The first Lockheed Martin (LM) F-35 Joint Strike Fighter (JSF) has now been grounded for more than six months. But program officials hope to get the world’s largest combat aircraft program airborne again by the end of this month. “We have a very aggressive flight test schedule to get everything done by October 2012,” Bill Coutts, LM’s F-35 site director, told AIN at Fort Worth recently.

On the 19th test flight of aircraft AA-1 in early May, when it flew to near-40,000 feet for the first time, the right horizontal tail actuator failed. LM test pilot Jeff Knowles was able to recover safely, as back-up systems worked. The F-35 uses a unique electro-hydrostatic actuation system, provided by Moog Inc. But the fault turned out to be a relatively simple matter of arcing in the electronics controller, a condition that had not been apparent during prior altitude chamber testing.

A fix was devised, but before AA-1 could fly again, two separate failures occurred at test sites elsewhere in the JSF program.  In Florida, a Pratt & Whitney F135 engine running on a test stand experienced a blade failure. In California, an anomaly was identified in the aircraft’s Integrated Power Package (IPP)–another system unique to the F-35, by providing both emergency and start generation, as well as thermal cooling. The IPP problem was also fixed, but the blade failure has now forced LM to swop AA-1’s original engine for one of the other three flight test engines available at Fort Worth.

Until that 19th flight, the aircraft proved very reliable, according to Knowles. “The jet handles really well, and it was Code One after 15 of those flights, ” Coutts told AIN. ‘Code One’ means that the aircraft needs no rectification work before flying again. However, Knowles added, a six-week pause in flight testing had already been scheduled after the 19th flight, for a major software upgrade. “The longer layoff has allowed us to push other planned items forward,” he noted.

A total of 15 F-35s will fly in the demonstration phase. AA-1 is an F-35A, the conventional takeoff and landing version (CTOL). It is proving the aircraft’s basic flying qualities, but does not have the weight-saving structural changes that were designed in 2004-05. All subsequent development aircraft incorporate these, starting with BF-1, the first short takeoff and vertical landing (STOVL) version.

It is now due to fly next May, three months later than originally planned, after tethered hovering trials. BF-1 will be closely followed by BF-2. Both will demonstrate transition to the STOVL mode, and 120-knot landings, during flights at Fort Worth. Then they will move to the U.S. Navy Test Center at Patuxent River by the end of next year for what Coutts described as the first “true STOVL flights.”

Also due to fly next year are the first three representative CTOL aircraft, AFs-1 through  AF-3; CV-1–the first aircraft built for conventional carrier-borne operations–and BF-4, a STOVL aircraft that will become the first F-35 equipped with a mission system.

Pentagon and LM F-35 program managers have drawn criticism in the U.S. Congress and elsewhere for requesting funds for initial production aircraft, before the flight test program has demonstrated sufficient maturity. LM flight test officials at Fort Worth insisted to AIN that the complaints aren’t warranted. “We’re applying a lot of our experience from the F-22 program,” said Knowles. Various parts of the mission system already fly on testbed aircraft, added Coutts. Fort Worth also has a huge mission systems laboratory.

LM’s trump card–the ‘CATbird’–is a highly modified Boeing 737 designed to prove the F-35’s mission systems in the air, ahead of them flying in the actual combat jet. The CATbird can carry the fighter’s radar in its nose, and the canard-like surfaces aft of the cockpit are exact copies of the F-35’s leading edge. The 737 carries an F-35 pilot station and enough consoles for flight test engineers and avionics specialists to operate any or (eventually) all of the F-35’s five avionics subsystems. Later this month, it is due to begin test flights of the first of these–the communication, navigation and information (CNI) system.

“The CATbird will also be a great tool for the test pilots from the international partners who will be joining us, giving them initial experience of the aircraft’s capabilities,” Coutts told AIN.