United Airlines will have the option to keep its fleet size flat, or even reduce it, following today's order for 150 Boeing 737s.
The airline's chief executive Jeff Smisek says the Star Alliance carrier has some flexibility with its plans for the new aircraft.
United's order comprises 100 737 Max 9s and 50 737-900ERs, powered by CFM International Leap-1B and CFM56-7B engines respectively.
The -900ERs, delivery of which will begin in late 2013, will replace United's 757-200s currently operating domestically, while the Max 9s will replace other less fuel efficient aircraft, says Smisek.
"We can keep our fleet flat or shrink our fleet," he adds.
Currently, United operates 134 757-200s, Flightglobal's ACAS database shows. The aircraft's build dates range from the early to late 1990s. United says the -900ERs will burn up to 15% less fuel per seat than the aircraft they will replace.
All 150 new aircraft will come with the Boeing Sky Interior, says Smisek, who describes the order as part of the airline's continued investment.
"These airplanes are not only more fuel efficient, more reliable but they are also more environmentally responsible," he adds.
United had extensive discussions with Boeing and Airbus on a new narrowbody order, says Smisek, but the airline chose the 737, which he believes is "the best airplane" for the carrier.
The airline also operates Airbus A320-family aircraft.