FRANK CARLUCCI, the deputy secretary of defense, has now instructed the military services to get busy and help sell American fighter planes to foreign governments. The idea is, apparently, that salesmen in uniform are more effective than mere civilians working for the aircraft companies. It's not enough just to let foreign nations know what the new fighters can do. "We must now go further," he has written, "and actively plan with the nations for sensible acquisitions."

The Carlucci order strengthens the impression that this administration is rapidly losing any sense of proportion in all questions of weapons sales abroad. To thrust this salesman's job onto the services requires an unwholesome blurring of distinctions between military responsibilities and commercial favors for private companies. It's bad enough to allow the two to become confused in the minds of foreign officials dealing with American military officers. It's worse to encourage that confusion in the officers' own minds.

That line, it is hardly necessary to say, has occasionally been badly blurred in the past -- and those experiences testify to the risks. You would think that the Reagan administration might remember, in particular, the AWACS imbroglio last year with Saudi Arabia and Israel. That originated in a suggestion by overeager Air Force generals to the Saudis about the desirability of having their own AWACS planes. When the Saudis began pressing the idea in Washington, the Israelis bitterly protested that the planes and their radars would constitute a dire threat. At that point, the actual military capacities of the planes became irrelevant. As is customary in these affairs, it became, for both governments, a test of which of them the United States would accommodate in preference to the other. That's a wretched way for the United States to conduct a foreign policy.

Mr. Carlucci's instructions, incidentally, identify 11 countries as promising prospects, measured in terms of their sense of danger, their ambition and their ability to pay. Six of the 11 are in the Middle East, where there are two wars currently in progress. Mr. Carlucci may recall that a succession of Defense Department officials, through most of the 1970s, justified the sale of an enormous fleet of F4 Phantom fighter-bombers to Iran on grounds that it was necessary to guarantee the stability of the Persian Gulf region. The Iranians currently seem to be getting good mileage out of their Phantoms.

The Carlucci order is evidently intended to do a favor to one company in particular. The Northrop Corp. has invested heavily in a new fighter, the F5G, now in production, for which it has no buyers here or abroad so far. Mr. Carlucci seems to think that the military services ought to give Northrop a little help. But he's offering more than he, or anyone in the Defense Department, ought to deliver.