Until recently, the ability of U.S. pilots to wage war unrestrained by darkness was considered one of America's main advantages over Iraqi forces in the Persian Gulf.

But now a series of after-dark crashes of U.S. military aircraft has exposed unanticipated problems in deseret navigation and raised doubts about whether American forces in Saudi Arabia are fully prepared for possible night combat.The latest accident Wednesday brings to 31 the number of U.S. servicemen killed so far in Operation Desert Shield. To put that figure in perspective, it means that U.S. troops in Saudi Arabia have been dying in aircraft accidents at about 10 times the recent peacetime rate.

Much of the problem can be easily explained - but not easily repaired without calling off flights after sundown and depriving U.S. forces of what was supposed to be an advantage.

The trouble is that the desert, not easy to fly across even by day, becomes even more hazardous at night when shifting sand dunes suddenly loom seemingly from nowhere and the dark floor of the desert is hard to distinguish from the dark night sky.

At the same time, sand blown by low-flying helicopters can incapacitate pilots by obscuring the stars and moonlight on which their night-vision goggles depend.

So much for the common boast of American pilots that "we own the night." So much, too, for U.S. contingency plans that relied heavily on the assumption of superior ability to launch attacks after dark.

Clearly, Operation Desert Shield is going to test much more than just America's nerve and determination. It is also testing the Pentagon's ability to learn quickly and make effective repairs in the unexpected chinks that have been found in this nation's armor.