BOSTON -- Once seen as a luxury of the corporate world, private planes are becoming increasingly common at U.S. colleges and universities as schools try to attract athletes, raise money and reward coaches with jet-set vacations.

Some schools spend millions of dollars a year flying their coaches and executives on scores of trips around the country, and some pass the cost on to students and taxpayers.

The Associated Press requested documents from dozens of public universities and found that at least 20 own or share ownership of planes for school business, often employing a few full-time pilots to fly them. Many others charter private flights through outside companies.

Flight logs show that, at times, the aircraft are used for purposes unrelated to university business.

At Ohio State University, which leases one plane and partly owns another, football coach Urban Meyer and members of his family took 11 personal trips last school year, including a vacation in Florida, a weekend getaway to Cape Cod and a spring break in South Carolina. The university's cost: $120,000. Add Meyer's 15 recruiting trips in the same planes during that period, and the price tag jumps to more than $350,000.

Iowa State president Steven Leath caused damage to the university airplane he was piloting when it made a hard landing at the airport in July 2015. Bloomington Normal Airport Authority via AP, File

Meyer declined to comment.

Some private colleges, which aren't subject to open-records laws, also own planes.

Colleges defend the costs, saying coaches and top administrators need to travel more than ever, while commercial airlines are offering fewer flights. Some say it's economical for officials who often fly on short notice or to towns that are far from a major airport.

But some critics condemn such spending as a luxury at a time when tuition continues to rise.

"The students are paying for it or the taxpayers are paying for it, and it's usually the students," said Richard Vedder, an economist and director of the Center for College Affordability and Productivity in Washington, D.C.

Universities often use planes for athletic recruiting, mostly football and basketball, and to shuttle administrators on trips to woo donors or lobby lawmakers.

Some of the nation's largest schools, such as Penn State and the University of Texas, own planes, as do many smaller schools, including the University of Wyoming and the University of Central Missouri.

The price for a private plane usually reaches into the millions, climbing as high as the $8.4 million that the University of Florida's athletic association paid for an eight-passenger jet in 2011. Then there are operating expenses such as fuel and maintenance, which at Ohio State cost $1.6 million last school year.

Each flight often averages more than $1,000 an hour, far exceeding the cost of a commercial flight.

Purdue University, for example, sent a plane to Providence, Rhode Island, last year to bring alumnus and former NFL lineman Matt Light to Indianapolis for an athletics meeting and then flew him back, at a cost of $15,000. A commercial flight between those cities typically costs less than $400 round trip.

The University of Kansas chancellor and two staff members were flown to the NCAA basketball tournament in Louisville, Kentucky, for $10,000 last year. Officials at the University of Tennessee routinely fly between Knoxville and Nashville, a drive of less than three hours.

"With our executive administration, their time is valuable enough that certainly the plane use is warranted," said Ron Maples, interim treasurer for the University of Tennessee. He added that the school's yearly spending on flights, about $700,000, is "hardly a blip" in the overall budget.

Costs for chartered flights can add up fast, too. The University of Minnesota doesn't own a plane but spent $2.9 million chartering flights last year.

To some critics, big spending on airplanes is fine as long as taxpayers don't foot the bill. And some schools, including Ohio State, say private donations and athletic revenue pay for flights. But at many, such as Kansas and Tennessee, they are covered by budgets that include tuition and tax dollars.

"If they're being funded by taxpayers at all, it falls outside the lines of what's proper and ethical, in my opinion," said Lawrence McQuillan, a senior fellow at the Independent Institute, a conservative think tank in Oakland, California. "Some of this is just personal vacations, which to me is completely improper for taxpayers to fund."

Private flights have also become a perk used to draw coveted coaches. At the University of Oklahoma and at Ohio State, among many others, top coaches and their families are given yearly shares of flight hours that can be used for vacations and other personal trips.

Three coaches at Ohio State topped $220,000 in personal trips last school year, records show, with destinations including Las Vegas and Marco Island, Florida.

"It's where the market is now from a compensation standpoint," said Martin Jarmond, the executive associate athletic director at Ohio State. "Coaches have families, too. They've got ballgames they need to get back for. They have important things they need to do outside of the job."

Colleges say they regularly audit their flight records to monitor for misuse, but some shield travel information from the public.

At the University of South Florida, spokeswoman Lara Wade said the flight logs for the school's plane do not have to be released because the aircraft is owned by an outside company.

Penn State said it owns two planes used by senior officials but would not provide records because the school is mostly exempt from the state's public disclosure laws.

"Many universities, including those within the Big Ten, have planes," Penn State spokeswoman Lisa Powers said in a statement. "It is not uncommon for major research universities with a statewide, national and international profile."

As some colleges add planes, others are scaling back or reconsidering their flight programs.

Last year, the University of Minnesota found that men's basketball coach Richard Pitino had overspent his budget for chartered flights by more than $200,000 over three years. Since then, the school has hired a new athletic director whose job will partly be to rein in the budget.

Iowa State University president Steven Leath, a pilot, acknowledged last year that he used a school plane for trips that mixed personal and university business, a practice that came to light after he damaged the aircraft in a hard landing. Iowa State has decided to sell one plane and is reviewing whether to continue using another.

Other schools say they simply have little need for private planes. The University of California at Los Angeles sometimes charters flights for sports teams, but not for recruiting or for executive travel.

"Purchasing a private airplane does not best serve our student-athletes," spokesman Tod Tamberg said.