Two more Lockheed Martin F-35s are set to join the flight-test fleet as programme officials work to recover from a setback on 9 March.
Carrier variant flight-test aircraft CF-2 is scheduled in early April to become the eleventh member of the active flight test fleet, Lockheed says.
The final addition, CF-3, is in the company's "final finishes" position on the assembly line in Fort Worth, Texas, but should achieve first flight before June.
With the original AA-1 prototype inducted into live-fire testing last year, the arrival of CF-2 and CF-3 means the test fleet will finally reach full strength in a few months.
The new F-35s will join a fleet that is just overcoming an electrical power shutdown during a test flight of the AF-4 test aircraft on 9 March.
© Lockheed Martin
That incident forced the programme to ground all 10 flying aircraft for seven days. Seven of the 10 flight-test aircraft were cleared to return to flight on 16 March. The last three F-35s in flight test resumed flying on 25 March after being grounded for nearly 16 days.
An internal investigation revealed the cause of the power failure to be a maintenance procedure. A small amount of "extra" oil was allowed after maintenance service to remain inside the generator, according to the F-35 joint programme office.
"Under some conditions, the extra oil that is churning inside a narrow air gap," the office says.
This causes the temperature within the generator to increase. On the 9 March flight, both generators over-heated, forcing the F-35's second back-up - the integrated power-pack - to take over. The F-35 needs electricity to move its flight controls.
The problem affected the alternate engine starter/generator, which is installed on AF-4, BF-5, CF-1 and all low-rate initial production aircraft. The other seven flight-test aircraft operate with a different engine starter/generator, and were not affected by the grounding.
Despite the flight-test disruption, programme officials do not expect the overall schedule to change. "Flight-test schedules are built with a margin for precautionary safety stand-downs," the programme office says.