Flight tests starting next week will determine whether a key milestone for the US Air Force version of the Lockheed Martin F-35A will be delayed only four months or perhaps even longer, programme officials say.

The USAF had planned to stand-up the first F-35A squadron and declare initial operational capability (IOC) in August. But a planned computer processor upgrade late last year destabilized the software that connects mission systems, such as the radar, to the flight computer.

“What the pilots are seeing is when they take-off and they need to use the sensors, particularly the radar, the communication between the radar and the computer is mistimed,” says Lt Gen Chris Bogdan, the F-35 programme executive, speaking at the House Armed Services subcommittee on tactical air and land forces on 23 March.

As timing delays pile up, the radar enters a degraded mode or shuts down completely, he says, requiring several minutes to restart. The software causes a sensor to shut down an average of once every 4h, he adds.

The US Marine Corps declared IOC with the F-35B model last summer with Block 2B software, offering the bare minimum of weapons and manoeuvring authority. A few months later, Lockheed introduced the Block 3I standard, which re-hosts the Block 2B software on a more advanced computer processor. The USAF plans to announce IOC with the next software upgrade called Block 3I. A follow-on upgrade called Block 3F would enable the F-35 to carry a full complement of weapons and enable the full flight envelope.

Although no new functionality was added, rehosting the Block 2B software on the Block 3I hardware caused the failure rate to soar from once every 30h to once every 4h. “The complexity of that re-hosting should not be understated,” says Sean Stackley, assistant secretary of the navy for acquisition.

In the long-term, the programme hopes to return to the Block 2B level of stability, but in the interim the goal is more modest. For IOC, the USAF would accept Block 3F software with an average system failure rate of once every 8-10h, Stackley says.

A software patch developed for the Block 3I standard will enter flight testing next week. If failure rate improves to the 8-10h threshold, the impact on the USAF IOC schedule will be “minimal”, Bogdan says.

But the next challenge will be introducing the Block 3F software, which adds significant new functions for operating the sensors, weapons and flight controls.

“We are wary that further issues will emerge,” Stackley says.

Source: FlightGlobal.com