Cheaper air fares to Florida, along with 5-cent beer - the first tangible benefits here of the Airline Deregulation Act - will be offered to Washington-area travelers next month.

Air Florida yesterday announced plans to begin service to Fort Lauderdale and Miami from Dulles International Airport on Dec. 14 with three daily round-trip flights, using special "Florida fares." The route had been unused and Air Florida won it under terms of the deregulation bill.

Mondays through Wednesdays, the one-way fare - offered on all seats on all flights - will be $53, compared to the current on-way coach fare of $106 on other airlines. Thursdays through Sundays, the fare will be $80 each way.

Travelers can get lower than coach fares on the other airlines by using the capacity-controlled Super Savers, but these have advance-booking and minimum-maximum stay requirements, and are blacked out at certain times. The Super Saver to Fort Lauderdale? Miami is $127 round trip Mondays through Thursdays, and $148 Fridays through Sundays, but this is to increase 5 percent Dec. 1.

Air Florida said it plans to use Boeing 737s seating 120 each. The airline said its service would feature free lunch and neckel beer.

The Fort Lauderdale-Dulles route is the first interstate route for the Miami-based carrier, which has extensive routes throughout Florida and recently got authority to serve the Bahamas.

Air Florida hopes to have some of the three round-trip flights a day use Miami and not Fort Lauderdale, but does not yet have Civil Aeronautics Board approval for that.

Allegheny Airlines also has plans to inaugurate new service to Florida, courtesy of the deregulation measure, next month. It will begin limited service between Baltimore-Washington International and Tampa on Dec. 16. Allegheny will have five flights a week to Tampa from BWI between Thursdays and Sundays; five flights a week from Tampa will arrive between Fridays and Mondays. Most of the flights are scheduled for very late night and very early mornings. Service will be increased Feb. 1.

From start-up until Feb. 1, Allegheny said it will offer an unrestricted 25 percent discount on coach fares. Also applicable to the new service will be Allegheny's Liberty Fare discount, which allows almost unlimited travel on Allegheny's system for 10 days for $179.

Piedmont Airlines will begin new service from here to Greensboro/High Point and Raleigh/Durham, N.C., on Dec. 15.

In a related development, the CAB was said to have decided to give airlines an extension of time in which to start the new routes awarded in the dormant-authority method. Originally required by law to begin in 45 days, the CAB noted that airlines were appearing to disrupt pre-Christmas schedules in order to start the service by the Dec. 24 deadline. To spare travelers from possible disruptions of service during the holidays, the CAB is planning to extend the deadline to Jan. 20.