AUSTRALIA'S proposed new Joint Strike Fighter combat aircraft can't fly near lightning storms, or easily turn to evade enemy fighters or missiles.

US Air Force Lieutenant General Chris Bogdan, who heads the US military's $16 billion JSF program, admits the F-35 Lightning II jet can't fly within 40km of a lightning storm because its fuel tanks could ignite.

"Will this problem occur in the future? No, because we have the known fixes for it and we will fix it," Lt-Gen told ABC TV.

The Lockheed Martin JSF is a fifth-generation combat aircraft featuring stealth capability - low visibility to radar - and advanced sensors to detect enemy aircraft.

But the program has experienced technical problems, rising costs and delays.

Australia is looking to acquire up to 100 JSFs to form the backbone of the RAAF air combat fleet.

So far, the federal government has firmly committed to just two.

But in last year's budget the government deferred signing a deal for the next 12 aircraft for two years.

A JSF critic, US defence analyst Pierre Sprey, told the ABC the JSF was inferior to the widely used F-16 - an aircraft dating from the early 1970s which remains in production.

"We have an airplane that can't turn to escape fighters, can't turn to escape missiles, sluggish in acceleration because it's so big and fat and draggy and doesn't have enough motor for the weight," he said.

"My prediction is the airplane will become such an embarrassment that it will be cancelled before 500 airplanes are built."

General Bogdan maintains the JSF is a formidable aeroplane.

"I have no doubt if you went head-to-head with this airplane with any other airplane in the world with the capabilities that I know it has, it will do very, very well," he said.

General Bogdan also said the costs of JSF aircraft were decreasing.

"I expect and will demand that the future lots will continue on that trend to cost less and less," he said.

Responding to the television program, a spokeswoman for Defence Minister Stephen Smith said earlier reviews of Australia's air combat needs concluded the JSF was the preferred aircraft.

"Deferring our project by two years reduces our exposure to increasing costs and provides increased time for resolution of the remaining technical challenges," the spokeswoman said.

Originally published as Our new fighter is scared of storms