MANUFACTURING FLAW FOUND IN STOVL JOINT STRIKE FIGHTER LIFT FAN ROTOR

Inside the Air Force
Officials: Testing should remain on track
MANUFACTURING FLAW FOUND IN STOVL JOINT STRIKE FIGHTER LIFT FAN ROTOR
Date: January 4, 2008
Program officials have discovered a manufacturing defect in the lift
fan rotor of the short take-off vertical landing F-35 Joint Strike
Fighter test aircraft that will require the installation of new
components before flight and ground trials of the stealth aircraft
begin later this year. The lift fan rotor is part of a complex
propulsion system that allows the next-generation fighter to hover, a
technology that is essential to the capabilities of the JSF STOVL
variant, the F-35B.
Inside the Air Force has learned that during ground tests this fall,
the JSF program office discovered problems in a key part of the lift
fan built by Pratt & Whitney and installed in STOVL aircraft BF-1,
flaws that will require the lift fan rotor to be replaced. Bill
Gostic, vice president of the F135 engine program for Pratt & Whitney,
said in a Jan. 3 interview that during manufacturing, a process he
said is "not trivial," some of the fan blades welded to a fan disk
were problematic. "Sometimes in the early stages in your manufacturing
processes," Gostic said, "you have some dimensional non-conformances.
We had one in this case." Defense Department officials said they are
convinced that this problem reflects a manufacturing glitch, not a
more significant design flaw, and that correcting the defect will not
force a delay to the flight test plan or any other part of the
program. "We were on a schedule where [JSF prime contractor] Lockheed
Martin really needed a lift fan, and they were not planning on
operating the lift fan, so, we went ahead and delivered a full-up lift
fan assembly with this non-conforming, first stage lift fan
rotor . . . with the plan that we would go and replace that rotor
before we go actually operate the lift fan in any sort of flight test
program," Gostic said. At that time, Pratt & Whitney did not have
another rotor to swap out with the non-conforming one, he said. The
non-conforming rotor currently in the jet is being used primarily for
lift fan fit checks. The "current lift fan will be swapped for another
lift fan," said Cheryl Limrick, a spokeswoman for the JSF joint
program office, in a Dec. 20 e-mail. "The current plan for STOVL
engine qualification and flight test engine delivery does not impact
the JSF program schedule and or first flight of the STOVL aircraft."
Flight testing on the STOVL F-35 is slated to begin by mid-2008. "The
first F-35B is scheduled to begin its flight test program on time,
with no delays foreseen," John Kent, a Lockheed Martin spokesman, said
in Dec. 20 e-mail. "The flight propulsion system (lift fan and engine)
will be installed on schedule in early 2008." Since the JSF's
inception, there only have been two non-conforming rotors produced,
which Gostic characterizes as "typical in a development program."
"There is no design-related issue," he said. "It basically arises from
the fact that we need to fine-tune the manufacturing process with
regards to [the] machining of those fan blades in that first stage
rotor." The current plan is to remove the lift fan rotor in the first
STOVL test jet in February and replace it with a dimensionally
conforming rotor, which has already been built. The Marine Corps will
be the primary operator of the F-35B, however, the Air Force has not
entirely ruled out the possibility of purchasing the STOVL variant.
Anticipating possible delays of the STOVL variant in fiscal year 2008,
the Navy and Marine Corps in August 2006 cut funding from their
portion of the Joint Strike Fighter program in their FY-08 to FY-13
spending proposal to factor in a 14-month slip, which would have
pushed the purchase of 35 aircraft beyond 2013. Deputy Defense
Secretary Gordon England later reversed this cut, restoring funding
and keeping the program on its current schedule. The lift fan
generates a column of cool air that provides nearly 20,000 pounds of
power using "variable inlet guide vanes" to modulate the airflow,
along with an equivalent amount of thrust from aircraft exhaust,
according to an Air Force fact sheet. The fan utilizes a clutch that
engages the shaft drive system for STOVL operations. "Because the lift
fan extracts power from the engine, exhaust temperatures are reduced
by about 200 degrees compared to traditional STOVL systems," the
document notes. On Dec. 18, Lockheed Martin executives hosted a
ceremony at the aircraft assembly line in Ft. Worth, TX, to roll out
the F-35B, attended by Marine Corps Commandant Gen. James Conway as
well as foreign military representatives. The press release from the
event quotes senior officials extolling the promise of the new
aircraft, its STOVL capabilities in particular. No mention of the
difficulties with the lift fan was noted. The Air Force version of the
stealth fighter successfully flew four times in December after a seven-
month hiatus from flight testing, due to engine and electronic issues.
It is unclear how these delays will impact the overall program
schedule, Limrick said. "The plane was ready to go on time, each time
scheduled this month," she said of the December flight tests. Engine
turbine design changes are being worked that would be complete by late
2008, she said. "The F135 engine has been exceptionally reliable
during the 23 flights conducted in the F-35's first year of flight
testing, and F135 engines have completed more than 8,500 hours of
ground testing," Kent said. During engine ground testing last year, an
issue with the third-stage turbine blade arose. This issue has been
resolved in the short term, he said. "In the meantime, we will proof
test engines to validate turbine durability," Limrick said. "A proof
test will be performed on each flight test engine to ensure that the
engines are not susceptible to the issue experienced on the 3rd stage
turbine of a prior test engine."