If West and East ever clash in Europe, the toughest challenge facing allied pilots may be evading antiaircraft fire from their friends.

War games have revealed that as many as half of all NATO aircraft could be shot down by NATO troops, such as Belgians who don't recognize Dutch planes or Americans misidentifying West Germans, according to a senior NATO official, whose information was confirmed by an American general.

That problem, Defense Secretary Caspar W. Weinberger noted dryly in a report to Congress earlier this year, will have "serious adverse impacts on combat effectiveness."

It is symptomatic of a larger breakdown that could occur when allied forces cannot refuel each other's planes, share each other's ammunition, load each other's bombs or unscramble each other's codes.

To NATO, those problems are "interoperability shortfalls." To a blunter Sen. Sam Nunn (D-Ga.), they are "war stoppers, a collection of Achilles heels."

They are rooted in the desire of each nation, including the United States, to protect its defense contractors and the jobs, foreign earnings and technological advances the firms provide.

David M. Abshire, U.S. ambassador to NATO, recently pointed out that 18 companies in seven allied countries are producing ground-to-air weapons. Each country's determination to produce an entire arsenal means smaller production runs and costlier tanks, planes and missiles for everyone.

As a result, Nunn said, the United States and its allies consistently spend more on arms than the Eastern Bloc -- and consistently get less for their money.

"The various wastes we read about, which are of concern, pale in comparison to the waste going on in the alliance," Nunn said. "That's where the money goes."

Beyond the raw numbers -- the Warsaw Pact producing twice as many tanks as NATO, for example -- the fractured effort could lead to fatal confusion in battle.

Weinberger said the inability to recognize friendly aircraft could force each air force to stay within an assigned sector and thus "diminish a major advantage potentially held over Warsaw Pact air forces."

Weinberger claimed some progress in promoting international cooperation in arms making and research. But a senior NATO official said the alliance is creating a new generation of electronic weaponry, repeating all the same mistakes -- and at a higher price.

"It's getting worse," the official said. "And the guys on the other side of the Elbe are having a field day."