WASHINGTON — All F-35 fighter jets should be retrofitted within 90 days with a fix intended to solve a potential engine vibration problem, the F-35 Joint Program Office said Thursday.

The JPO issued an order Wednesday evening recommending the fleetwide retrofit — globally, not just American aircraft — over the next three months, as well as immediately putting it in place for a “small number” of fighters that have been grounded since December.

In a statement to reporters on Thursday, the JPO said it is not grounding other F-35s, aside from the newly built fighters believed to be susceptible to the vibration problem in their Pratt & Whitney-made F135 engines and that have been grounded for more than two and a half months.

The office also said it plans to work with the military services flying the F-35 and international partners to ensure they understand the technical order. “The safety of flight crews is the JPO’s primary concern,” the JPO said.

F-35 deliveries were halted in mid-December after a mishap involving a new F-35B in Fort Worth, Texas. That F-35B, which was undergoing a quality check flight, was videotaped bouncing, tipping forward, and spinning around on the ground before its pilot ejected safely.

After that mishap, Lockheed Martin, which makes the aircraft, stopped acceptance flights for new F-35s. Those flights must occur before the company delivers the jets to the U.S. government. Those groundings had the effect of halting deliveries.

Lockheed confirmed to Defense News on Thursday that it has not yet resumed flight operations or deliveries of new F-35s, most of which are constructed at its Fort Worth, Texas, facility. Lockheed Martin has delivered more than 890 F-35s around the world.

The Pentagon as well as Pratt & Whitney halted engine deliveries later in December. An investigation found a vibration issue in the engine led to the mishap. The JPO said that vibration issue was a “rare occurrence” and announced in February that Pratt & Whitney, which is owned by Raytheon Technologies, and other engineers had developed a fix for it.

The JPO on Thursday said the vast majority of F-35s are not experiencing this engine vibration problem. But it is retrofitting the entire fleet because the fix is “inexpensive [and] non-intrusive,” and will mean all F-35 engines have the same configuration.

This fix can be done at the operational level in four to eight hours, the JPO said. But the office did not detail what this short-term fix will include, besides saying it “mitigates the risk of loss of aircraft should the harmonic resonance occur.”

A longer-term fix is also in the works to keep the vibration problem from happening again, the JPO said, but it did not say what those solutions might be.

In a Tuesday conference call with reporters, Jen Latka, vice president of F135 programs at Pratt & Whitney, said the company is shipping new engines with the fix already installed.

The JPO said Feb. 24 that it had cleared Pratt & Whitney to resume delivering engines.

Latka said the problem is “very rare,” and pointed to the F135′s 600,000 flight hours before the vibration issue happened. Nothing had changed in the F135 engine before this vibration issue happened, she added, and “multiple parameters” had to take place for the dangerous vibrations to show up. She did not identify those factors.

When asked why the vibration issue only cropped up now, the JPO responded: “The harmonic resonance is caused by other systems performing their normal function. Root cause investigation is still ongoing to determine where these system sensitivities intersect with the excitation frequencies.”

Latka said Pratt & Whitney had dealt with “resonance” issues with the F135 engine before the Dec. 15 mishap. But what happened with that F-35B involved “new aspects,” she explained.

“There was new learning there,” Latka said.

The JPO as well as Pratt & Whitney declined to comment on who would bear the cost of the retrofit and other issues with the F135 engine and the delivery halt.

“We will not comment on financial responsibility,” the JPO said. “We are focused on returning the fleet to full safe flight operations.”

Stephen Losey is the air warfare reporter for Defense News. He previously covered leadership and personnel issues at Air Force Times, and the Pentagon, special operations and air warfare at Military.com. He has traveled to the Middle East to cover U.S. Air Force operations.

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