Air Force photo The F-35 over California.

Darn that gravity. Without it, airplanes would be easier to design (yes, there’s be other problems, like people floating off into space, if you’re going to get technical).

But gravity is what makes designing warplanes so tough. Every ounce added to a plane detracts from its performance.

That’s what the Pentagon’s F-35 program is now grappling with.

The three versions of the plane – the F-35A for the Air Force, the F-36B for the Marines, and the F-35C for the Navy – have reached their maximum weights, about 30,000 pounds each.

But F-35 engineers know that required tweaks down the runway will force them to add weight to the plane — to strengthen parts that need to be stronger, add protection of one kind or another, or if components end up weighing more than predicted. So they’ve been whittling away at the plane’s evolving design to make it as light as possible.

That can be a problem, as the Pentagon’s Operational Test and Evaluation shop has just revealed in its 2012 report on the F-35.

Two changes designed to cut the plane’s weight by 11 pounds have made it 25% more vulnerable to exploding in mid-air, and other unfavorable outcomes.

It also makes it more vulnerable – gulp – than the airplanes it is replacing. That’s not good for a program whose price tag of $396 billion for the planned buy of 2,457 aircraft makes it the most costly weapon ever built.

Recent tests show that removing nine pounds of fueldraulic fuses and two pounds coolant shutoff valves “results in a 25 percent increase in aircraft vulnerability.”

Notes the DOT&E report about those 32 ounces of valves:

The aircraft uses flammable PAO [Polyalphaolefin] in the avionics coolant system, which has a large footprint on the F-35. The threat in this ballistic test ruptured the PAO pressure line in the area just below the cockpit, causing a sustained PAO‑based fire with a leak rate of 2.2 gallons per minute (gpm). The program assessed that a similar event in flight would likely cause an immediate incapacitation and loss of the pilot and aircraft. The test article, like the production design, lacks a PAO shutoff system to mitigate this vulnerability.

The F-35’s developers are reconsidering that decision.