hitting people.

Planes dumping fuel while in the air is a typical procedure, done to reduce the plane’s weight before landing

Planes carry not only the fuel that is needed to reach its destination, but extra fuel in case the aircraft needs to divert or re-route and make the flight longer, as well as an extra fuel reserve in case of an extreme emergency.

Lees ook op Business Insider €50.000 bij je ouders lenen voor een verbouwing: familiebank kan gunstig zijn, maar kunnen pa een ma een boeterente opleggen als het misgaat?

This extra fuel is heavy, and can cause problems when landing because the plane is supposed to have used a lot of that fuel up by the time it puts its wheels down.

Foto: Multiple children and adults were treated for jet fuel exposure at Park Avenue Elementary School in Cudahy on Tuesday.sourceScott Varley/MediaNews Group/Torrance Daily Breeze via Getty Images

Patrick Smith, an airline pilot and author of Cockpit Confidential, told The Telegraph in 2017: “The maximum weight for takeoff is often considerably greater than the maximum weight for landing.”

Delta said in its statement that: “The aircraft landed safely after a release of fuel, which was required as part of normal procedure to reach a safe landing weight.”

Smith said 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.”

This means that planes dump fuel when they have to turn back to their origin airport – as the Delta flight did – or end up making a significantly shorter trip than what was planned when the fuel was allocated.

Foto: Delta Air Lines Flight 89 dumping fuel over Los Angeles on Tuesday.sourceAP Photo/Matt Hartman

This means that planes can end up releasing thousands of gallons into the air before landing. And as these airports are sometimes close to people and built up areas, there are set practices to avoid soaking communities in the fuel.

Planes are supposed to try dump fuel over unpopulated areas, and high enough for the fuel to disperse before reaching the ground

The FAA said on Tuesday: “There are special fuel-dumping procedures for aircraft operating into and out of any major US airport.

These procedures call for fuel to be dumped over designated unpopulated areas, typically at higher altitudes so the fuel atomizes and disperses before it reaches the ground.”

The FAA’s requirement say that planes should fly at least 2,000 feet above the nearest obstacle within five miles of the plane’s route, and also advises planes to be a significant distance from any other aircraft in the air.

Foto: The path taken by Delta flight 89, which shows it was far from any unpopulated area.sourceScreenshot/FlightRadar24

Unless it is in an emergency, planes dumping fuel “may be requested to fly a different route,” the FAA says.

Smith told The Telegraph: “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.

“It sounds terrible but one way or another that fuel is going into the atmosphere.”

LAX is in a built-up area of the US, but it is not yet clear why the plane did not fly higher so the fuel would disperse.

Delta’s statement did not elaborate on what level of emergency the plane was facing, but said it needed to “return quickly to LAX.”

Video footage of the plane in the sky has audio of one eyewitness asking: “Why is it so low?”

You can watch that here:

A Delta Airlines jet making an emergency landing at Los Angeles International Airport dumped fuel on the playground of an elementary school in the flight path, inflicting minor injuries on 17 children and nine adults https://t.co/pVdQ5ed1MV pic.twitter.com/IIlgSGapTj — Reuters (@Reuters) January 15, 2020

Delta said it is “in touch with Los Angeles World Airports and the LA County Fire Department as well as community leaders, and shares concerns regarding reports of minor injuries to adults and children at schools in the area.”