The Lockheed Martin F-35 hasn’t flown a combat mission yet, but its taxed developmental aircraft need a service life extension.

The Joint Strike Fighter programme intends to award F-35 prime contractor Lockheed Martin a sole source contract in the third quarter of fiscal year 2017, according to a September announcement posted on the Federal Business Opportunities website. Under the SLEP contract, Lockheed will examine the aircraft’s airframe, hardware, software and structure, the Joint Programme Office tells FlightGlobal. The contract, which applies to US and international partners, will also bring the test fleet’s hardware configuration to parity with the new fleet.

“The goal will be to resolve any modifications done to the new production aircraft and operational fleet that the DT aircraft were too busy to accomplish until the end of System Development and Demonstration without major impacts to the SDD schedule,” he says.

Lockheed designed the DT aircraft with a service life of 8,000 flight hours. But unlike the production aircraft, the DT aircraft fly at the extreme ends of their design limits for extended periods in order to gather flight test data, JPO’s spokesman says.

“Extended flight in those regimes will wear out an F-35 faster than F-35s flown in their operational [flight] envelope,” he says. “The operational F-35s can go into the same corners of the envelope, but if so, spend brief times in those areas as it is not tactically feasible to remain there.”

By October, individual aircraft within the DT fleet had clocked between 314 and 1,200 flight hours. The JPO plans to use the full life of DT aircraft for systems aircraft, but will not use the maximum 8,000 flight hours for flight science aircraft that fly at the most stressful corners of the flight envelope. The DT aircraft will be used beyond Block 4 testing.

The test aircraft have gone through a drawn-out process as F-35 developmental test has progressed more slowly than expected. In an October memo, the Pentagon’s top weapons tester revealed that several quick reaction capability software builds pushed back flight test for Block 3F 10 months later than originally planned. And although additional developmental testing is planned, the JPO has planned to truncate parts of that process, Michael Gilmore states in the memo.

In August, Gilmore warned the programme was running out of time and money to complete planned flight testing and modifications.

“Flight test is making progress, but has fallen far behind the planned rate to complete System Development and Demonstration with the remaining time and funding,” the director of Operational Test and Evaluation stated. “Despite needing to continue developmental testing at full capacity for at least another year to complete the planned testing of the new capabilities and attempted fixes for the hundreds of remaining deficiencies, the program is already beginning to reduce the number of test personnel and defer required fixes to beyond SDD due to funding constraints.”