Stealth bomber is invisible ... as long as it doesn't rain

It is one of the most feared weapons in the US arsenal: an airplane invisible to radar, which can fly in and drop its bombs before flying elusively away.

But a new US government report has put a dampener on the B-2 "Stealth" bomber's reputation. If you leave the airplane in the rain, the report reveals, its special powers rapidly erode: the composite coating that absorbs radar signals is destroyed by water.

Even worse, according to the US Government Accounting Office (GAO), the special plastic and metal composite coating the $2.2bn B-2 bombers also loses its invisibility if exposed to humidity or excess heat. To be functional abroad, B-2s would have to be kept in giant air-conditioned hangars - and even then, where maintenance crews will find themselves spending 39 per cent of their time repairing the damage caused to the material which covers the aircraft. To repair properly, the material needs a cool, dry environment to "cure" correctly.

A report, published this week by the GAO, which monitors public spending, notes that the 29 B-2 bombers ordered, at a total cost of $44.7bn, "cannot meet their intended deployment requirements because the low observability features are more sensitive to climate and moisture than expected".

Ideal conditions would be a desert - but even that carries hazards. Night temperatures can drop below freezing in the desert - and the GAO learned that "if moisture or water freezes in the B-2 it can take 24 hours to thaw and drain".

So far, the US Air Force has spent about $43bn of the budgeted $44.7bn it will cost to get the 21 aircraft into the sky by 1999. But the GAO warned that the need for special hangars will drive up costs even further.

In a response to the GAO report, the US Department of Defense managed to look on the only bright side remaining.

"Sheltering the plane facilitates maintenance," it said. "It also protects the low-observation surfaces from damage." It had no comment on the cost of air-conditioned hangars.

John Pike, an analyst with the Federation of American Scientists, said: "At this point the B-2's got an awful lot to do with money and politics and not much to do with defence."