Three years ago, Congress passed a law requiring the federal government to tell Americans about overseas airports prone to terrorist attack.

But the lawmakers stopped short of ordering Washington to tell the public about threats like the one that preceded the mid-air explosion of a Pan Am jetliner over Scotland in an apparent act of terrorism that killed 280 people.This tragedy was preceded by a telephone call last Dec. 5 to a U.S. diplomatic facility in Europe. The anonymous caller warned that sometime within the next two weeks there would be a bombing attempt against a Pan American aircraft flying from Frankfurt to the United States.

Should the public have been told about this threat? Plenty of the friends and relatives of those who died in the Pan Am crash think so. And a review of the present policy is underway.

Unless that review turns up some major considerations that aren't already known, it would be a mistake to change the policy.

In just the past three months, the State Department has received 105 warnings of potential terrorist attacks on U.S. airlines. All but a few of them are crank calls. The State Department had no reason to think the Dec. 5 call warranted measures beyond the warnings it gave to Pan Am. That's why three State Department employees were aboard the Pan Am flight that crashed, killing them.

After receiving the warning, Pan Am promptly tightened security. But chaos could be created if such warnings were extended to the general public. The resulting panic could seriously impair an airline's business, giving enormous power to disgruntled ex-employees, the emotionally unstable, and chronic malcontents.

Besides, how is the government to distinguish a serious threat from a crank call? After the government warned the public, how could it determine when the danger to a particular airline was over? How could it formally declare the danger was over without inviting more threats?

This week's Pan Am crash, a tragedy that looks like the result of a terrorist attack, underscores the vulnerability of the international transportation system. While security certainly could be improved at many airports, the fact remains that total security cannot be achieved in a world that has come to rely upon easy access to air travel.