The F-35 jet’s design puts it at risk at losing both of its hydraulic brake lines when a tire is blown upon landing, and although the issue has been corrected in the F-35C carrier version, the "A" and "B" models may not ever get a full fix.
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Staff Sgt. Mark Freeman, 33rd Aircraft Maintenance Squadron, performs a post-flight check on an F-35A Lightning II tire before refueling it at Eglin Air Force Base, Fla. (Samuel King Jr./U.S. Air Force)
The issue came to light in documents exclusively obtained by Defense News, in which the problem is labeled as a category 1 deficiency by the U.S. Defense Department, the designation given to serious technical problems that affect safety, mission effectiveness or some other requirement.
But not all such problems are created equal, and the Pentagon may be able to downgrade the deficiency to category 2 status without a fix in place.
According to the documents, the concern is that a blown tire “will result in the loss of one or both hydraulic systems, which may degrade directional control during landing rollout and could lead to runway departure,” presenting a loss-of-aircraft risk.
The integrated test force at Naval Air Station Patuxent River, which documented the deficiency in 2014, was originally “highly concerned” that the proximity of hydraulic lines could make the probability of a dual failure "nearly as likely as a single failure,” the documents state.
As a result, the brake lines were relocated in the F-35C, starting in the tenth lot of aircraft.
The F-35 Joint Program Office did not respond by press time to a detailed list of questions about the problem, submitted months in advance of publication, including why the issue has been corrected in the "C" variant and not the A and B models.
However, a risk assessment is ongoing that will help determine whether the problem can be downgraded to a category 2 deficiency, and whether further modifications are needed.
Greg Ulmer, Lockheed Martin’s F-35 program head, said that having two hydraulic lines provides redundancy if one line is ruptured, and that there has never been a case of both lines being impacted.
He also noted that the Defense Department’s program office has instituted some workarounds that have decreased the probability of blowing a tire.
“Brake control software updates and pilot training have alleviated this concern and resulted in a significant drop in blown tire events,” he said. “Additionally, we made minor adjustments to the location placement of hydraulic lines on the F-35C that has resolved the potential for line breaks. We believe the item is resolved and are standing by for additional customer feedback.”
Another source familiar with the program said while the frequency of blown tires made this a concern about four years ago, the rate of such events has slowed down “and it’s really been very quiet ever since, which shows good progress.”
In the past, when there has been a failure of a single hydraulic line, the incident has resulted in no injuries and less than $50,000 worth of damage to the aircraft, according to the documents.
Usually “such an event requires some repair work to the landing gear,” the U.S. Government Accountability Office explained in an April 2019 report, which added that both Lockheed and the government’s program office do not consider the issue a safety concern.