The 50-seat jets once prized by carriers such as Delta Air Lines are being culled from U.S. fleets as higher fuel and maintenance bills make them too expensive to fly.

By 2015, U.S. airlines will have about 200 jets with 50 or fewer seats, down from about 1,200, said Michael Boyd, president of consultant Boyd Group International Inc. in Evergreen, Colo. More than 80 have been scrapped in 2010, he said.

“These are litters of aluminum kittens – nobody wants them,” Boyd said. Their only value is for recycled metal, he said. “The next stop is the Budweiser factory because that’s all they’re good for.”

Delta’s Comair unit underscored the turnabout last week with its announced move to get rid of three-fourths of its 50-seaters after pioneering their use in the 1990s. Regional jets flew about twice as fast as turboprops, and crude oil at about $20 a barrel made them affordable to operate.

The drawback was spreading costs over about a third as many seats as in a Boeing 737. With oil averaging $77.93 this year through Thursday, up 39 percent from 2009, airlines favor regional jets that can carry 70 or more people and fly less often, or new turboprops.

Comair’s move to shed 53 Bombardier CRJ-100 and CRJ-200 jets is a “defining moment on the long road to 50-seat oblivion,” said Richard Aboulafia, an analyst at consultant Teal Group in Fairfax, Va.

“The economics are awful, especially in a time of high fuel prices,” Aboulafia said. “It makes sense if you’re focused on market share, hub preservation and other really outmoded concepts. But if you’re focused on profitability, then 50-seats begin to look awful.”

Comair President John Bendoraitis told employees in a memo the Cincinnati carrier needed to “dramatically change course” with steps that include chopping the fleet to 44 planes by 2012. Before cuts in the 2008 recession, the total was 131. Comair’s oldest CRJ-100s average 14 years old, according to Ascend Worldwide Ltd., adding to maintenance expenses.

U.S. passengers and airlines embraced regional jets when Bombardier and Empresa Brasileira de Aeronautica entered the market in the 1990s. Use of models with 50 or fewer seats peaked in 2007 at 1,420, up from 110 in 1997, according to London-based Ascend, which compiles fleet data.

“The growth in this aircraft type was far too many, far too fast,” said Douglas Runte, managing director at Piper Jaffray & Co. in New York.

More-comfortable turboprops such as Bombardier’s Q400 and labor contracts favoring bigger regional jets helped erode the advantages of the smallest planes, he said.

Embraer and Montreal-based Bombardier are now selling or planning models able to carry more than 100 people, part of what Bombardier predicts will be a $393 billion global market for jetliners with 100 to 149 seats in the 20 years ending in 2029.