Lockheed says F-35 to begin short-takeoff testing

* First vertical landing seen before end of year

* Delays underscore problems with STOVL version

WASHINGTON Nov 16 (Reuters) - The first short-takeoff, vertical landing version of Lockheed Martin Corp's (LMT.N) F-35 fighter plane arrived Sunday at a U.S. air base in southern Maryland, paving the way for flight testing of the new capability soon, company spokesman John Kent said on Monday.

Kent said the short takeoff vertical landing (STOVL) propulsion system would be operated during flight "very very soon," followed by a first vertical landing before year's end.

"The plan is to complete the first vertical landing before the end of the year," Kent said.

The STOVL testing had been scheduled for May but was pushed back to first September, then late October.

Kent attributed the delay to issues with delivery of some parts, a needed design change in the STOVL engine built by Pratt & Whitney, a unit of United Technologies Corp (UTX.N), and more recently, and rainy and cloudy weather that prevented remaining flight tests in the jet's conventional mode.

One congressional aide, who was not authorized to speak on the record, said the STOVL flight tests were more than two years later than initially planned, which underscored the difficulty with that version of the plane.

He said some Pentagon officials still were unsure if the first vertical landing would occur before January.

Cheryl Limrick, spokeswoman for the Pentagon's F-35 program office, said there would be about 12 flights before the plane moved into STOVL mode.

Dan Crowley, Lockheed executive vice president and F-35 program general manager, said the company had "high confidence in the capabilities of this aircraft, and we fully expect that it will meet or exceed the expectations of our customers."

The plane's STOVL propulsion system has already been tested on the ground, but analysts and lawmakers have been anxiously waiting for the plane to finally begin flight tests.

The F-35B will replace the U.S. Marine Corps' AV-8B STOVL fighters, F/A-18 strike fighters and EA-6B electronic attack aircraft. The air force and navies of Britain and Italy also plan to use the F-35 STOVL version. (Reporting by Andrea Shalal-Esa)