On Monday evening, a passenger died on board Delta Air Lines flight 583 from Detroit to Shanghai. Well, although the passenger -- Yury Rubel, who suffered from hypertensive cardiovascular disease -- was quite dead on the airplane as it returned to Detroit, it wasn't until after the plane landed and Yury was taken into the airport for examination that he was officially declared dead by on-site medical staff.

You see, there's one thing that Delta, United, American, Southwest, or any other airline will ever let you do while you're in the air: Die. At least not officially.

It's no secret that plenty of people fall ill on airline flights every year. While colds are common and relatively benign (according to one study, people sitting in close proximity to someone with a cold have up to an 80 percent chance of catching it themselves), other illnesses are quite serious. Heart attacks, strokes, seizures, alcohol poisoning, and more are not uncommon.

According to a study published in the New England Journal of Medicine, one in every 604 flights reports a medical emergency, which works out to about 44,000 in-flight emergencies a year.

And, sometimes, these in-flight medical emergencies end up with a passenger death. The leading cause? Cardiac arrest at a whopping 86 percent.

Flight attendants receive extensive training in dealing with medical emergencies, and they can consult with medical professionals on the ground for assistance with difficult cases. But, according to one report, when a passenger dies on a flight, the airplane's crew is not allowed by the airline to consider the passenger to be deceased "unless pronounced so by the local medical authorities after the aircraft has landed."

As a Southwest Airlines spokesperson explained in an interview, "We technically don't have any mid-flight deaths."

Regarding Yury Rubel's flight, a Delta spokesperson said, "The aircraft landed without incident and the passenger in question was removed." He was then brought into the airport where he was officially declared dead.