Pratt F135 Woes

By William Alibrandi, Forecast International

Pratt & Whitney’s F135 program has been under great scrutiny after several high-profile incidents, the latest involving an engine fire. While these have been a dark cloud hanging over the program, any new engine could have issues like this in development testing; it’s actually par for the course. The bigger problem may be with the F-35 Lightning program. The Pentagon decided to design, build, test and fly the F-35 all at the same time rather than in consecutive order as in past aircraft programs. Now, any problems with the F135 are exacerbated because the test schedule is so compressed, and any problems or flaws in the engine mean expensive rework – setting back the entire program. Arguably, if the normal development process had been followed, we might not be seeing as many engine-related issues with the Joint Strike Fighter.

The issue that resulted in the July 3rd grounding of the F-35 fleet has turned out to be an interference problem in the third-stage fan of the integrally bladed rotor (IBR) in the engine’s low-pressure compressor section. Normally an abradable strip maintains the tight clearances necessary to reduce pressure loss in this area, but the teardown has determined the rubbing to have generated excessive heat, causing micro fissures in the blades. The resultant failure caused the blades to come apart, resulting in the fire. The official word from the Pentagon is that the problem appears isolated, and all 98 F135 engines in service have been inspected. Pratt & Whitney’s president, Paul Adams, said the issue is not related to any previous incident. Pratt was already redesigning the first stage fan of the IBR and is switching from a hollow blade design to a solid blade. Plans for the redesign have been submitted to the Pentagon’s F-35 Joint Program Office for approval.


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