Lockheed Martin prepares to return F-35 to flight

Washington DC
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Lockheed Martin has begun ground runs of the first F-35 Joint Strike Fighter and is aiming to return to flight in November, six months after the aircraft's 19th test flight was cut short by a brief electrical failure.

Issues with electrical power supplies, the integrated power package, flight-control actuators and the Pratt & Whitney F135 engine combined to keep the aircraft, AA-1, on the ground longer than expected. Ground runs finally began on 9 October.

"Software updates and all hardware repairs resulting from the May flight anomaly have been completed," says Lockheed. "Over the next several weeks we will replace several more hardware items, based on laboratory and ground testing results."

Interruption of the May test flight, caused by arcing within electrical units in the 270V power system, has been tackled by increasing separation between wiring and casings. The JSF joint programme office (JPO) says AA-1 will return to flight with reworked and requalified "Dash 9" units and switch to redesigned "Dash 10" units as they become available.

An issue with the integrated power package (IPP), which combines auxiliary and emergency power units and environmental control system, was discovered during laboratory testing. Honeywell delivered a replacement IPP to Lockheed on 17 October after the unit had been disassembled and rebuilt to verify proper permanent-magnet rotor-to-stator clearance, the JPO says.

The non-flightworthy IPP in the aircraft was used for the initial ground runs, allowing aircraft system leak checks, flight software check-out and transition between normal and emergency power, says the JPO.

Lower than expected fluid reservoir levels in some flight-control electro-hydrostatic actuators (EHA) also caused concern. The JPO says AA-1 will return to flight with the existing EHAs, adding: "An investigation is under way to determine the cause of hydraulic fluid penetration through the EHA seals." The JPO does not expect any impact on flight testing because adequate spares are available, "and leakage is expected to decrease as the aircraft is exercised routinely".

The pacing issue is now root-cause corrective action following the30 August failure of an F135 engine during ground testing. Analysis indicates the primary cause was a third-stage low-pressure blade failure that led to bearing and shaft failures, says the JPO, and P&W says it has made sufficient progress to implement risk-mitigation actions.

A safety assessment cleared the way for ground runs on AA-1 with flight-test engine number 1, with inspections between runs, and the engine has since run at full augmented power in the aircraft, says the JPO. A further safety assessment for return to flight is expected to be completed by the end of October.

"Future test aircraft will be positively affected by the changes we've made on AA-1 that has, to date, performed as an effective programme testbed," says Lockheed. The next aircraft will be the first STOVL F-35B, which is scheduled to fly in May 2008.