Lockheed Martin's Fort Worth, Texas plant is beginning to look like a final assembly line. The first short take-off and vertical landing F-35B, aircraftBF1, is taking shape and behind it is B G-1, the first ground-test article. Across the partners, the programme has reached a build rate of about one a month - a momentum they want to maintain.

Mating of major components for BF-1 began on 1 June, and the aircraft is to roll out in December and fly in May 2008. It is the first F-35 with the "optimised airframe". Redesign to reduce the weight of the STOVL variant changed the mating concept, and BF-1 has a new centre wing section. Its assembly differs from AA-1.

"It has proved easier to assemble than we expected," says Bobby Williams, air vehicle team lead. "It's a huge piece of structure, and it was a hefty effort to get it built, but we made the scheduled goal of 1 June ." This included delays caused by additional machining steps required to produce a new bulkhead introduced by the redesign.

While assembly of the STOVL development aircraft gathers pace, drawing release for the conventional take-off and landing F-35A is nearing completion. Assembly of the centre fuselage for the first optimised CTOL flight-test aircraft, AF-1, has begun at Northrop Grumman, with its maiden flight planned for the- first quarter of 2009.

The F-35C carrier variant has just completed its critical design review and the first flight-test aircraft, CF-1, is scheduled to fly in May 2009.

Weight, a major programme concern, appears to be under control. "The F-35B's weight is exactly where it needs to be," says Williams. "We are meeting the key performance parameter for vertical landing bring-back. We're 200-250lb [90-110kg] below the weight limit - that's a considerable margin for a STOVL aircraft."

Over the last six to eight months, Williams says, there has been significant weight reduction on the F-35C as the carrier variant enters detail design. "We took about 200lb out," he says. "There isn't a weight requirement for the CV, but early on we agreed a weight target because the government wanted to assure long-term growth."

The F-35C is the third iteration of the JSF design, involving significant changes to the landing gear and structure to handle the loads introduced by catapult launches and arrested landings.

Integrating a carrier tailhook into the airframe was a challenge, Williams says, as the required structure competed for volume with the engine and stealth demanded the hook to be stored behind a door.

Recent changes to the CV design include an increase in wing area to reduce the power approach velocity. "Resizing the wing bought back considerable margin," Williams says.

Although detail design of theF-35C has involved structural changes, the commonality that is key to JSF affordability has been retained.

"On the structural side the benefit of commonality is in assembly, in common tools structural commonality part by part is not as critical," he says.

Software development is progressing in parallel with air vehicle design and assembly. "The software blocks are all tracking schedule," says Williams. Block 0.5 is "the heart of the mission-system integration", he says, with subsequent Blocks 1 to 3 adding functionality. "All the hardware for 0.5 is in the laboratory and operating on 0.5 software," he says, while the definition of Block 3, the standard for initial operational capability, has just been agreed with the JSF programme office.

Lockheed and its partners are preparing to begin low-rate initial production, beginning with a first batch of twoF-35As. The first international orders, test aircraft for the UK and the Netherlands, are expected in LRIP 3. With a slowing of the planned ramp rate pushing up the cost of early production aircraft, programme officials are working on a scheme to allow international partners to place multi-year orders to amortise pricing.

While all this goes on the first F-35, aircraft AA-1, is being prepared for a return to flight in August after a hiatus in testing following a brief electrical fault on its 19th flight, in May.

Source: Flight International