The X-35B — the Marine Corps version of the new Joint Strike Fighter F-35. Lockheed Martin, Peter A. Torres/AP
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A test pilot preparing for takeoff in what is billed as the world’s most advanced military aircraft made an unsettling discovery last week: A cockpit signal warned him of a fuel problem and closer inspection revealed a hose that carries jet fuel had come loose in the engine compartment.

The result was a scramble to investigate the incident and the grounding over the weekend of twenty-five F-35 Joint Strike Fighter being tested at air bases in Florida and Arizona, as well as Lockheed Martin’s production factory in Fort Worth, Texas. The decoupled hose was only the latest of many glitches in the costliest weapons program in U.S. history.

Just days earlier, the Pentagon’s Operational Test and Evaluation office’s annual report to Congress had identified the “fueldraulic” lines at the heart of the incident as a potential fire hazard.

The report said that a 2008 decision to remove certain fire protective systems from the plane to save weight, including those associated with the fueldraulic lines, resulted in “a 25 percent increase in aircraft vulnerability.” It warned that removal of these systems meant the F-35 did not meet a requirement that it be less vulnerable to damage from fires than older, similar military aircraft.

The risks associated with the fueldraulic lines were just one of a number of problems the OT&E report detailed in the Marine, Air Force and Navy versions of the plane, all currently undergoing testing. They include ongoing problems with the plane’s millions of lines of computer code, a particular vulnerability to lightning, and continuing defects in its sophisticated helmet, with its see-through data and symbol displays.

According to the test report, the F-35’s mission is to be ready to attack targets day or night “in all weather, and in highly defended areas of joint operations.” But because of concerns about the fuel tank’s vulnerability to lightning, the report said, current flight operations by the aircraft are not permitted within 25 miles of lightning-producing storms.

The plane’s use of “fueldraulic” line, installed only in the Marine Corps’ version of the F-35 aircraft, is just one of its many technically-challenging features. Its aim is to save weight by using some of its fuel as the hydraulic fluid that helps swivel the plane’s jet exhaust system during short take-offs and vertical landings. Weight is a major factor affecting the speed and agility of the 6 ½-ton airplane.

The report revealed that a prototype system for protecting pilots from chemical or biological weapons – things they could conceivably encounter in places like Syria – was developed but judged “too complex for field use.”

Following the incident at Eglin Air Force base in Florida, the Marine Corps launched a review of what caused the hose to decouple, which officials said could be finished quickly. The two other variants of the F-35, one designed for the Air Force and the other the Navy, were not affected by the glitch because they don’t have fueldraulic lines. Both are designed only for horizontal, not vertical takeoffs and landings.

“There were no injuries to the pilot or ground crew,” said Commander Kyra Hawn of the F-35 program office in an e-mailed statement. “The jet was then safely towed to a maintenance hangar and secured.” Pratt & Whitney spokesman Matthew C. Bates said a team of Pratt & Whitney and Rolls Royce engineers were working with Lockheed Martin and the Pentagon’s F-35 Joint Program Office to fix the problem as quickly as possible.

But the latest problems come after nearly seventeen years of development and seven years of production, while testing remains incomplete. The aircraft is years behind schedule and more than half a billion dollars over budget.

Pierre Sprey, a critic of the F-35 who played a major role in the design of the F-16 fighter and A-10 ground attack jets, said the detached line could have posed a fire hazard in flight. “They’re damn lucky,” he said.

As the Center reported in June the F-35’s helmet, a critical piece of technology for the futuristic aircraft, has long been a headache for developers. The plane is more or less designed around the helmet, which is supposed to let the pilot look through data projected the visor while maneuvering.

Jitter caused by aircraft vibrations is still making the helmet see-through display hard to read, the latest report affirmed. Night vision isn’t as good as required by the specifications.

The report also disclosed that flight testing has uncovered a problem with what is called “green glow,” where light from the cockpit displays “leak” into the helmet display, making it harder to see through the visor in low-light conditions.

Sprey said he was particularly troubled by the report’s mention of scorching in the the trailing edge of the plane’s horizontal tail fins by the jet’s engines. “It’s not hard to figure out whether the engine exhaust is going to touch and burn the tail,” he said.

The report also disclosed that some of the planes have suffered cracks, experienced excess vibrations at high speeds, and have recurring problems with their complex software.

In response to rising costs and other problems, the Department of Defense put the F-35B, the version used by the Marine Corps, on “probation” in early 2011.

A year later, then-Secretary of Defense Leon Panetta lifted that probation, pending resolution of outstanding issues with the builders, saying the aircraft was performing with “the kind of performance and maturity” of the other two versions — the Air Force plane designed for conventional landings and a Navy version adapted to landing on aircraft carriers.

“I want you to know that as secretary of defense, my department is committed to the development of the F-35,” Panetta told military personnel at Patuxent Naval Air Station in Maryland in January 2012.

The F-35 is known as the Joint Strike Fighter, because it was originally conceived as a cost-saving new fighter that could be shared by the Air Force, Navy and Marines. But during the plane’s lengthy and complex development, the three designs have diverged.

Many experts have questioned the Pentagon’s decision to design, test and build the complex aircraft at the same time, rather design it first and test it later. Officials argued that this dual track approach could save time and money, and that most major design problems could caught by advanced computer modeling and simulation.

But Maj. Gen. Christopher Bogdan, now the F-35’s executive program manager, last fall called the decision to begin to manufacture the plane while it was still being designed “the greatest of all sins in the Joint Strike Fighter Program.”

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