Travelers check in at the United Airlines ticket counter at Terminal 1 at O'Hare International Airport in Chicago. (AP Photo/Nam Y. Huh)

In the airline business, filling seats is everything.

Whether it’s packed or empty, airlines have to pay for fuel, flight crews and everything else it takes to get a plane off the ground. So to boost profits, airlines try to get as many people on board as they can.

Efforts to fill up planes have spawned seat designs that look like torture devices, a federal investigation into potential collusion and a constant peeve of travelers: no leg room.

In fact, the percentage of seats that fly empty has been dwindling for more than a decade, federal data show — that is, it’s been getting harder and harder to luck out and get a row to yourself.

But if you flew between Newark and Columbia, S.C., back when United offered that route, you had a pretty good chance of being able to spread out.

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On average, in the year and a half the route ran, the flights were just over half-full — 51 percent. By comparison, at the time, flights nationwide were hovering above 80 percent full.

At their peak in the latter half of 2013, demand for the Newark-Columbia flights picked up, getting close to 80 percent full, but plummeted below 50 percent in the months before the route was cut in 2014.

That’s part of why the flight is now the focus of a federal probe. Investigators are looking into whether the airline reinstated the flight to curry favor with David Samson, then the powerful chairman of the Port Authority, who vacations in South Carolina. The investigation led United’s chief executive, Jeff Smisek, and two other senior executives to step down Tuesday.

From when United started the route in September 2012 to when it was cut in April 2014, an average of 400 people a month would get on board one of the weekly flights in either direction.

During winter months, the flight from Columbia to Newark was especially sparse. In seven of the 19 months United flew the route, more than three seats in four were empty.

Federal data don’t show how much United was making from the flight, but ticket prices between Newark and Columbia don’t appear to have been especially expensive at the time.

Including passengers with connecting flights, a one-way ticket between Newark and Columbia cost an average of $230, or 36 cents a mile, on the major airlines at the start of 2014, according to Transportation Department data. Federal data show that on a per-mile basis, the route was more expensive than most, but it was in line with similar regional flights.

But the profitability of the flight may not have been a major worry for United. The flight ran at a time of growing profits for the company, which netted $1.1 billion in 2014, the year the route was cut, after posting a $723 million loss in 2012, when it started up.

Eventually, the route, known inside the airline as the “chairman’s flight,” was cut. The last flight took off April 1, 2014, said Columbia airport spokeswoman Kaela Harmon, just days after Samson resigned.

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