F135 Fix Nears Completion As Production Ramps Up

Aviation Week // April 07, 2015

Pratt & Whitney is ramping up retrofits to operational F135 engines with a fix to the problem that led to a catastrophic engine fire last year in the F-35A Joint Strike Fighter, and aims to modify the entire fleet by the first quarter of 2016.

The engine maker also aims to define a long-term solution by the middle of this summer, Pratt & Whitney Military Engines President Bennett Croswell says. “We have several candidates, including the current fix, and we’ll work with the [F-35 Joint Program Office] to identify what the long-term fix will be.”

The interim fix—called “pre-trenching”—deepens the gap in the polyimide foam lining between the tips of stators and the knife-edge seal forward of the third-stage integrally bladed rotor (IBR). The pre-trenching deepens the gap, enabling the spinning seal to pass through the end of the stators without creating friction. The company is also evaluating a couple of long-term solutions which include adding a hard coating to the seal plate to resist microcracks that occurred as a result of the frictional heating, or a combination of trenching along with a coating, Croswell adds.

All but one of the F-35 System Development and Demonstration (SDD) aircraft have been modified, apart from one F-35B currently undergoing evaluation in the U.S. Air Force 96th Test Wing’s McKinley Climatic Laboratory located at Eglin AFB, Florida. “We either have the pre-trench configuration installed or, on a couple of the jets—mainly the Flight Sciences jets—we have the burn-in procedure,” Crosswell says, referring to a series of specific flight maneuvers that have been devised to gradually wear in the trench between the stator and polyimide foam.

Pratt also says opening up clearances by pre-trenching has had a negligible effect on performance. Mark Buongiorno, head of the F135-engine program, describes the effect as “insignificant,” while Crosswell says, “we only gave up a couple of degrees of turbine temperature. Operability-wise it was no impact, so it was a pretty good solution.”

Tests to help confirm the configuration of the final fix are underway at a rub rig in West Palm Beach, Florida. “We still wanted to evaluate whether we could create a rub-in system that would give us the full capability of a matched set,” Buongiorno says. “We will use that to inform what will be the long-term solution,” comments Crosswell. “We are doing lots of parametric studies on polyimides to see if there’s any impact on orientation, and our clearance tools are much improved.”

Pratt also is gearing up for engine-production rates that were originally expected to have occurred two to three years before development delays began to seriously affect the F-35 program. The engine maker, which expects to deliver 60 F135s in 2015, is currently completing deliveries of engines for the seventh low-rate initial production (LRIP) batch of aircraft and will complete the eighth set in the second quarter. The company has submitted a proposal to the JPO for the ninth and 10th LRIP sets. “For the LRIP nine proposal we have submitted for 60 engines again, and LRIP10 goes to 100. So in 2017, we are finally going to start seeing the ramp that we have been waiting for,” Crosswell says.

The rate increase “is going to be very beneficial to us from a fixed-cost absorption perspective and we have continued to come down the ‘war on cost’ curve. But it has certainly been a challenge with the volume moving to the right,” he admits. Since the first production-representative engine was delivered in 2009, the cost of the F135 has been reduced by 55%, Croswell says. Officials from Pratt still decline to provide a dollar figure for the price of the engine, citing ongoing competitive pressures.

Pratt is now laying out a production plan for “post T300,” the milestone engine Pratt aims to be producing for the same cost as the F119 engine, from which the F135 is derived. “That will form part of the ‘blueprint for affordability’ objective and we are in process of working that now with [the] JPO,” says Crosswell, who adds that as of March 31, the company has delivered 217 F135s, including special upgrade test units and SDD powerplants. At the time of Aviation Week’s visit to Pratt’s Middletown F135-production facility in early April, the company was set to deliver production engines 146 and 147.