Lockheed Martin has pushed back the resolution of a manufacturing problem plaguing F-35 Joint Strike Fighter final assembly schedules, but key suppliers are making progress building components as the programme prepares for the next leap in production orders.

In October 2009 government audit reports showed that Lockheed expected to eliminate the "wing-at-mate overlap" problem for the F-35's four-piece wing with final assembly of BF-13, the thirteenth short take-off and vertical landing (STOVL) in production.

The overlap means that key parts are delivered after the wing has entered final assembly, requiring workers to partially disassemble the structure.

The Defense Contracts Management Agency (DCMA) identified the resulting delays and inefficiency in the wing manufacturing process as one of the key drivers for production delays ranging from four to six months during the first two years of low-rate initial production (LRIP).

F-35 supplier Northrop Grumman has released a photograph showing a nearly completed centre fuselage for the BF-13 airframe, meaning the structure will soon be shipped to Fort Worth to be mated to Lockheed's forward fuselage and wing sections, as well as the BAE Systems empennage.

But Lockheed acknowledges the wing-at-mate overlap problem lingers as BF-13 enters final assembly, with the amount of completion before the wing is mated to the fuselage improving by only 2% during the past six months.

Lockheed has not disclosed when it will achieve its stated goal of eliminating wing-at-mate overlap, saying only the issue is now expected to "expected to continue past BF-14".

The company also points out that the problem's severity has softened significantly since October, when completion rates ranged from 70-75%. Lockheed made dramatic progress on the problem from October to February, raising completion to 90%. The rate is now at 92%.

Lockheed is steadily reducing parts shortages and late deliveries that created despair at the DCMA in April 2009, when the agency's on-site inspectors warned in a monthly assessment report that the programme may never "achieve or sustain" full-rate F-35 production if delivery trends continued.

Northrop has announced that the F-35 centre fuselage is now being delivered at a rate of more than 24 a year. That signals the company's output is outpacing the schedule. The supply chain is still building F-35 ordered under the third LRIP contract, when only 17 jets were ordered.

Maintaining that rate of progress will be critical in the next few years. Yearly orders increase from 32 to this year to perhaps 43 next year. The programme had planned to more than double the order to 110 jets in fiscal year 2012. A restructuring ordered earlier this year by Secretary of Defense Robert Gates is likely to soften the ramp's steep one-year jump.

Source: Flight International