WASHINGTON, Feb 24 (Reuters) - Lockheed Martin Corp's F-35 Joint Strike Fighter will start "ski-jump testing" at a Maryland air base this week, while another B-model jet wraps up six months of tests at temperatures as low as minus 40 degrees Fahrenheit (minus 40 Celsius) to as high as 120 F (49 C).
Two UK pilots will test the ability of the new warplane to take off from upward-sloping ski-jump ramps used on aircraft carriers like those operated by Britain and Italy. The ramps launch the jets forward and upward, reducing the thrust needed.
Sylvia Pierson, spokeswoman for the Pentagon's F-35 program office, said two British pilots, one from BAE Systems Plc and the other from the British Royal Navy, would use the first UK F-35B jet to complete the testing through late May.
The F-35 is also finishing six months of tests at the McKinley Climatic Laboratory in Florida, another key milestone before the U.S. Marine Corps declares the jet ready for initial combat use in July.
After 14 years of development, early cost overruns and schedule delays, the $400 billion F-35 fighter jet program is becoming an operational reality for the U.S. military. Over 120 jets are flying at nine U.S. bases.
Richard Aboulafia, analyst with the Teal Group, said the program has stabilized but it is unclear whether the Air Force will stick to plans to buy 1,763 F-35 A-models through 2037 even as it ramps up work on a new bomber and a "sixth generation" jet.
The F-35 climate testing has been closely watched by the U.S. military and nine other countries that have placed orders: Britain, Italy, Turkey, Australia, Norway, Japan, South Korea, the Netherlands and Israel.
Pilots, technicians and F-35 program officials have tested the jet's performance in heat, driving rain, hurricane-force winds, snow, sleet and other extreme conditions.
Industry and government officials say the testing has gone better than expected, although final assessments will wait until after the testing is concluded next month.
Billie Flynn, a former Canadian Air Force pilot who tested the F-16 fighter 23 years ago in the same climate chamber, said he expected the F-35 to face some issues since it is so much more complex and software-driven.
But Flynn, now a Lockheed F-35 test pilot, said the jet surpassed his expectations. "This just gives us so much more confidence about when and where we operate the jets."
F-35 program officials and enginemaker Pratt & Whitney, a unit of United Technologies Corp, said oil in the engine became too thick at very cold temperatures, but the problem had been resolved by switching to a thinner oil used by the Air Force.
They said it was unclear if the issue would require a minor modification to an external component of the engine. (Editing by Matthew Lewis)