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Sneezes on a PlaneWhen do they divert a flight for a health reasons?

By Brian PalmerPosted Monday, May 4, 2009, at 5:52 PM ET

Masks on a planeA Washington-bound United Airlines flight was diverted to Boston on Friday when a passenger complained of flulike symptoms. Who decides whether to land a plane for passenger health reasons?

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The captain. In most cases, the pilot follows the recommendation of a passenger with medical training or a ground-based medical consultant. He or she also consults with the dispatcher at the airline's operations center. The company that flies the jet will shoulder the substantial cost of an unplanned diversion, as well as any legal liability for medical catastrophes that an untaken diversion might have prevented. Still, the captain of a particular flight always has the final say.

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Sydney Spiesel reminded us that avian influenza is still around. Michelle Tsai discussed the problem of developing a universal influenza vaccine. Keelin McDonell explained why airlines have to divert planes carrying passengers on the "No-Fly List."

Each airline maintains its own guidelines for how crew members should respond to ill passengers. If a case is too severe to be treated with a basic first-aid kit, the flight attendants notify the pilot and request that physicians onboard identify themselves. (Medical professionals respond in 69 percent of cases, according to a 2000 study [PDF].) The physician is granted access to an enhanced medical kit, including injectable drugs to treat severe allergic reactions, hypoglycemia, and heart attacks. The advanced kit can stabilize some patients who might otherwise need immediate treatment on the ground. If the patient or other passengers are still in danger, the pilot can make the decision to divert the plane.

That decision is more complicated than merely assessing the severity of passenger illness. The pilot must also consider the quality of medical care at the diversion site, the local weather, the aircraft's fuel stores, and the state of the target runway. In 1996, a Virgin Airlines 747 landed in the city of Iqaluit, on Canada's Baffin Island, in response to a passenger heart attack. The runway in Iqaluit was not set up to handle such a large aircraft, and one of the engines struck a fuel pump as the plane tried to take off. The passengers spent 15 hours in a curling rink waiting for charter flights to evacuate them.

The cost of diversion is hard to predict and can sometimes be staggering. At a minimum, an emergency landing will cost the airline $2,000 in jet fuel to land the plane and bring it back to cruising altitude. If the diversion requires rerouting passengers or bringing in a new aircraft and crew members, the cost can easily exceed $100,000.

Friday's incident was extraordinary, because infectious diseases very rarely cause flight diversions. Suspected heart attacks and strokes account for 63 percent of all unplanned landings for medical reasons; fainting is a distant third. Viral infections are responsible for just 1 percent of diversions, and post-hoc analyses usually conclude they were unnecessary. Guidelines on suspected communicable diseases [PDF] from the International Air Transport Association recommend moving passengers with flulike symptoms to a more isolated area and providing them with a mask, a designated crew member attendant, and a personal lavatory. Emergency landing is not recommended. A United spokesperson claimed the CDC advised diverting the plane, but the public-health agency has issued no guidelines for dealing with potential swine flu cases in-flight and does not have the power to order a diversion.

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Explainer thanks Shelly Sikes Diaz of the Centers for Disease Control, Alison Duquette of the Federal Aviation Administration, and Steve Lott of the International Air Transport Association.