‘I Need to Go to the Hospital,’ Delta Pilot Tells Air Traffic Control

The altitude read 39,000 feet. The speed was hovering around 600 miles per hour. And the compass pointed due east.

But just after crossing over Lake Michigan on Tuesday, a Delta Air Lines flight from Los Angeles to Boston declared an emergency, leaving its course hundreds of miles short of its destination. Flight 437 had to land.

The captain of the Boeing 757 told air traffic controllers that someone was experiencing a medical episode, not one of the 147 passengers, but himself, the rare occurrence playing out over radio transmissions from the cockpit.

The transcontinental flight diverted safely to Detroit under the control of the first officer before another crew completed it, according to Delta. The trip normally takes between five and five-and-a-half hours.

On its final descent into Detroit Metro Airport, a Delta hub, the pilot explained his predicament to the tower.

“This is the captain,” the pilot said, “I’m the one that’s ill. I’m going to taxi the jet to the gate, and then I need to go to the hospital.”

The pilot did not specify what kind of medical issue he was dealing with during the exchange with the air traffic controller, who said the Delta flight was clear to use taxiway Yankee 2 so that it could reach the terminal faster.

But after the plane landed on Runway 22 Left, some confusion emerged about the gate that had been assigned to the flight.

“It says the gate is Alpha 13, but it doesn’t look like it exists,” the pilot told the controller, who responded that A-13 was not large enough to accommodate a Boeing 757 in the first place.

The flight was directed to gate A-10.

In a statement, Delta Air Lines apologized for the disruption and said the flight had landed “uneventfully.”

The airline confirmed that the captain had handled communication with air traffic controllers while the first officer landed the plane, which it described as standard operating procedure.

Delta did not answer questions about the pilot’s condition or his symptoms.

Kit Darby, a retired United Airlines captain and a former instructor pilot for Delta and Boeing, said in an interview on Thursday that instances of commercial airline pilots experiencing a medical episode and declaring an emergency were rare.

Airlines have safeguards in place when that happens, he said: the other pilot and the autopilot.

“The airplane can be flown, landed and taxied with one pilot,” said Mr. Darby, an aviation consultant who has flown several aircraft models, including the Boeing 757 and the Boeing 767. “It’s redundant.”

He said that pilots are trained to check for signs that another pilot might be in distress, such as if a plane starts to deviate from the course it has been cleared to fly by air traffic controllers.

“The guy sounds perfectly normal,” Mr. Darby said of the pilot’s exchange with the airport tower.

On the Boeing 757, the controls for steering the plane while it is on the ground are positioned next to the left cockpit seat, which would require the pilots to switch seats if one could not perform his or her duties, Mr. Darby said. It was not clear if the captain and first officer of the Delta flight had switched seats.

Before Flight 437 departed Los Angeles International Airport, both the pilot and the first officer would have been required to sign a release attesting that they were fit for duty that would have been submitted to an airline station agent at the gate.

“It states that they are physically and mentally fit to fly the flight and they’ve done the things necessary to make the flight safe,” Mr. Darby said.