The U.S. Court of Appeals in the District of Columbia, frequently criticized for long delays in handing down decisions, has approved several changes in its internal operations which the court hopes will cut the time it takes to announce the outcome of cases.

Chief Judge Spottswood W. Robinson III said here today that at the start of the court's term in September, judges who have three or more written opinions pending for more than six months will not be assigned to hear any new cases until their backlog is at least down to two cases.

In addition, Robinson said, the appeals court this fall will expand its schedule of oral legal arguments, Robinson, speaking at the 42nd annual judicial conference of the D.C. Circuit, said the extra court sessions could help dispose of an additional 100 to 200 cases each year.

The appeals court earlier agreed to a new procedure in which the court will dispense with written opinions altogether, if lawyers on both sides of the case agree, and instead issue a simple announcement of the court's decision. The court also plans to crack down on attorney's requests for extensions of time and other "afterthoughts of counsel," Robinson told the conference.

Robinson, who recently took over as chief judge, said the appeals court ultimately hopes to reach the point where it can issue decisions in cases within the same term of the court in which written briefs and oral arguments are completed, a procedure similar to that followed by the U.S. Supreme Court. The court's one-year term begins each September.

The changes were recommended by a committee of judges and lawyers appointed after complaints at last year's judicial conference about delays in announcements of decisions, and particularly delays in opinion writing.

About 1,600 new cases are filed each year with the federal appeals court in Washington. The court disposes of about 1,500 cases a year. However, the court expects to have a backlog of 1,700 pending cases when its new term begins in September, work that is left over from previous years. It currently takes a year from the time an appeal is filed to the time it is decided.

Robinson told the 275 lawyers and judges at the conference that the court's backlog is aggravated by the fact that 80 percent of the court's caseload involves complex cases from federal administrative agencies, such as disputes over environmental and communications regulations, and other civil cases involving the federal government.

The judicial conference is convened by law each year to consider the business of the federal appeals court, the U.S. District Court, and joint activities conducted with members of the bar.

The conference overwhelmingly approved a resolution for continued support of the Legal Services Corp., which the Reagan administration has proposed to shut down. The corporation provides free legal assistance to low-income persons in non-criminal cases.