What is a fuel imbalance and why did it force a Delta flight to turn around mid-air?

• A Delta flight had to make a U-turn over the Atlantic Ocean after experiencing a fuel imbalance.
• A fuel imbalance happens when the amount in the tanks in the left and right wings becomes unequal.

A Delta flight had to make a U-turn over the Atlantic Ocean after the crew realized there was a fuel imbalance and was unable to resolve the problem.

The plane departed from John F. Kennedy International Airport in New York for Accra in Ghana.

What is a fuel imbalance?

A fuel imbalance is when the amount of fuel between the left and right fuel tanks in the wings of an aircraft is unequal and could affect the plane's handling, according to Boeing.

They can occur for many reasons, but the plane maker said the most frequent causes were variations in the performance of fuel system components, variations in fuel burn, component faults or faults that cause fuel leaks.

Boeing says troubleshooting can be relatively easy on two-engine airplanes because the fuel system can be isolated to the left and right sides of the airplane. That is not the case for a four-engine Boeing 747, however, which would require an inspection of the fuel tanks to determine the cause.

What happens when there's a fuel imbalance?

The crew on Boeing plans gets a notification when the imbalance reaches a specific level.

If the crew can confirm there is no leak, the can then crew will proceed by moving some fuel from one tank to the other. Boeing says that if the warning sounds again, the same balancing procedures should be repeated.

If it is not able to adequately balance the fuel tanks, pilots will sometimes decide to turn the plane around – like the Delta flight.

While some might find the experience scary, like this YouTuber who said he was "sobbing" in his seat and thought he was going to die, pilots can decide to keep flying.

Boeing states: "If following appropriate maintenance procedures has failed to identify a cause, but no external fuel leaks, internal fuel leaks from a pressurized line, or other system component failures are indicated, continued operation of the airplane may be acceptable."

"However, this is acceptable only until the earliest available maintenance opportunity when more detailed troubleshooting can be performed."

According to Boeing, the most recent fuel imbalances have occurred on the 747-400 and 777 models.

The Delta flight to Accra was on an older Boeing 767 plane.