THE LINE BETWEEN art and illustration is slippery as a buttered noodle. The reason it comes up is that the National Air and Space Museum has just mounted a show of William S. Phillips' oil paintings of airplanes, which look like pictures of airplanes until you look at them.

Phillips, 42, is a self-taught artist who never got over his boyhood fascination with airplanes. He was going to be sensible and study law, but then a guy bought four of his airplane pictures. Soon Phillips was working as a fireman in Ashland, Oregon, while studying the techniques of Luminists Albert Bierstadt, Thomas Moran and Frederick Church.

He was a quick study. The twoscore works in the show include several Phillips did in his early 30s, when he was already very nearly a master of the medium.

But he couldn't sell his work if he couldn't show it, and he couldn't show it because the people who patronize fine-art galleries don't buy pictures of airplanes. So Phillips painted wildlife art, which in recent decades has gained grudging critical attention, and sneaked airplanes into the background.

It was competent but uninspired work. Phillips finally was plucked from between the rock and the hard place by the American armed forces art programs, an outgrowth of the combat-artist movement of World War II. He was given not only show space and re- cognition, but flight time in the aircraft he had only been able to watch from the ground.

Commissions followed, including from the Jordanian royal family, and now Phillips paints aircraft full-time. Many are story paintings, such as "Into the Teeth of the Tiger" (1984), which shows pilot Don Lopez in his "Flying Tiger" P-40 just after a mid-air collision with a Japanese "Oscar" over China in 1943. The enemy plane went down. Lopez, minus a yard of his port wing, found somebody else to shoot at, and went on to become a combat ace and deputy director of the Museum.

It's an exciting picture that would make a good cover for a paperback war book. It is in fact the cover of Lopez's memoir by the same title. But is it simply a very good illustration?

Go to Gallery 211 and look at it. Ask yourself where you last saw such painterly use of texture, light and composition, and what Impressionist ever rendered a more subtle landscape. The story's not the point of the picture, it's a bonus.

INTO THE SUNLIT SPLENDOR --

The Art of William S. Phillips. Through December 31 at the National Air and Space Museum.