Yet another problem has grounded one of the most expensive aircraft in U.S. military history. The Pentagon has grounded the the F-35 while it investigates possible engine fires. The F-35 will not return to the air until the issues are resolved.

On June 23, a Lockheed Martin F-35 Joint Strike Fighter was attempting to take off from an Air Force base in Florida in a routine training flight when its engine caught fire as it taxied down the runway. The pilot aborted the flight and managed to leave the fighter jet uninjured. However, the incident at Eglin Air Force Base has raised Air Force's concerns about the plane.

The stealth fighter jet aims to replace the armed forces' aging fighter fleet and has been designed to meet the requirements of the Air Force, Navy and Marine Corps. Its complicated design, which combines stealth technology and vertical landing with takeoff capabilities, has plagued its construction and inflated its price tag. The Government Accountability Office has said the cost per plane has risen to $135 million per plane, and the program overall will cost taxpayers around $400 billion.

On June 26, the USAF grounded all of its F-35s. According to Bloomberg News, the Air Force will investigate the engine fire and that grounding affected only the Air Force's "A model" aircraft. However, the Pentagon decided Thursday to suspend the whole fleet across the three branches that operate the plane.

McClatchy reports that the entire fleet of 79 F-35s used by military (including those being used by the Navy and Marine Corps) will be inspected to see what caused the fire.

Pratt and Whitney make the plane's engine, and this is not its only problem. Last month, a plane stationed at Marine Corps Air Station Yuma in Arizona had an oil leak, which led to an investigation, and that may not be the last.

"Sadly, this is part of a larger pattern of teething issues that we've come to expect with this program," Teal Group Corp. aerospace analyst Richard Aboulafia told the Los Angeles Times. "It's bad, but it will be even worse if the grounding lasts more than a week or two."

The Times reports that the plane is assembled at Fort Worth, Texas, but subcontractors all over the country manufacture parts, with 277 in California alone. This investigation could deal another blow to the program's overextended schedule, since the Pentagon has ordered 2,457 planes overall.