WASHINGTON — The F-35 joint program office has completed development of the Block 3i software the Air Force will use to declare its jets operational later this year, and will soon begin retrofitting the fleet with the improved software load.

The JPO and the F-35 test team have been working overtime for months to fix major bugs in the original Block 3i software. Pilots found that jets' systems would shut down midflight about once every three or four hours and have to be rebooted. This "choking" effect, which pilots are seeing both in the air and on the ground during startup, is caused in essence by a timing misalignment of the software of the plane's sensors and the software of its main computers.

But the new, improved version of 3i is a significant improvement, according to the JPO. The new version showed approximately twice the level of stability as the previous load, Block 2B, and three times better stability than the original 3i software, JPO spokesman Joe DellaVedova wrote in a Monday email.

The JPO will begin to upgrade the F-35 fleet with the new 3i software beginning this week, DellaVedova said. The same fixes have also been incorporated into a new version of the Block 2B software, which the Marine Corps used to declare initial operational capability last year. The new version of 2B will be used to start retrofitting the earlier F-35s by the end of May, he noted.

The entire fleet of fielded F-35 jets will eventually be upgraded to the latest 3i and 2B software versions by the end of this year, according to DellaVedova.

The F-35 program is now focused on completing Block 3F, the software load that will give the fleet its full war-fighting capability, he wrote.

Although the JPO has given the new 3i software the green light, test pilots at Edwards Air Force Base, California, are still seeing frequent stability issues on the ground. Officials told Defense News last week that development test pilots at Edwards have trouble booting up their jets about once every three flights.

"The best analogy is you are starting up your computer and you want to use Word, Excel, PowerPoint and Outlook, and you are trying to get your work done for the day, and PowerPoint and Outlook came up but you are having an issue with Excel," Brendan Rhatigan, director of engineering and test operations for the F-35 ITF, told Defense News last week at Edwards. "So you say, I don't know what's going on, so let me x out of that, let me restart it again."

Twitter: @laraseligman