RAF FAIRFORD, England – Three F-35A Lightning IIs arrived here last week, the first U.S. Air Force Joint Strike Fighters to make the transatlantic journey. But watchers of the project were more interested in another milestone: When will the plane be ready for war?

“We’re getting very close to initial operational capability,” said Gen. Herbert “Hawk” Carlisle, the head of Air Combat Command, using the military’s term for battle-ready. Carlisle spoke to reporters Thursday at the Royal International Air Tattoo, a military airshow in central England, where the jets are to perform. “I’ll make the call. I’ll talk to Secretary [Deborah Lee] James and Gen. [David] Goldfein, [the Air Force chief of staff], and we’ll make the decision on when we’re going to declare IOC.”

The F-35 project office had previously set an Aug. 1 target date for IOC. In recent months, Air Force leaders have said the jet — specifically, a squadron at Utah’s Hill Air Force Base, north of Salt Lake City — would be declared battle-ready sometime between August and December.

But there are a number of benchmarks the plane must hit first — or as Carlisle put it, “It’s capability-based, not time-based.”

At least a dozen individual F-35 must demonstrate their ability to drop bombs and shoot down other planes. Each jet must be upgraded to a specific software package, and plugged into the complex logistics cloud that manages maintenance.

The Marine Corps said its F-35s were ready for war last summer. So, naturally, dozens of reporters here at the Royal International Air Tattoo pressed Carlisle for details.

“If you talk to the young men and women at Hill, Aug. 1 is their plan,” Carlisle said. “No, it’s not off the table.”

So when?

“I personally believe that we’re going to be able to declare IOC at the leading edge of that window,” Carlisle said. “So as we look at things, we’re getting close.”

The Air Force recently deployed seven F-35s from Hill to Mountain Home Air Force Base in Idaho. There, the planes fought mock battles against F-15E and F-16 fighters. The F-35s also dropped real bombs and flew all of the planned 88 sorties, exceeding Air Force officials’ expectations.

“Based on everything I know today and how this deployment went, I believe it will be the front end of that window of August to December,” Carlisle said.

So, maybe, expect it before Oct. 15.