A F-35C conducts a test flight over the Chesapeake Bay on February 11, 2011. Inspections of F-35 engines have been ordered after a runway fire took place at Eglin Air Force Base on June 23. The F-35 Lightning II, still in its development and testing phase, has been beset by delays and cost overruns in the years since its introduction. 
F-16 Fighting Falcons are parked at the Aerospace Maintenance and Regeneration Center in Tucson, Arizona, on December 11, 2004. General Dynamics (which was later sold to Lockheed) delivered the U.S. Air Force its first F-16As in 1979. More than 4,500 of the fighters have been built and are used by more than 20 nations in addition to the United States. 
An F/A-18C Hornet becomes the first aircraft to be catapulted from the USS George H.W. Bush on May 19, 2009. The F/A-18 Hornet, a late-'70s contemporary of the Air Force's F-16 Fighting Falcon, became the workhorse of U.S. carrier-based air power, and still supplements the Navy's and Marines' more current fleet of F/A-18E and F/A-18F Super Hornets. It is designed as both a fighter and attack aircraft. 
An F-15E Strike Eagle flies by during a U.S. Air Force demonstration on September 14, 2007, near Indian Springs, Nevada. The F-15E was designed for long-range, high-speed interdiction without relying on escort or electronic warfare aircraft. It was derived from the F-15 Eagle, which was developed to enhance U.S. air superiority during the Vietnam War. 
Pilots perform daily flight checks on their F-5E/F Tiger aircraft in Key West, Florida, on January 7, 2005. The Vietnam-era aircraft -- one of several offshoots of the original Northrup F-5s that went into service in the early 1960s -- is used to simulate adversary aircraft in training. 
A F-22 Raptor flies over Marietta, Georgia, home of the Lockheed Martin plant where it was built. The F-22 is the only fighter capable of simultaneously conducting air-to-air and air-to-ground combat missions. 
A AV-8B Harrier lands on board the USS Nassau on April 14, 1999, following a strike mission into Kosovo. The AV-8B Harrier is a single-engine ground-attack aircraft capable of vertical or short takeoff and landing. Though production of the aircraft ceased in 2003, the U.S. Marine Corps is looking at systems enhancements and plans to continue using Harriers well into the next decade. - Fire takes place June 23 at Eglin Air Force Base
- The F-35 fighter was grounded last year because of a crack in an engine component
- The F-35 program has been beset by cost overruns
(CNN) -- The Pentagon's most expensive weapons program ever, the F-35 warplane, is grounded again.
Developed at a staggering cost of nearly $400 billion, and beset for years by cost overruns and delays, the so-called Joint Strike Fighter was put down temporarily this week following a runway fire in Florida.
Pentagon spokesman Rear Adm. John Kirby said the cause of the June 23 incident remains under investigation, and the timing of when they might take off again is unclear.
"Additional inspections of F-35 engines have been ordered, and return to flight will be determined based on inspection results and analysis of engineering data," Kirby said.
Pentagon grounds F-35 fighter jets Engine maker Pratt and Whitney said in a statement that it was working with Air Force investigators to inspect all engines in the fleet. It declined further comment.
Contractor Lockheed Martin is producing variants of the plane for the Navy, Marines and the Air Force. The Pentagon wants more than 2,400, while hundreds more are expected to go to allies like Italy, Japan and Australia.
Among other things, the F-35 is designed to hit supersonic speeds at times and blend in features that make it stealthy, or hard to detect.
The military says the fighter will be "the most affordable, lethal, supportable and survivable aircraft ever to be used by so many warfighters across the globe."
But its development has been controversial for its cost and for setbacks. Test flights began in 2007 and are ongoing.
Last year, it was grounded because of a crack in an engine component.
There is precedent for the move.
Last year, the U.S. military grounded the F-35 because of a crack in an engine component that was discovered during a routine inspection in California.
The nearly $400 billion Joint Strike Fighter is the Pentagon's most expensive weapons system program.
It has been beset by cost overruns and various technical problems during development.
The U.S. military describes the jet as the "next generation strike aircraft weapon for the Navy, Air Force, Marines and our allies."
It also says the fighter will be "the most affordable, lethal, supportable and survivable aircraft ever to be used by so many warfighters across the globe."
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