United Airlines is ordering 50 new airplanes the way some people ease into a cold lake. Toes first, and very slowly.

The deal announced Tuesday has several conservative pieces. United split its order between the Boeing 787 and the Airbus A350, and waited until the depth of a recession so it could press for better prices. No planes will be delivered until 2016, and United has extensive rights to defer the orders. Perhaps most importantly, United only has to come up with $152 million over the next five years, putting off the day it will need to line up loans for its new planes.

At list prices the new jets would be worth more than $10 billion, with about $4 billion for Boeing's 787-8 and around $6 billion for the Airbus planes. United President John Tague said the carrier got a discount, which is common for jet orders, though he didn't specify how much.

"We felt that we had a significant opportunity by timing the order with the backdrop of the current economic environment," he said.

The planes will replace Boeing 747 and 767s. Deliveries are expected between 2016 and 2019.

Both of the new aircraft models are made with composites, meant to be lighter (and thus more fuel-efficient) than the usual aluminum construction. Neither plane has ever flown. Boeing hopes its 787 will make its first flight by the end of this year, while Airbus is aiming to deliver its first A350 in 2013.

Both of the planes have two aisles and would usually be used on international flights. The 787-8 seats up to 250 people, while the Airbus A350 seats 314. Both can fly more than 9,000 miles _ enough to go nonstop from United's hub in Chicago to Shanghai.

"They give us access to virtually every region in the world from all of our hubs," Tague said.

United could have picked the much larger A380 or 747-8. Although it might be able to fill those massive planes during boom times, United would risk having many empty seats in a recession. Tague said the company would rather reduce its risk in future tough times, rather than "capture every last dollar of opportunity that an extremely large aircraft might create for a short period of time."

So why split the order? Both planes are made of lightweight materials, but their sizes are different and United thinks having some of each will let it match the size of the plane to demand on individual routes. That flexibility outweighed the benefits of sticking with one plane or the other, the company said.