Three of the five developmental Lockheed Martin F-35Bs have developed tiny cracks in a lift fan-related component which prevent the flight-test aircraft from reconfiguring in flight and landing vertically.
Two flight-test aircraft - BF-1 and BF-2 - are now being modified with a redesigned actuator support beam, according to the joint programme office.
BF-4 has also developed "hairline" cracks in the same part, but is continuing to fly in conventional mode only until the part is modified, the programme said.
The potential for cracks to develop in the actuator support beam was identified several years ago. A redesigned beam was installed on the fifth short take-off and vertical landing (STOVL) test aircraft during final assembly, the programme said. That means the BF-5 test aircraft can continue to make vertical landings as part of the flight-test programme.
The only remaining test aircraft - BF-3 - has accumulated fewer flight hours. No cracks have yet been found. BF-3 is not technically restricted from completing vertical landings, but has only flown in conventional mode since the cracking problem was discovered.
The overall impact on the flight-test programme is expected to be light, programme officials said. BF-5 was the only STOVL variant scheduled to complete more vertical landings this year.
The cracks were discovered about a month after BF-2 and BF-4 completed a series of shipboard vertical landings on the USS Wasp, an LHD-class amphibious carrier.
The 18-day period aboard the Wasp was hailed as a landmark in the F-35B flight-test programme, symbolising its recovery from technical issues that had limited the STOVL variant to 10 vertical landings in 2010.
US Secretary of Defense Robert Gates placed the F-35B variant on a loosely defined "probation" earlier this year, warning that further problems could result in the programme's termination.
The F-35B appeared to rebound after Gates' announcement on 6 January. Three of the five propulsion system glitches have been fixed, with the remaining two expected to be cleared by February. A cracked bulkhead discovered in durability tests last November has also been redesigned.
However, the F-35B continues to face higher scrutiny amid the US military's uncertain budget situation.
Winslow Wheeler, director of the Straus Military Reform Project at the Center for Defense Information, and a noted F-35 critic, links the newly-discovered cracks on the actuator support beam to his overall case against the F-35B.
"The least capable, most complex, most expensive, most problematic F-35 is amply demonstrating its doomed-from-the-start heritage," Wheeler said.