By Emelie Rutherford

HARTFORD, Conn.–The Marine Corps’ variant of the developmental Joint Strike Fighter (JSF) is expected to fly for the first time in short-take-off-and-vertical-landing (STOVL) mode either late summer or the end of the year, Pratt & Whitney [UTX] officials said this week.

Final prep work is being done now for the F-35B STOVL variant aircraft to fly its first test flight– in a conventional mode–in late-May, William Gostic, Pratt & Whitney’s vice president for F135 engine programs, told Defense Daily Tuesday at the company’s headquarters here. Pratt & Whitney develops the F135 engine for the Lockheed Martin [LMT] three-variant JSF effort.

Following that anticipated conventional flight of the F-35B, Gostic said his company’s ultimate goal is to “get that engine flight-approved for STOVL flight operations.”

Before the F-35B can perform an actual STOVL flight, though, officials have to learn more about engine blade failures.

During proof testing in February, a third-stage turbine blade of the F135 engine vibrated while at very high frequencies at a stress level than can cause it to crack, and cracks and holes were discovered, Gostic said.

A similar fatigue issue had been discovered last August, during an early phase of the qualification program, he said. A test initiated after the incident last August, intended to identify whether a turbine blade was susceptible to cracking, helped officials discover the blade problem in February, Paul Adams, senior vice president of engineering at Pratt & Whitney, told Defense Daily in a separate interview Tuesday.

Going forward, if the STOVL engine in April has a “very successful” test, then the F-35B potentially can have a STOVL test flight in or around September, Gostic said. If engine retrofit work is needed, the debut STOVL flight operations instead will likely be in December, he said.

The fatigue fracture associated with the third-stage blade was not abnormal, Adams said.

“It’s not atypical in a development program to have a failure like this,” he said. “We’ve actually done some risk-mitigation design work, recognizing that this might be an area where we need some further design margin. And we actually accelerated that design work up toward resolving the fix.”

Gostic dubbed the F135 “a remarkably smooth development program,” which he said is “largely because we’re building off of a F119 engine, which gives us a good foundation to build this improved engine.”

He noted the F135 STOVL engine is like no other.

“There has never been an engine that goes from running a very high speed condition in a conventional mode to flipping a switch and engaging a lift fan,” he said.

The JSF effort–which is developing variants for the Air Force, Marine Corps and Navy–is the Air Force’s largest acquisition program.

The Government Accountability Office released a report last week that said JSF’s cost has increased by more than $23 billion over the past year because of a seven-year program extension, future price increase and increases in the price of materials (Defense Daily, March 12).

The JSF STOVL variant is expected to reach initial operating capability in 2012.

Lockheed Martin is developing the F-35 with partners Northrop Grumman [NOC] and BAE Systems. In addition to Pratt & Whitney, a team made up of General Electric [GE] and Rolls-Royce is developing a second engine–the F136–for the JSF, though it is not as far along in development as Pratt & Whitney’s F135.