WASHINGTON — In at least two cases, sudden spikes in cockpit pressure have left F-35 pilots with searing ear and sinus pain, Defense News has learned.

But F-35 prime contractor Lockheed Martin is confident that is has a fix ready for flight test that will correct the problem, which has been known since 2014 and categorized as the most serious type of technical deficiency by the U.S. Defense Department.

“There is no indication from the fleet that this ear and sinus pain issue is widespread,” read one document obtained exclusively by Defense News. Only two Air Force test pilots using early versions of the F-35 have experienced extreme incidents of barotrauma — the word given to ear injuries that occur due to changes in air pressure.

“[The] pain has been described as excruciating, causing loss of in-flight situational awareness, with effects lasting for months,” the document states.

Incidents of barotrauma aren’t unique to the F-35 fighter jet and could occur in any aircraft. The seriousness of such events can vary widely, with symptoms ranging from sinus pain and headaches to ruptured eardrums and hearing loss.

The pressure spikes in the F-35 seem to be of a more serious nature, forcing both pilots to abort test missions and inflicting “lingering symptoms” of “significant ear and sinus pain,” the document revealed. This presents the services with risks not only to pilot safety but to its own mission effectiveness, and all variants of the aircraft are susceptible to the problem.

The F-35 Joint Program Office believes it has identified the root cause of the problem: Sensors on the outer mold line of the aircraft are detecting “rapidly changing static pressures” that, in turn, drive very quick changes of the cockpit pressure regulator valve.

While there is no workaround for dealing with pressure spikes, the problem will eventually be addressed with a hardware modification to the cockpit pressure regulation system, with a proposed design change expected in 2019, a second document states.

The fiscal 2018 budget funded the development of a test rig to evaluate the proposed fix.

“We have an update that performed successfully in lab testing and will now be flight tested for future integration, based on customer timing priorities,” said Greg Ulmer, Lockheed’s vice president for the F-35 program manager.

One source close to the issue told Defense News that the program is working to identify exactly when the fix can begin flight testing.

“My goal would be to try and get it done if not this year, very early next year. And it is mostly a scheduling issue more than anything,” the source said. From there, the fix can either be rolled into the production line or delivered via retrofit kits for military maintainers to install.

Vice Adm. Mat Winter, the Defense Department’s F-35 program executive, said the government’s program office has taken “incredible steps” to ensure the jet’s life support system is robust.

“The other elements are canopy seals, the air system, ventilation system that allows the continued flow of air — all of those have been checked, rechecked and triple checked to ensure that we do not have a design issue that will have a systemic pressurization change,” he said. Pilots have also been given training on various physiological events that could occur in the cockpit “so that in an event that there is an overpressurization, they’re fine, they can handle it.”

A history of trauma

The documents reviewed by Defense News state that an informal safety assessment concluded there is low risk of the problem occurring more frequently, and no formal risk assessment ever occurred. The Defense Department also discussed flight restrictions that would limit the maneuvers that pilots may conduct while in the F-35, but these measures were not pursued.

Mark Gunzinger, a senior fellow at the Center for Strategic and Budgetary Assessments, said it was telling that the services had not imposed flight restrictions across the F-35 fleet. He characterized the problem as “certainly distracting, certainly not optimal, could certainly cause mission abort.”

“But the fact is it doesn’t appear to be a huge safety of flight problem, and it seems to meet requirements today,” he said. "If this was more endemic, if many more episodes occurred out in the force itself, I would be concerned. But this happened in a pretty significant flight regime. It’s not something you would probably do in day-to-day, real-world operations.”

Get a review of U.S. Air Force barotrauma events.

One Air Force F-35 pilot, speaking on condition of anonymity, said that the barotrauma problem wasn’t something that was causing anxiety among F-35A operators.

“I’ve been flying the airplane for three-plus years and haven’t had any issues with cabin overpressurization,” the pilot said.

While incidents of barotrauma occur throughout the military’s aircraft inventory, they are not as common or well-known in the public compared to other physiological conditions such as hypoxia, the medical term for oxygen deprivation.

According to data provided by the Air Force Safety Center, there have been 37 incidents of barotrauma in the service’s test, training and operational fleets between 2010 and 2018. Only one of those documented events involved an F-35A — an episode in 2018. It was unclear why the inciting incident in 2014 was not recorded, but a spokesman for the Office of the U.S Air Force Surgeon General said that if it was not reported to the flight surgeon, it may not have been recorded.

In terms of which platforms were most responsible for symptoms of barotrauma, the phenomenon appeared more common among older aircraft. There were multiple occurrences noted for a wide variety of legacy platforms, including the U-2 spy plane, KC-135 tanker, F-15 and F-16 fighters, A-10 Warthog plane, and C-130H military transport aircraft.

But barotrauma does not always occur due to a flaw in an aircraft. From fiscal 2003-2007, the Air Force reported 143 physiological incidents related to ear and sinus pressure issues, according to a December 2007 article in Air Force Flight Safety Magazine. Forty-three percent of those cases involved pilots who acknowledged preexisting cold symptoms or congestion, which can exacerbate symptoms.

These incidents of barotrauma aren’t the only physiological episodes that have been experienced by pilots while flying the F-35. In June 2017, Luke Air Force Base, Arizona — where all U.S. Air Force and international F-35 pilots train — temporarily stood down operations due to incidents where five pilots reported symptoms of oxygen deprivation. One additional hypoxia-related incident occurred at Luke AFB during the summer of 2017.

Investigations into the F-35′s life support systems and maintenance practices at Luke never produced a single root cause for the physiological episodes. However, a report by the U.S. Government Accountability Office laid out several problems with life support systems that were discovered during evaluations.

Aside from the barotrauma problem, the GAO noted that a breathing regulator on the pilot’s seat is failing at a higher-than-expected rate, which could increase the risk of oxygen deprivation. The unnamed supplier has made “slow progress” on improving that component, so alternative manufacturers are being considered by the program office.

When that breathing regulator fails, an anti-suffocation valve is supposed to open to allow the pilot access to air. That does not happen on a consistent basis, “creating a risk that unconscious pilots ejecting over water may drown,” the GAO stated in June 2018.

The valve’s manufacturer is assessing how it can improve the valve. But in the meantime, F-35 units have been directed to periodically inspect and clean that component.

To address these issues, the F-35 program office has made updates to change how air moves up the seat and to the pilot, Winter said. It also implemented a carbon monoxide filter and made adjustments to the onboard oxygen generating system, he added.

“Our previous software would allow the oxygen concentration to vary between an upper limit and a lower limit, and what's recommended by the air and medical community was to tighten that tolerance to keep a more consistent concentration of oxygen, and so we've done that,” he said.

Winter is confident the barotrauma problem will be downgraded from the most serious category 1 designation to category 2. However, the deficiency will stay on the books forever, as there’s no way to prove without a doubt that the issue is fixed, he said.

“The problem is that I’ll fly this jet all the way to 2077, for another 50, 60 years, and never have one [barotrauma incident], and there will be people saying: ‘Yeah, but you could have one tomorrow,’ ” he said. “There’s no way to prove the negative.”