But it happens.
An estimated 44,000 in-flight medical emergencies occur worldwide each year, and once such a situation occurs, it starts a complex, orchestrated and often expensive response that airlines train constantly to handle.
For frequent business travelers, if you haven't already been on a flight where a medical emergency has been declared, chances are you will at some point.
"If somebody has to absolutely get on the ground ASAP to the nearest ambulance, we do what we have to do," said Capt. Michael Sharpe, a pilot and flight instructor for Southwest Airlines.
There are no exact figures on how much it costs to divert an aircraft from its planned route, but it's thousands of dollars. A Boeing 737-700, for example, holds 6,875 gallons of jet fuel. At $3 a gallon, that's nearly $21,000 to fill the tank. Once a plane is diverted and lands, it almost always has to be refueled.
"It is a major expense to the airline, but we just look at it as a cost of doing business," said Sharpe, who has been with Southwest 30 years.
"It's not that person's fault" they became ill, he said. "Some people have health conditions they are not even aware of."
Once a medical emergency is declared, landing becomes the priority. Landing, though, takes time.
"Even if you have someone who needs absolute, immediate medical attention, you can't just push the 'down' button like you're on an elevator," Sharpe said.
It takes at least 25 minutes to get from altitude to the ground. "You have to plan your descent (so) that you don't get down too early -- that burns a lot of fuel -- and that you don't get down too late -- you'll overfly the airport," Sharpe said.
Doctor on board
At 35,000 feet, the first responders in the cabin are the flight attendants. By law, they are trained to deliver basic first aid as well as begin the process for getting a sick person help.
Say someone collapses after a plane is airborne.
Flight attendants notify the cockpit crew that there is a medical emergency in the cabin. The cockpit crew then establishes a communication link with doctors such as Christian Martin-Gill, assistant professor of emergency medicine at the University of Pittsburgh Medical Center, which operates the STAT-MD Communications Center and provides emergency medical guidance to 17 commercial airlines.
Martin-Gill is one of the authors of a New England Journal of Medicine study this year that looked at 34 months of in-flight medical emergency data -- nearly 12,000 in flight medical emergencies.
The study found that in about three-fourths of those cases, there was a medical professional such as a doctor, nurse or paramedic on the flight.
"That's a big reason we wanted to publish this -- to provide some guidance to health care providers who might find themselves in that kind of situation," Martin-Gill said.
"The key piece is, it doesn't matter if you are a cardiologist or an orthopedist or a pediatrician," he said. "Often what is necessary is being some eyes and ears, and being able to provide a medical assessment or just the administration of some medications."
Flight and aircraft cabin crews aren't picky when an in-flight medical emergency occurs, Sharpe confirmed.
"A nurse. A firefighter. A paramedic. A dermatologist. Any type of doctor. When you have a situation like that, you don't care what their area of medicine is," he said.
Emergency medical kits on commercial aircraft are stocked with a variety of emergency medical equipment, medicines, IV fluids and defibrillators.
While flight attendants have basic training in first aid and cardiopulmonary resuscitation, if someone needs to start an IV, "you're really dependent on having a medical volunteer on board," Martin-Gill said. "The good news is that, in a majority of cases, that person is available."
Deciding to divert
Patient care on the aircraft is only part of the equation. Where the airplane is at the time someone is stricken also plays a role in what takes place during an in-flight medical emergency.
"Over the middle of the Atlantic, it's certainly much more concerning," Martin-Gill said.
The decision to divert a flight is made in concert with the flight crew and the physician on the ground who answers the call for help.
"They will say, 'Captain, I think this man is in very serious condition. I would advise you to get him to a hospital as soon as you possibly can,'" Sharpe said. "We know what that means. We start getting really busy in the cockpit."
So do airline staffers, air traffic controllers and airport emergency medical crews on the ground.
"It happens several times a month," said Assistant Chief Terry Czajkowski, who is in charge of emergency medical services for the Milwaukee County Fire Department, which serves Mitchell International.
Sometimes aircraft are diverted to Milwaukee because it is the closest major airport. Sometimes planes are diverted here because the skies are too crowded over Chicago. "It all depends on the condition of the patient," Czajkowski said.
Once an in-flight medical emergency is declared, air traffic controllers give the aircraft priority in the airspace.
Then, a whole bunch of questions have to be answered.
Where is the nearest airport that can handle jet aircraft traffic? What are the weather conditions at that airport? Once the plane lands, is there a medical facility nearby that can treat the symptoms the patient is experiencing?
"They are discussing which airport might be more appropriate to divert to," said Barbara Martin, a registered nurse who is responsible for aircrew and passenger health services at Delta Air Lines.
"Sometimes it makes more sense to fly a little farther rather than divert only to have the person have to be helicoptered to the next city" where critical care is available, she said.
"It's quite exquisite coordination" among everyone involved, Martin said.
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