Pan American World Airways is planning to offer a $99 introductory fare for full service roundtrip flights between New York and Los Angeles - an enormous savings over the regular coast-to-coast roundtrip coach fare of $420.38.

Now for the bad news. This "sneak preview" fare will be available, starting Nov. 15, only on one flight in each direction weekly, Pan Am said yesterday.

Until now, the large international carrier was permitted to carry only overseas travelers on its weekly flights between New York and Australia or New Zealand by way of Los Angeles. Empty seats on the New York-Los Angeles leg of the international journies could not be filled with domestic travelers.

But the Civil Aeronautics Board recently approved Pan Am petitions to carry local passengers on domestic segments and Pan Am is now planning to start daily transcontinental service next May 18.

To promote the new U.S. service until then, Pan Am will offer the $99 ticket to passengers who buy them from 7 to 14 days before departure. The number of seats available will be limited, vice president A. B. Magary said yesterday. The weatbound flights leave New York on Thursday evenings while flights east from Los Angeles depart at noon Wednesdays.

Pan Am also will offer a $170 oneway fare for passengers without advance reservations and a $136 one-way fare for tickets purchased 14 days in advance with limited refundability if the passengers don't show up.

Trans International Airlines also announced plans for $99 one-way fare between New York and Los Angeles, starting Dec. 15 for weekly flights that may be increased in frequency to twice a week next year.

Eastern Air Lines, meanwhile, advised the CAB it will begin nonstop service next month on six new routes acquired under provisions of the air deregulation act: Miami-San Francisco; Orlando-Los Angeles; St. Louis-Salt Lake City; Houston-Austin; and Ft. Myers, Fla. New York. Eastern earlier said it will start Atlanta-San Francisco service on Dec. 13.