Long range flight burns though a lot of jet fuel.
When a long haul flight takes off they are often thousands of kg heavier than when they land.
A Boeing 777-200 Long Range plane has a maximum takeoff weight of 340,500kg but a maximum landing weight of 223,168kg.
The planes materials and structural design are engineered to land on a much lighter load at their final destination, and coming in with a full load of fuel can be dangerous.
This is why when for any reason a plane has to cut a flight short – as with Tuesday's Delta flight 89 – or make an emergency landing, they must find a way to dump excess weight.
The way of doing this is by dumping fuel.
On Tuesday flight 89 was just five minutes into a service from Los Angeles to Shanghai when the pilot reported a problem with the plane's right engine.
Shortly after the plane circled and made to return to LAX airport, still dangerously overweight.
A fuel jettison is normally perfectly safe.
Normally other air traffic is warned and, at an altitude of over 2000m, dumped jet fuel evaporates long before reaching the ground. This is normally done over water or open, uninhabited areas.
However on Tuesday this did not happen. In flight data obtained by the LA Times it was shown that the plane was at an altitude of just 700 metres when it dumped hundreds of litres of jet fuel above Park Avenue Elementary school.
Communications between air traffic control and the plane requesting a turnaround following the "engine" issue.
After checking local traffic, the control tower told the plane to come back to LA "immediately"
When asked if he needed to "hold to burn fuel" to reduce weight, the pilot said he had "got it under control".
When asked a second time "OK, so you don't need to hold to dump fuel or anything like that?" The pilot responds: "Negative."
Just 20 minutes later the plane had dropped to under 3000ft (900m) while still dumping fuel, dousing a local primary school in fuel.
Though no serious injuries were reported fifty people had to receive medical treatment, many of them children.
The damning recording of the radio communication raises further questions as to what happened on Tuesday.
Though costly and with environmental impacts, fuel jettisons are normally safe for both plane occupants and those on the ground.
Overweight planes landing heavier than the maximum landing weight can encounter structural damage leading to months of expensive repairs or even a total structural write off of the aircraft.
In the worst case scenario, the overweight plane could overshoot the runway on landing.
Coming in to Los Angeles Airport after just five minutes into the abortive flight, the jet would have burned through less than a tonne of potentially 45 tons of fuel.
There are minimum speeds and a dump rate of around 2.5 tons per minute (2.26796 tonnes per minute) to make sure that fuel safely dissipates into the atmosphere.
It would take almost 20 minutes to dump the fuel following jettison protocol and reach safe landing weight.
Given the short duration of the flight of less than 25 minutes, it is unclear if the plane will have further breached guidelines on fuel dumping.
The FAA said it is "thoroughly investigating the circumstances" around the incident to see why the plane did not dump in one of the "designated unpopulated areas" and at a safe altitude.
A spokesperson for the airline said at the time: "Shortly after takeoff, Flight 89 from LAX to Shanghai experienced an engine issue requiring the aircraft to return quickly to LAX.
The aircraft landed safely after a release of fuel, which was required as part of normal procedure to reach a safe landing weight. Delta 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."