WASHINGTON (AP) — One in five airline pilots lives at least 750 miles from work, according to a study by scientific advisers to the government, raising concerns that long commutes to airports could lead to fatigue in the cockpit.

The calculations were based on home addresses of more than 25,000 pilots. Six percent of pilots listed a primary residence at least 1,500 miles from the airline base where they begin flights, according to a National Research Council report released Wednesday.

Although a significant share of pilots list addresses hundreds of miles from their base, it’s not clear that they routinely begin their commutes to work from those addresses, the report said.

The council acknowledged it is difficult to determine the safety risk associated with long commutes without more information about the practices of individual pilots.

Pilot unions and airlines have long maintained that pilots can safely commute long distances to work if they act responsibly. For example, a pilot might fly across the country to reach his airline’s base but then sleep overnight in a hotel before showing up for work the next day well-rested.

“There are lots of stories and anecdotes but no systematic information,” Indiana University professor Clinton Oster Jr., chairman of the panel, told The Associated Press.

Congress directed the council to study the issue in response to a regional airline crash that killed 50 people in February 2009 near Buffalo, N.Y. The flight’s co-pilot had commuted overnight from her home near Seattle to her airline’s base in Newark, N.J., in order to make the flight. The flight’s captain, who regularly commuted from Florida to Newark, had spent the night before the flight in an airport crew lounge where sleeping was discouraged.

The National Transportation Safety Board concluded the accident was caused by pilot errors. The NTSB said it was also likely both pilots were suffering from fatigue, but the board wasn’t able to determine if fatigue contributed to crash without more information. The decision was a controversial one, with board chairman Deborah Hersman arguing in favor of finding that fatigue was a contributing factor. The vote was 2-1.

The accident drew attention to the salaries of regional airline pilots, who say they sometimes need to commute long distances because they can’t afford to live in more expensive communities where they are based. The salary of Rebecca Shaw, the co-pilot in the Buffalo crash, was less than $16,000 in the year before the accident.

“The pilots that can afford it often will fly in earlier and have a cheap crash pad,” said John Goglia, a former NTSB board member and a member of a Federal Aviation Administration fatigue advisory committee. “The pilots for the commuters and other low-paying operations suffer because they can’t afford to do that.”

Pilot unions and airlines oppose suggestions that the FAA limit commuting. The ability to live where they choose and commute to work by air at no cost has long been a cherished pilot prerogative. It’s the pilot’s responsibility to show up for work well-rested, they point out.

“I don’t think there is a ready-made solution to the commuting problem, but awareness has to be raised,” said Kevin Kuwik, whose girlfriend, Lorin Maurer, 30, was killed in the Buffalo crash.

Fatigue has been cited as a cause or a contributing factor in only nine out of 863 aviation accidents the NTSB investigated between 1982 and last year, the report said. Yet the board has said it considers fatigue one of the most serious safety issues facing aviation, as well as other modes of transportation.

Last year, the FAA proposed a major overhaul of regulations governing pilot work schedules to reflect the latest scientific understanding of the causes of fatigue and how to prevent it. The proposed regulations, which are expected to be made final later this year, don’t address commuting.

The study recommends airlines include commuting in their computerized “fatigue risk management” programs, which are designed to prevent flight crew schedules likely to cause fatigue.