SOMEONE HAS FINALLY gotten stern with the Professional Air Traffic Controllers Organization (PATCO), one of the most arrogant federal unions around. Federal District Judge Thomas C. Platt in Brooklyn has just found PATCO in contempt of court and ordered it to pay a $100,000 penalty for staging slowdowns at Washington National and three other airports in May and June. The union acknowledges its action, but plans to appeal the ruling that an 8-year-old injunction against slowdowns, sick-outs and the like is still in effect.

We hope Judge Platt is right about the law. He was certainly right in reminding PATCO that its members "have the lives of many thousands of people in their hands" every day and should never "trifle with such awesome responsibility."

Slowdowns inflict inconvenience and added costs on airlines and their passengers. (In this case the $100,000 penalty is to go to the Air Transport Association of America, the airlines' representative and the plaintiff in the original suit. Passengers who suffered losses have to swallow them.) Beyond that, of course, are the dangers that arise from the slightest disruption of operations at any airport as intricately scheduled as Washington National.

The recent slowdown was doubly offensive because of its cause. The controllers were peeved because three major airlines had refused to guarantee that they could take free "familiarization" flights to Europe and the Far East whenever they chose. The controllers have been given some free domestic trips for years, on the theory that they should be familiar with cockpit operations. The Federal Aviation Administration recently agreed to international flights, too - even for controllers at airports with no overseas service at all. But that was not enough for PATCO; its members did not want to go standby. they wanted guaranteed space - perhaps so they could plan their vacations and get their families on the same plane.

That's why traffic at National Airport got so snarled on June 7, and that shows how overbearing the union has become. No one begrudges them some special compensation for what is undeniably vital, highly skilled and nerve-racking work. But their demands and tactics have gone far beyond reasonable bounds. They are public employees, after all. As Judge Platt suggested, the FAA and the Justice Department ought to crack down on those who slight their obligations to the public or scoff at the law against strikes.