For the third time in less than a year, the Pentagon has grounded all F-35 joint strike fighters because of a mechanical problem.

The 20 operational test and training aircraft were ordered parked Wednesday until engineers and technicians can find why a power system that starts and cools the aircraft failed during an engine ground test Tuesday at Edwards Air Force Base in California.

Suspending flights "is the prudent action to take at this time until the F-35 engineering, technical and system safety teams fully understand the cause of the incident," the F-35 Joint Program Office said in a statement. The program office oversees contractors and military test teams.

Few details of the incident were released, but the program office said that once the power system failed "the engine was immediately shut down and the jet was secured. No injuries to the pilot or ground crew occurred."

In March, all 12 test planes then flying were grounded until engineers could sort out why a dual electrical power generator failed in flight, creating a hazard. Some planes were released to fly within a week and the rest after two weeks, after the problem was tied to improper maintenance.

In October, planes were grounded for a week because of a software problem that could shut down fuel pumps in flight.

The system that failed this week, the integrated power package, was on the same plane whose electrical generator failed in March. The power system is used to start the main jet engine, provide air conditioning for the cockpit and numerous electrical systems, and generate backup electrical power.

Lockheed Martin spokeswoman Laurie Quincy referred questions to the program office.

Lockheed is the prime contractor on the F-35, the Pentagon's costliest weapons program ever. The F-35 development effort, based in west Fort Worth, has come under intense scrutiny in recent years from Pentagon leaders and Congress because of numerous delays and cost increases.

To read the complete article, visit www.star-telegram.com.