Lockheed Martin has rolled out the fifth aircraft for the Joint Strike Fighter flight-test programme, but one that carries special significance.
The second flying short take-off and vertical landing F-35B to roll out of Lockheed's Forth Worth, Texas factory, BF-4 will be the first asset used for mission systems testing.
After its first flight in the middle of this year, BF-4 will be used to evaluate and certificate the F-35's sophisticated sensor suite, including its Northrop Grumman APG-81 active electronically scanned array radar, Lockheed electro-optical tactical targeting system, BAE Systems electronic warfare systems and avionics boxes.
As such, the STOVL prototype will be the first to put the roughly 7 million lines of software code on board the F-35 to the test. BF-4 will begin flight tests using the Block 0.5 software code.
Two more flight-test aircraft will eventually join BF-4 for dedicated mission systems testing.
Meanwhile, Northrop has started flight tests of the first parts of the mission system using the JSF programme's Boeing 737-based co-operative avionics testbed, or CATBird.