The professional Air Traffic Controllers Organization began informational picketing here and elsewhere yesterday after expiration of the union's three-year contract woth the Federal Aviation Administration.

A spokesman for the 17,000-member PATCO said picketing would continue through today at Washington National and Dulles International airports and at the Air Route Traffic Control Center in Leesburg.

Other informational picketing will be scheduled on an "as-needed basic," the PATCO spokesman said.

However, he emphasized the union has not planned a strike or work slowdown to protest the delay in arriving at a new agreement. Strikes by federal employes such as air traffic controllers, who are responsible for the safe conduct of commercial and other air traffic in and out of the nation's airports, are forbidden by law.

"We're continuing to meet with the FAA in an attempt to resolve our problems through negotiations," the spokesman said. He said the contract that expired at 12:01 a.m. yesterday contains a "continuation clause" that, in effect, keeps the old agreement active until a new one is reached.

However, PATCO and the FAA have agreed on at least one thing -- both refuse to discuss specific issues on the bargaining table in order to avoid, as the PATCO spokesman put it, "negotiating through the press."

Still, it is known that the hardy poerennials of collective bargaining -- wages and working conditions -- figure heavily in the talks. An increased employe voice "in the procedural aspects of air traffic control" -- who does what and how -- also is at issue, according to one source.

Negotiations between the union and the FAA are taking place on a rotating basis at PATCO and FAA headquarters here.