THE FUNDAMENTAL question raised by the air disaster over San Diego last week is whether it makes sense to let commercial airliners and small, private planes use the same airports and the same part of the airways at the same time. The answer, clearly, is no - especially when the pilots of the small craft are practicing. The trouble is that the policy that flows logically from this conclusion - bar small planes from airports used by big airliners - is easier said than done.

That doesn't mean that steps need not be taken to reduce the chances of another mid-air collision. But it does mean there are difficulties. The pilot of the small plane involved in the San Diego collision, for instance, was practicing instrument landings at the commercial airport because it was the only airport in the vicinity equipped to handle them. So one begins with the fact that some mixing of large and small airplanes is inevitable, at least until more secondary airports have better equipment than they now possess. The question is: How much and under what safety precautions? The Federal Aviation Administration, which is charged with making the airways safe, has closed some - though not many - major airports to practice flights. It has also forbidden small planes to fly near several major airports unless the air traffic controllers know of their presence and unless they are equipped to receive instructions from the controllers. But it has been reluctant to take the ultimate step of reserving some airports exclusively for big airliners, partly because few airports have enough commercial traffic to keep them busy and partly because small aircraft under tight air traffic control are not perceived as a major hazard. Dulles Airport, for example, would be vastly underused if small planes were excluded and, if they were, facilities for them would have to be provided elsewhere.

The FAA is most open to criticism for not having provided enough such facilities at other airports. Even before the San Diego crash, the National Transportation Safety Board charged that the FAA was dragging its feet in putting safety equipment at those fields. Once they are fully equipped to handle the needs of general aviation, the process of reserving more air space for large airliners can begin. In the meantime, the FAA can - and must - expand the number of airports around which the activities of small planes are sharply limited. Such restrictions now exist over only 21 cities in the nation - Washington is one of them, but San Diego is not - although almost 600 are served by commercial airlines.