EDWARDS AIR FORCE BASE, Calif. – F-35 critics often point to the Pentagon's decision to start building the fifth-generation fighter before design and testing is complete as the root of the program's problems. Even now, as the Air Force prepares to declare its F-35A jets operational this year, so-called "concurrency" remains an obstacle.

These ongoing challenges were on full display at Edwards last week during a development test flight of an Air Force F-35A, when the jet's team was on the ground troubleshooting for nearly two hours before the aircraft finally launched.

The problem, which revolves around a glitch in the next increment of F-35 software, is a recurring one that causes the plane's systems to shut down and have to be rebooted – sometimes even mid-flight.

Officials say development test pilots here have trouble booting up their jets about once out of every three flights, but downplayed the problem, pointing out that the goal of test flights is indeed to test, find problems, and work to fix them.

The F-35 joint program office and test team have largely resolved the software stability issue, which impacts jets loaded with the 3i software package, while the jets are airborne, but pilots are still frequently seeing shutdown events on the ground. And as the Air Force prepares to declare the F-35A operational before the end of the year, such problems are under a microscope.

Defense News was invited to Edwards to view the F-35 development test, or DT, program first hand. In conversations with officials, maintainers and pilots, those involved in the program acknowledged challenges with the software, but expressed optimism about the jets generally.

Although test pilots here often see shutdown events when trying to boot up a cold airplane, Lockheed Martin and government officials stressed that the number of these events that occur during DT is not representative of an operational environment. The DT startup sequence is unique in that the team has to use a special system, the data acquisition recording and telemetry pod, to monitor test data, according to Dan Osburn, F-35 integrated test force (ITF) deputy director and director of projects for the 461st flight test squadron.

Over the course of 40 flights using 3iR6.21 software, a version of the Block 3i software the Air Force needs to declare its F-35A jets operational, 15 experienced one or two shutdown issues while on the ground, said Brendan Rhatigan, director of engineering and test operations for the F-35 ITF. Test pilot and 461st flight test squadron commander Lt. Col. Raja Chari's experience matches up with the raw data: he told Defense News May 3 in a separate interview that he has to re-boot his jet during about half of his flights. 

"For a lot of these things, you go re-cycle the particular sensor or computer and the system comes back up, it doesn't mean you couldn't launch." Rhatigan said. "What's important is how quickly you can get out of the chocks, so when they get into the jet, turn the battery on, start the engine and power up the mission systems, how quickly can they taxi out to the runway?"

Gen III Helmet, 3 F-35 Pilots on Rogers Dry Lakebed, Air Force Test Center, Edwards AFB, CA. Oct. 1, 2015. Mr. David
Gen III Helmet, 3 F-35 Pilots on Rogers Dry Lakebed, Air Force Test Center, Edwards AFB, CA. Oct. 1, 2015. Mr. David "Doc Nelson, LtCol Jon "Miles" Oman and Maj. Chris "Sasquatch" Nations

Three F-35 Pilots wear the new Gen III helmet on Rogers Dry Lakebed, Air Force Test Center, Edwards Air Force Base, Ca.

Photo Credit: Tom Reynolds

The mission impact is minimal, Rhatigan and Osburn said, stressing that the start-up issues never prevented the aircraft from eventually taking off. Chari backed up the claim, adding that when he flew F-15s, it was "rare to not have an event."

"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," Rhatigan said. "So you say, I don't know what's going on, so let me x out of that, let me restart it again."

From that perspective, the team saw an improvement from the prior software release to the 3iR6.21 load, which incorporated solutions meant to fix the stability issues in the air, Rhatigan said. The fix shaved about six minutes on average from pilots' start-up time.

The shutdown events never prevented testing, said Rhatigan. In fact, all six 4-ship missions the team flew, which are much more representative of an operational mission, launched without needing backup planes, he stressed.

Despite the ongoing shutdown issues, the DT team recently gave the latest software load the green light. They finished testing 3iR6.21 April 26 and handed it over to the operational test, or OT team to continue testing. In the meantime, JPO Chief Gen. Bogdan has sent a "red team" to Edwards to investigate the root cause of the ground start-up and stability events, he told reporters recently.

Now the DT team is testing the 3F software, including critical tests of weapons delivery accuracy, in preparation for the Navy's planned deadline to declare its F-35C jets operational in 2018. They are aiming to be finished with DT by June 30, 2017, in time for the OT team to begin testing the full-up jets – a key test period that is officially called initial operational test and evaluation, or IOT&E – later in the fall, Osburn said. 

However, this timeline could see a delay due to software maturity and unplanned discoveries during testing, Osburn acknowledged. The team has used up "a majority" of its schedule margin, he said, but is working hard to find efficiencies in order to get done on time.

Meanwhile, the Pentagon's top weapons tester has been warning for months that the F-35 program will not be ready for IOT&E until 2018 at the earliest.

"Because aircraft continue to be produced in substantial quantities (all of which will require some level of modifications and retrofits before being used in combat), IOT&E must be conducted as soon as possible to evaluate F-35 combat effectiveness under the most realistic combat conditions that can be obtained," Director of Operational Test and Evaluation J. Michael Gilmore wrote in recent testimony before the Senate Armed Services Committee.

Twitter: @laraseligman