DoD Buzz

F135 Beset By 'Screech;' Fix Found


It's not often the Pentagon's top testers use the term "screech" to describe a problem with a weapons system, but that's just what they are calling a problem with Pratt & Whitney's engine for the Joint Strike Fighter.

"The [JSF] program began implementing plans to modify test aircraft to rectify the afterburner 'screech' problem, a problem that prevents the engine from sustaining full thrust. These modifications are necessary for the test aircraft to complete envelope expansion at the planned tempo," said the annual report by Michael Gilmore, director of Operational Test and Evaluation. My colleagues at Defense News first reported the screech problem.

With the continuing political battle over funding for the F136, the screech may become a factor in the debate. However, GE's often feisty spokesman, Rick Kennedy, was uncharacteristically soft-spoken in his email reply about how this would affect the program.

"Will it affect the debate? With Congress, GE/RR will use this is another example of how difficult it is to develop next-generation fighter engines. As you know, we believe the F136 -- because it was developed later than the F135 and was specifically designed for the JSF -- has inherent airflow advantages, and is demonstrating better temperature margins that translate directly into better performance and maintenance costs. That's being played out in the test cells right now," he said.  Then he softened. "But beyond the chest pounding, the F136 and F135 are both going to continue to experience technical challenges. It's the nature of fighter engines. And given the production volume anticipated for this single-engine F-35, you have a scenario incredibly similar to the F-16." Then he hauled out the standard line from GE and partner Rolls Royce: "Putting the aircraft's engines into a highly competitive environment is the only way these complicated technical problems -- like screech or any other issue faced by P&W or GE/RR -- is going to get fixed in the most aggressive manner possible."

According to Pratt spokeswoman Stephanie Duvall, a fix has in fact been found. And she provided a fairly detailed description of the problem:

"During development testing of the F135 in the May 2009 time frame, P&W found that at low altitude and high speed, certain pressure pulsations occurred when operating in full afterburner. This phenomenon, known broadly in the industry as screech, has been addressed with design modifications that have been validated to eliminate the issue. The modifications include minor hardware changes to the fuel system, reduced aerodynamic leakages, and upgraded software. The design of these modifications benefited greatly from the tools and processes developed in the design of the F119 engine that powers the F-22. The F119 and the F135 are the only two production engines that have provided augmented, stealth capability. With the modifications identified and implemented, the F135 now provides full max augmented thrust throughout the flight envelope. For the SDD program, a kit has been developed that brings these modifications to the engines that are powering the flight test program. Two engines have been modified to date with the design showing excellent results. The production configuration is being validated this year in both the CTOL/CV and STOVL variants of the F135. Confident that the F135 was providing the full required thrust throughout the flight envelope was just one of many reasons the government certified the CTOL/CV engine for Initial Service Release (ISR) in March 2010 and the STOVL engine achieved ISR in December 2010."

Perhaps Kennedy is right about the forces of competition? Or Pratt, an honorable company, drove ahead, powered by the desire to turn out the best solution for the taxpayer and the pilots. Or a bit of both.

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