WASHINGTON — The most widely used variant of the F-35 Joint Strike Fighter is currently unable to fly in thunderstorms after the discovery of damage to one of the systems it uses to protect itself from lightning, its prime contractor Lockheed Martin said Wednesday.

To safely fly in conditions where lightning is present, the F-35 relies on its Onboard Inert Gas Generation System, or OBIGGS, which pumps nitrogen-enriched air into the fuel tanks to inert them. Without this system, a jet could explode if struck by lightning.

However, damage to one of the tubes that distributes inert gas into the fuel tank was discovered during routine depot maintenance of an F-35A at Hill Air Force Base’s Ogden Logistics Complex in Utah, Lockheed said in a statement.

Lockheed temporarily paused F-35 deliveries June 2-23 as the company validated whether it was properly installing OBIGGS systems. However, “it appears this anomaly is occurring in the field after aircraft delivery,” Lockheed said in a statement.

Lockheed has since delivered two F-35s, company spokesman Brett Ashworth said.

Because it cannot be confirmed that the OBIGGS system would function properly if the jet was hit by lightning, the F-35 Joint Program Office has opted to institute flight restrictions.

“As a safety precaution, the JPO recommended to unit commanders that they implement a lightning flight restriction for the F-35A, which restricts flying within 25 miles of lightning or thunderstorms,” Lockheed said. “We are working with the F-35 Joint Program Office (JPO) on a root cause corrective action investigation to determine next steps.”

The issue only seems to affect the F-35A conventional-takeoff-and-landing variant, which is used by the U.S. Air Force and the majority of international customers. The OBIGGS design is slightly different on the F-35B short-takeoff-and-vertical-landing variant due to the aircraft’s lift fan, and the problem has not been observed on F-35C carrier-takeoff-and-landing aircraft, Ashworth said.

Bloomberg, which obtained a JPO memo dated June 5, reported that flawed tubes were found in 14 of the 24 “A” models inspected.

The JPO did not respond immediately to a request for comment.

For a plane nicknamed “Lightning II,” the F-35′s lightning protection systems have, ironically, become an embarrassing problem issue for the jet at times throughout its development.

The F-35 was prohibited from flying within 25 miles of lightning in the early 2010s after the Pentagon’s weapons tester discovered deficiencies with the original OBIGGs system in getting enough inert gas into the fuel tanks. Those restrictions were rescinded after the OBIGGS was redesigned in 2014.