Of all the F-35 Lightning II variants currently in testing, the most troubled has been the F-35B STOVL fighter. The aircraft has had recurring issues with sub-components that are failing at higher than expected rates leading to problems and delays in the flight program.

The F-35B fighter was also dealt a blow when the program was recently put on a 2-year probationary period. Lockheed is still hard at work on the F-35B and the aircraft has shown some progress recently. Defense News reports that the aircraft has had a series of five vertical landings over the last eight days that have come off without a hitch.

The vertical landing tests were performed between January 6 and 13. The five successful landings are part of the 42 that must be completed before the aircraft can be tested at sea on an amphibious assault ship. When the remaining tests will happen is unknown; so far the 2011 flight test schedule has not been published.

"I think it does [signal that the program is getting back on track]. This program has never been quite as troubled as many critics thought. I think it's probably progressed more smoothly than other fighter development program with the possible exception of the F-16," said Loren Thompson, an analyst at the Lexington Institute (Arlington, Virginia). The F-16's development proceeded so smoothly because of the simple nature of the original version of that aircraft, he said.

Thompson also notes that the issues the F-35 has faced so far are common teething problems that can be found in new aircraft programs. By comparison, the issues that the F-22 Raptor faced were much worse.

Teal Group analyst Richard Aboulafia said that the challenges left in the F-35B program can be addressed in the 2-year probationary period.