F-35 Program Moving Forward to Repair Cracks in Engine Component
The military is honing in on the root cause of an engine fire that temporarily grounded the fleet of F-35A joint strike fighters in June, and a fix could be decided in October, its program executive officer said.
"Once we get to the root cause by the end of this month, we've got six different options on the table for how we're going to fix the engine,” Air Force Lt. Gen. Christopher Bogdan, head of the F-35 Joint Program Office, said in a Sept. 15 speech at the Air Force Association’s Air and Space conference at National Harbor, Maryland. “I would say by the end of October, we will downselect to what appears to be the best solution to fix the engines.”
Bennett Croswell, president of Pratt & Whitney military engines division, which manufactures the joint strike fighter’s F135 engine, said he believes that weeks before the fire, a pilot performed a “relatively aggressive maneuver” that may have caused the seal plate within the third-stage fan to overheat and form “micro cracks.” In time, those micro-cracks propagated to other third-stage fan components, including a titanium arm that broke apart from the engine and pierced through the fuel tank, starting the June 23 fire.
"These type of seal systems are on legacy platforms ... and we've never had an issue,” he said. “This was not an area that we thought we needed to do recurring inspections on until we had this event," Croswell said.
Croswell stressed that although the maneuver “had a lot of pitch, yaw and Gs to it,” it was still within the flight envelope of the jet.
The military expects that a fighter’s engine components will rub together and produce heat during flight, Bogdan said.
"Fighter engines [are ] actually not static things when you put them in an airplane,” he said. "When you pull Gs on the airplane or you yaw or you roll the airplane, the engine actually flexes."
What was unexpected was the excessive heat caused by the “hard rub,” he said. The seal plate normally functions at about 1,000 degrees Fahrenheit, but the cracks were formed by temperatures closer to 1,900 degrees.
Pratt & Whitney is considering numerous fixes to the problem, Croswell said. The company has built “rub rig” at its West Palm Beach, Florida, headquarters to help determine the root cause of the hard rubbing, he said.
"We have to validate that we can generate those kind of temperatures, and that those kind of temperatures create those kind of micro-cracks,” he said. Testing is expected to wrap up this month.
The rig uses titanium and a foam-like polyimide to replicate different kinds of rubbing in a controlled manner, Croswell said. "We're going to test different densities of foam to see if that makes a difference. Were' going to look at the orientation of how the polyimide is formed ... to see if that is a contributing factor."
The company plans to conduct ground testing of an engine that has “pre-trenched” the polyimide rub strip in the third stage fan, which should eliminate rubbing, Croswell said. Currently, the normal rubbing of components inside the engine creates the trench after use. If ground testing shows that pre-trenching has no detrimental effect on engine performance, the company will begin flight tests in October.
Should testing be successful, Pratt & Whitney will retrofit the existing system development and demonstration aircraft F-35s with pre-trenched engines. That could begin as early as November and conclude by the end of the year.
"Pratt has agreed that we're going to be part of the solution to pay for that cost,” Croswell said. “It's not a hugely expensive process."
Pre-trenching could be adopted as a long-term solution, in addition to the other six options under consideration by the joint press office, he said.
The company is also working with F-35 prime contractor Lockheed Martin to validate a set of uniform maneuvers that can slowly build a trench without causing a hard rub, Croswell said. The companies are culling data from the rub rig, as well as other modeling and simulation assets, to develop those maneuvers.
F-35s are no longer grounded, as they were in the weeks following the fire, but flight envelope restrictions are still in effect for most of the system development and demonstration fleet, Bogdan said.
To minimize the impact of the flight test program, the JPO has expanded the envelope of five SDD aircraft.
Aircraft are also required to undergo an engine inspection every three hours, he said.
Three other engines were found to have hard rubbing, Croswell said. All of them probably pulled an aggressive maneuver early in the lifecycle of the engine.