By Guy Norris

First flight of the first short take-off and landing (STOVL) Lockheed Martin F-35 Joint Strike Fighter, BF-1, could be delayed following a repeat of the Pratt & Whitney F135 engine turbine blade failure that led to a hold up in flight tests last year of the conventional take-off and landing (CTOL) JSF.

The failure of the low pressure (LP) turbine blade is thought to have occurred in engine six which was undergoing flight clearance "proof test" ground runs prior to installation in the first STOVL F-35, BF-1. The tests were aimed at proving, on an engine-by-engine basis, that the specific unit was safe to use in BF-1, and that it did not exhibit the same combination of assembly characteristics, tolerances and other factors that led to a similar failure on a test stand engine on August 30.

The August failure, which contributed to a prolonged delay for both ground and flight tests of AA-1, the first conventional flight test F-35A, was traced to an unsteady flow regime in the wake of the stator upstream of the LP turbine third stage. P&W is developing a fix involving redesigning the stators and is planning to test the improved configuration by the middle of this year. In the meantime it cleared specific engines to allow flight test to resume of AA-1 in December 2007, and hoped tests of engine six would similarly clear the way for BF-1 to start tethered hover pit tests originally set for April.

To this end, P&W confirms it is "currently conducting proof testing of F135 CTOL and STOVL propulsion systems in order to validate low-pressure turbines for ground and flight testing. The proof testing was developed to specifically address a 3rd stage turbine durability concern previously identified during development testing."

According to information revealed at the December roll-out of the BF-1 in Fort Worth, Tex., the aircraft was expected to make its first flight from a conventional take-off roll in late May or early June. This was to be followed with a gradual "build-down" to STOVL tests by around the end of 2008. It's unknown right now if and by how much the recent failure, which occurred on Monday, will affect the F-35B test schedule.

Lockheed Martin won't comment on specific dates for the start of flight tests, although sources suggest the internal target date for first flight has, until recently, been May 19. Test results of the resdesigned stator are meanwhile not expected to be known until around the third quarter.

P&W says in a statement that "on February 4, during proof testing of a F135 STOVL engine variant on a Pratt & Whitney test stand in West Palm Beach, Florida, an incident occurred involving a single 3rd stage blade. The engine is being inspected and Pratt & Whitney is working in concert with the Joint Program Office and Lockheed Martin to determine next steps."

Ironically the test problem with the F135 occurred on the same day that the Pentagon once again proposed to cut the alternative General Electric Rolls-Royce F136 engine from the FY 2009 defense budget. The competing engine has twice previously been rescued by Congress from the brink of cancellation, the last time in 2007 on the heels of two F135 test incidents. The first was when damage was sustained during STOVL flight release testing in May 2007 when a deliberate hard stall of the shaft-driven lift fan caused the shaft to break, while the second on August 30 concerned the original LP turbine blade failure.