America's latest stealth fighter just got heavier, slower and more sluggish.

For the second time in a year, the Pentagon has eased the performance requirements of the F-35 Joint Strike Fighter (JSF). The reduced specs – including a slower acceleration and turning rate – lower the bar for the troubled trillion-dollar JSF program, allowing it to proceed toward full-rate production despite ongoing problems with the plane's complex design. Under the old specs, the stealth fighter, due to enter service in 2018 or 2019, probably wouldn't pass its Pentagon-mandated final exams.

At the same time, newly identified safety problems could force F-35-smith Lockheed Martin to add fire-suppression gear that will only increase the plane's weight and further decrease its maneuverability. The JSF is meant to be a jack of all trades, equally capable of dropping bombs and fighting other aircraft – the latter requiring extreme nimbleness in the air.

For the pilots who will eventually take the F-35 into combat, the JSF's reduced performance means they might not be able to outfly and outfight the latest Russian- and Chinese-made fighters. Even before the downgrades, some analysts questioned the F-35's ability to defeat newer Sukhoi and Shenyang jets. Despite the JSF's lower specs, Lockheed bizarrely claims its new plane is now more maneuverable than every other fighters in the world except the company's own F-22.

In short, the F-35 program is losing altitude as Lockheed's claims grow loftier. The result is a widening gulf between expectations and reality for a jet that's supposed to represent the backbone of U.S. air power for the next 50 years.

The latest bad news came in mid-January the form of the annual weapons-testing report (.pdf) overseen by J. Michael Gilmore, the Defense Department's Director of Operational Test and Evaluation. The report revealed that the government's F-35 program office had changed performance specs for all three JSF variants: the Air Force's F-35A; the vertical-landing Marine Corps F-35B; and the carrier-launched F-35C flown mainly by the Navy.

"The program announced an intention to change performance specifications for the F-35A, reducing turn performance from 5.3 to 4.6 sustained g’s and extending the time for acceleration from 0.8 Mach to 1.2 Mach by eight seconds," Gilmore's report stated. The F-35B and F-35C also had their turn rates and acceleration time eased. The B-model jet's max turn went from 5.0 to 4.5 g's and its acceleration time to Mach 1.2 was extended by 16 seconds. The F-35C lost 0.1 g off its turn spec and added a whopping 43 seconds to its acceleration.

The changes likely reflect higher-than-expected drag on the JSF's single-engine airframe, according to Bill Sweetman of Aviation Week. The implications for frontline pilots are pretty serious. Less maneuverability makes the F-35 more vulnerable in a dogfight. And the slower acceleration means the plane can spend less time at top speed. "A long, full-power transonic acceleration burns a lot of fuel," Sweetman explained.

This is not the first time the Pentagon has altered its standards to give the JSF a pass. In early 2012, the military granted the F-35 a longer takeoff run than originally required and tweaked the plane's standard flight profile in order to claw back some of the flying range lost to increasing weight and drag.

Despite the F-35 growing heavier, slower and more sluggish by the Pentagon's own admission, Lockheed insists its product is still the second most maneuverable warplane in existence. Company test pilot Billy Flynn told Flight's Dave Majumdar that the JSF accelerates better and flies at higher angles than every other fighter except the Lockheed-made F-22. "The F-35 is comparable or better in every one of those metrics, sometimes by a significant margin," Flynn said.

Majumdar promptly ran Flynn's claims past several active-duty military test pilots. The feedback was not surprising in light of Lockheed's history of overselling the JSF. One Navy aviator called Lockheed's boasts "fantastical." An F-22 pilot expressed his doubt that the jet manufacturer has accurate data on the F-35's flight energy and maneuverability so early in testing. "The reality is that I would be floored if they had accurate E-M diagrams right now," the F-22 flier said.

In any event, the F-35 is likely to get even less maneuverable as development continues. Gilmore's report warned that the F-35A's tightly-packed airframe has essentially zero room for weight growth without losing nimbleness. "The program will need to continue rigorous weight management through the end of [development] to avoid performance degradation and operational impacts."

But in the same report, the Pentagon admitted to a chain of safety problems that could force Lockheed to add weight to the radar-evading plane. Extra mass doesn't necessarily affect the JSF's ability to avoid detection, but it does impact maneuverability. Several years ago, to save around 50 pounds, the F-35's designers removed some fuel safety valves. As a result, the JSF is now 25 percent more likely to burn if struck by enemy weapons, making it "overall more vulnerable [to fire] than most” older warplanes, Jennifer Elzea, a DOT&E spokesperson, told Bloomberg.

The Pentagon should tell Lockheed to "immediately reinstall" the valves, Rep. James Moran, a Virginia Democrat on the House Defense Appropriations subcommittee, wrote in a letter to the Defense Department dated Feb. 5.

If and when that happens, expect yet another downward revision of the F-35's performance specs, as America's future jet fighter grows steadily more disappointing.