The airline introduced its first Boeing 737-800 on Wednesday during a launch party for several hundred employees in a hangar at its Dallas headquarters.
The plane holds 175 passengers, compared with 137 on the biggest jet now in Southwest's fleet, the 737-700. The extra 38 seats should mean more revenue per flight.
"It's going to make us more profitable from day one," said chief operating officer Mike Van de Ven.
The new plane has higher ceilings and more overhead bin space than other Southwest planes and will be equipped for wireless Internet access.
Southwest plans to get 33 of the Renton-built planes this year, and 41 next year, while retiring a similar number of older jets. Southwest has more than 550 planes, not counting its AirTran Airways subsidiary.
If it passes operating tests, Wednesday's plane will join the fleet on April 11. Southwest plans to use it and other 737-800s mostly on long-haul flights out of Baltimore, Chicago and Florida airports, then in Los Angeles and Las Vegas.
It could also be used at New York's LaGuardia Airport and Washington's Reagan National Airport, where Southwest has limited takeoff and landing slots, Van de Ven said.
Eventually the new plane could allow Southwest to fly to Hawaii and the Caribbean, said CEO Gary Kelly. First, the airline needs to negotiate Hawaiian-trip pay scales for union pilots and flight attendants, he said.
"As long as you have the demand for 175 customers, it's a really good business decision," Kelly said.
Extra seating is a recurring theme at Southwest, which needs more revenue to offset high jet fuel costs. The airline is installing new, thinner seats on its 369 Boeing 737-700s, making room for another row with six seats.
The 737-800 is Southwest's first new model since it added the 737-700 in 1997. The bigger plane means Southwest flights will require an additional flight attendant, but executives said that will be offset by more revenue and lower fuel and maintenance costs than with older jets.
Southwest has the world's largest all-Boeing fleet.
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