It's a traveller's worst nightmare. A potty fellow passenger makes a beeline for the emergency exit and yanks the door open.

That nightmare nearly became a reality in 2017 when a passenger in first class attempted to open the cabin door on board a Delta Air Lines service between Chicago and Beijing.

The crew managed to restrain the man before he was arrested.

But why would opening the cabin door be catastrophic?

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It's now been revealed that in that highly unlikely event, it would only take half a second before catastrophe struck, The Sun reports.

According to ASAPScience, anything not secured when the cabin door opens would immediately be pulled out of the plane and sent into the sky, due to the sudden and severe difference in air pressure.

But if you're strapped into your seat you are still in a life-threatening situation.

There would be a huge risk of oxygen deficiency to anyone who didn't have an oxygen mask on and even if you did do this, the plane would likely fall apart in the air.

However, luckily, the chances of being able to open the cabin door while mid-flight is almost impossible.

"You would need a hydraulic jack, and airport security doesn't allow those," Pilot and author Patrick Smith told the Telegraph.

"Think of an aircraft door as a drain plug, fixed in place by the interior pressure. Almost all aircraft exits open inward. Some retract upward into the ceiling; others swing outward; but they open inward first.

"At a typical cruising altitude, up to eight pounds of pressure are pushing against every square inch of interior fuselage. That's over 1100 pounds against each square foot of door.

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"A meagre two pounds per square inch is still more than anyone can displace — even after six cups of coffee and the aggravation that comes with sitting behind a shrieking baby.

The aftermath of the Delta incident.
The aftermath of the Delta incident.

"The pressure is so intense that it would be impossible to do it with your bare strength."

However, earlier in 2018 a man managed to open the emergency exit on a grounded plane before climbing out on the wing while allegedly suffering an asthma attack.

After landing in Spain, he took the drastic action when the plane was held on the tarmac for an extra 30 minutes following a previous one-hour delay in the UK.

"This man decided he wasn't going to wait any longer," stunned witness Fernando Del Valle Villalobos said.

"He activated the emergency door and left, saying, 'I'm going via the wing'. It was surreal."