FORT WASHINGTON, Maryland — Retrofitting a fix to the F135 propulsion systems in the 21-aircraft-strong F-35 test fleet could begin in November and be complete as soon as early next year, according to Bennett Croswell, president of Pratt & Whitney military engines.

This would finally get the program back into a solid pace of flight testing, which has been interrupted since the June 23 fire caused by an catastrophic engine failure in an A model of the Lockheed Martin aircraft just prior to takeoff at Eglin AFB, Florida. Aside from six flight test aircraft for which the envelope has been expanded owing to the status of their engines, the remainder of the fleet is being handled somewhat gently with low-g flight and limited rolls.

The probable retrofit schedule appears to be far later than hoped for by U.S. Air Force Lt. Gen. Christopher Bogdan, the F-35 program executive officer. He said two weeks ago that if he is unable to get the testing fleet back into full-envelope operation by the end of this month, "we will start seeing some delays in future milestones that we talked about that we haven’t pushed up against yet." He was referring to such milestones as the first arrested landing and catapult trials on the USS Nimitz planned for November, and possibly items leading to the U.S. Marine Corps initial operational capability date set for July 1, 2015. When asked about the Nimitz tests, he said they are still not off the table. Regarding Marine IOC, he said it is "fundamentally on track," though needed mods to the F-35Bs and the delivery of mission data packages could be late.

Program officials have narrowed their investigation to five possible root causes for the engine failure in aircraft AF27, Bogdan said during the Air Force Association Air and Space Conference and Exposition 2014 here Sept. 15. The culprit should be known by the end of this month, Croswell says. Testing of a fix is slated to take place in October and retrofits could begin as early as November, with emphasis on the test fleet, he says. Roughly 150 engines are fielded and Pratt has agreed to pay for the retrofits.

The company is conducting testing on a "rub rig," a piece of test equipment in its West Palm Beach, Florida, facility designed specifically to test different densities of the polyimide foam used to form a plate seal between the stators and the integrally bladed rotor (IBR) in the third stage of the compressor section; this is where microcracking eventually led to a catastrophic failure in AF27.

Thus far, engineers have traced the origin of the extreme friction in the third-stage IBR to a ridge crossing maneuver conducted by a pilot three weeks before the fire. "It was definitely within the envelope of the aircraft, and the engine should withstand that," Croswell said. During such a maneuver, a pilot rolls an aircraft onto its back to maintain low altitude when climbing a ridge rather than a more extreme nosing up of the aircraft; this is a common practice in low-level flight, says Gen. (ret.) Howie Chandler, business development president for Pratt & Whitney Military Engines. During this maneuver, a combination of roll, yaw and g forces on a relatively new engine created what Bogdan calls "flex in the engine" such that heat from the excess friction caused the microcracking.

Such a phenomena had not been seen earlier in other F135s because of this combination of a low-hour engine performing such a maneuver, Croswell said. Because of its low hours, a suitable trench for the stator tips had not been "burned in" to this engine’s polyimide foam for a proper seal. This would normally wear in through natural use, as happened with the test fleet because those aircraft gradually performed more demanding maneuvers in flight testing. "This was not an area we thought we need to do recurring inspections, until we have an event," Croswell said. "We now understand … We are seeing that we need to open the clearances up."

Bogdan previously said that section reached 1,900F, nearly double the expected temperature, which likely weakened the materials there. Similar seals with similar clearances have been used in earlier engines without similar rubbing, though Croswell said the F-22 engine seal experienced problems leading to an oil leak early in its life.

Once the fix is devised and validated through the rub rig, Pratt intends to retrofit it onto F-35A engine FX638 for ground testing followed by validation that the seal prevents backflow of air in the compressor section while also providing suitable clearance to prevent the rubbing seen in AF27. Three other engines were found to have suspect rubbing during inspections following the AF27 incident; they are likely to be retrofitted, Croswell said.

Retrofits should be straightforward, he says, and should involve opening up the third stage and replacing the stators. He says this is likely to take place at service depots rather than field locations, which are only now standing up regular operations.

Once validated, FX638 will be flight tested; officials have not yet selected which test jet will carry the newly retrofitted engine.

Croswell says there should only be a "negligible," if any, performance penalty associated with the fix. "It is going to be a nit, I believe," he said.

Meanwhile, Pratt and airframe prime contractor Lockheed Martin are studying whether a series of flying maneuvers can be used to safely "burn in" the trench needed for already fielded engines so that they may return to full envelope flight. Croswell said a date has not yet been set to start those flights; Bogdan hoped this process could be used on the remainder of the test fleet to get it flying sooner without having to await retrofits.

Until the engine problem, the flight test program was on track to finish the year 25% ahead of its test plan, Bogdan said. "If it were not for that engine problem in June, I would have told you that for 2014 it would have been an awesome test year."

Aside from this issue, Croswell said the F135 has been a solid performer boasting high "time on wing" between failures. "We certainly don’t like to have issues like this," he said. "By and large the engine has performed well on the F-35 program."

Pratt and the F-35 Joint Program Office are nearing a handshake on a production contract for Lots 7 and 8, he says, adding that there will be up to an 8% price drop between low-rate, initial-production lot 6 and the end of lot 8. This would bring the total cost reduction in the F135 since LRIP 2 (two F-35A engines) to 50%, Croswell said.

Pratt & Whitney does not release the price of the engine; executives say engine companies typically do not release such data for military programs due to competitive sensitivities.