News | Dec. 19, 2023

Simple Fix Prevents Catastrophic F-35 Accident

By David Byrd

A simple, “washer-like” design helped avoid a serious F-35 Lightning II accident in mid-October and enabled an uneventful landing at Marine Corps Air Station Cherry Point, North Carolina.

About the size of a fingernail, the conical device fits into the aircraft engine’s fuel line to mitigate the effect of a ruptured fuel line, such as the one that caused a Class A F-35B mishap at Naval Air Station Joint Reserve Base Fort Worth, Texas, in mid-December 2022. The DoD defines a Class A mishap as one that costs more than $2 million, causes a fatality or results in the total loss of an aircraft.

There were no fatalities during the mishap and engineers and specialists are still trying to determine if that aircraft is salvageable.

“The Pratt & Whitney F135 engine uses fuel pressure to control actuators in the engine. When you lose that fuel pressure, you lose control of the engine,” said Mike Reedy, the lead propulsion engineer with the F-35 Lightning II Joint Program Office. “The washer doesn’t prevent the fuel line—or what we more aptly call the fuel-draulic line—from cracking and failing, but sustains the pressure needed to maintain engine control.”

Reedy likened a failure without the washer to a car trying to merge onto a highway.

“It’s as if you were driving your car and stepped on the accelerator, but nothing happened, or nothing happened for five seconds. Plus, your steering wasn’t responsive.

“With your car, it’s dangerous, but with a single-engine aircraft, that is catastrophic.”

During the October Cherry Point incident, the pilot received an alert that manifests after the vulnerable fuel-draulic line ruptures with the washer in place. After an uneventful landing, ground inspection confirmed a fuel leak identical to the one that caused the crash at Fort Worth.

“Better a relatively benign event than an accident,” Reedy said. “We directly prevented a loss of aircraft.”

Following the Fort Worth accident in December, the F-35 JPO team immediately began work to determine the cause of the incident and produce a solution. The washer was the first thing the team came up with.

“It was so successful we didn’t need to try anything else,” Reedy said. “More exotic designs were proposed, but because of [the washer’s] simplicity and effectiveness—validated through extensive testing—and ease of installation, no other near-term mitigations were pursued.

“The only day we didn’t meet those first weeks was Christmas Day. I was blown away with how the team came together to design, fabricate and test the part. It was a tremendous effort. By March 2023, we started installation.”

“This was a team win that included JPO, P&W and Lockheed Martin. All had a hand in this success. The unfortunate circumstance that brought this team together yielded a model for government/industry working relationship,” he said.

Somewhat counter-intuitively, the JPO began to field the solution on its new and low-time engines rather than older engines.

“The Fort Worth incident and a similar occurrence in the engine acceptance phase in 2020 happened on low-time engines,” Reedy said. “If the line is going to rupture, it is going to rupture early.”

“After the 2020 incident, we incorporated a screening procedure during the engine acceptance test. The 2022 incident demonstrated that we needed a hardware fix in addition to the screening procedure.”

Since March 2023, the interim solution has been included in engine deliveries off the production line. A complementary retrofit campaign affected about 1,000 F135 fielded engines, new and old alike. Engines belonging to partner nation and international customers would also be retrofitted.

“The core engine is the same on the A, B and C models, so this work affects all of them,” Reedy said. “The [Short Take-Off and Vertical Landing] STOVL (B Model) is most critical considering the way it is recovered on a ship. During low speed approach and hover unique to the STOVL variant, it is reliant on the propulsion system for aircraft control because there is minimal-to-no lift created by the aircraft's wings.”

Despite its success, the device is an interim fix until a permanent solution is implemented beginning late this year. “The permanent solution to the vibration problem is a more robust fuel line. The washer is highly effective, but doesn’t prevent the line from rupturing. We will start installing the improved fuel line in December 2023 and plan to be finished in late 2024. That said, we are also going to include the washer in the new fuel line as well.” 

According to the Congressional Budget Office, last year the Department of Defense operated about 450 F-35 aircraft, with plans to fly 2,500 of them by the mid-2040s. 

David Byrd is editor-in-chief of Naval Aviation News.