A new quality issue with Pratt & Whitney F135 engines has upset a manufacturing system for the Lockheed Martin F-35 that is otherwise showing signs of reaching stability, according to the programme's top executive.

While previous manufacturing schedules wasted one day of work for every two days of calendar time, the latest approved timetable - internally dubbed "shop operating plan-5" - has not deviated by more than about two days cumulatively since being adopted six months ago, says Rear Adm David Venlet, F-35 programme executive officer.

Speaking to reporters on 21 April, Venlet predicted that the pace of deliveries would start increasing after Lockheed delivers the last three of 13 test aircraft that have long been associated with unforeseen complications and schedule delays.

Engine quality problems, however, have become a major issue for that F-35 manufacturing schedule in the last several weeks.

Pratt & Whitney confirms that a "small number" of F135 test and production engines have been replaced with spares since March. The replacements were ordered after a ground test engine was found to be mis-assembled after an overhaul, Pratt & Whitney says. Further checks identified the same problem on other test and production engines.

"These engines are being replaced by spare engines on site in Fort Worth, with no impact to the F-35 flight test programme," the company says.

However, Venlet says the engine problem has caused a short-term interruption to the new manufacturing plan.

"The engines are getting there and they are recovering to schedule," Venlet says. "The shop operating plan has some resilience in it - a few engine changes. We can't be changing every engine. So [the manufacturing schedule] will drop off but I think we'll see it recover."

Venlet's predecessor, Brig Gen David Heinz, also complained about quality control issues on the F135 engine, telling reporters in July 2009 that although individual turbine stages met quality specifications, the combined stack of stages in a completed engine did not meet tolerances for quality.

P&W officials have said that the "stack" problem cited by Heinz was corrected long ago.

The F135 is now the only F-35 engine after the Department of Defense terminated the General Electric/Rolls-Royce F136 alternate engine on 25 April.

Source: Flight International