An F-35 Lightning II flies overhead at Eglin Air Force Base in Florida. (Sr. Airman Julianne Showalter/U.S. Air Force via AP )

Four years before Pentagon officials discovered potentially life-threatening problems with the F-35 Joint Strike Fighter’s ejection seat, a top official warned in an urgent memo that the escape system should be more thoroughly vetted before pilots were trained on the plane.

In an unsolicited dispatch to the top defense officials overseeing the $400 billion F-35 program, J. Michael Gilmore, the Pentagon’s chief weapons tester, said he was concerned that training flights would proceed even though the ejection seat system had not been fully tested.

His warnings were rejected by Pentagon brass, who pressed on with the controversial program, according to internal documents obtained by The Washington Post. But a series of recent tests revealed serious problems with the jet fighter’s escape system, the Pentagon acknowledged this month, creating potentially hazardous circumstances, especially for lighter-weight pilots.

Those lighter-weight pilots face a “high” risk of danger, and the risk is deemed “serious” for mid-weight pilots, according to an internal risk assessment of the problem, which was obtained by The Post.

Lighter-weight pilots, those weighing less than 136 pounds, are now prohibited from flying the aircraft, officials said, until the problem is fixed.

The latest setback for the most expensive weapons program in Pentagon history has concerned some members of Congress who wondered why testers are still finding significant flaws in a fighter jet that has been in development for about 14 years.

“They pushed this system through recklessly, and now we’re seeing the costs,” said Jackie Speier (D-Calif.). “We are just lucky that the testing was done with dummies and not real Air Force pilots.”

Appearing before a congressional panel this week, Air Force Lt. Gen. Christopher Bogdan, the program’s executive officer, said that, “we take this deficiency with the ejection seat and the safe escape very, very seriously.”

The problem occurs because the force of ejection can be so great that a pilot’s head could suddenly snap forward or back, causing injury. But Bogdan said officials have already identified solutions—reducing the helmet weight, creating a switch on the ejection seat for lower-weight pilots and a head support panel in the parachute. Some of those fixes have already been in the works for six months, but it could be another year until they are resolved, Bogdan said.

The new F-35 helmet at Lockheed Martin's Fighter Demonstration Center in Arlington, Virginia. (Photo by Marvin Joseph/The Washington Post) The new F-35 helmet at Lockheed Martin’s Fighter Demonstration Center in Arlington, Va. (Marvin Joseph/The Washington Post)

“I’m confident the current risks will be resolved, and we will be able to overcome the current and future problems,” he said.

The issue, another in a long list with the F-35, is in part the result of how the program was structured. Instead of developing a new plane and then buying it, the Pentagon committed to the plane while it was still in the development phase, meaning problems would still be discovered and there would be costs to fix them. For years, critics have lambasted defense officials for going this route, saying they violated a central tenet of weapons procurement: “Fly before you buy.”

In 2011, Gilmore, the director of the Pentagon’s Operational Test and Evaluation Office, said he had “serious concerns” about beginning training flights before several safety issues had been addressed. He urged officials to upgrade the “ejection seat with a system that has completed qualification testing.”

In response, program officials said at the time that they did “not agree with the characterization,” and would proceed with the flights.

In his testimony this week, Bogdan said that testing of the ejection seat has been going on “for many, many years,” and has been methodical. It is also a complicated process, he said, because the seat is designed to handle pilots that range in weight from between 103 to 245 pounds, more than other fighter jet ejection seat systems.

The testing has pushed the edges of the envelope, he said, where conditions “become more severe and are harder to achieve in terms of safety.”

In August, when officials realized that pilots who weighed less than 136 pounds could face “potentially fatal whiplash,” the program restricted those pilots from flying. Problems with the ejection seat were previously reported by Congressional Quarterly and Defense News.

Out of the more than 200 F-35 pilots, only one was affected, said Joe DellaVedova, the program’s spokesman. And that pilot has since been transferred to another aircraft.

The troubles with the ejection system are just the latest to plague the F-35 program. Last year, for example, the entire fleet was temporarily grounded after an engine caught fire as a pilot was about to take off.

[Meet the most fascinating part of the F-35: The $400,000 helmet.]

In addition to the plane, which is manufactured by Bethesda-based Lockheed Martin, officials have also have had problems with the high-tech helmet worn by pilots. The helmet, which costs $400,000, can help the pilot see through the plane, giving pilots unprecedented 360-degree views. The pilots use six cameras embedded in the skin of the aircraft, so when they look in a particular direction, they can see through the corresponding camera. Twin projectors located inside the helmet then beam that outside images onto the pilots’ visor, which then acts as if it were a screen.

For years, though, designers struggled with the new technology. Images streaming onto the visor were jittery in turbulence. The night-vision technology created a “green glow” that obscured the pilot’s view.

And now the ejection seat testing has led to another troubling discovery: The helmet is about six ounces too heavy.

The fixes should be fairly straightforward, Bogdan said this week. But, he warned, it could be a lengthy process.

“I never want to say anything’s easy in the F-35 program, because nothing is ever easy,” he said.