The following were among the actions at the Sept. 12 meeting of the Fairfax County Board of Supervisors:

NATIONAL AIRPORT--Voted to go to court to halt the proposed "scatter plan" for flights out of National Airport. The board directed the county attorney to decide whether to join lawsuits filed by Arlington and Alexandria or to file a separate suit. The board also opposed the regular scheduling of flight arrivals after 10 p.m. Letters expressing opposition will be filed with the Federal Aviation Administration, Department of Transportation and Northern Virginia delegates to the General Assembly expressing.

POTOMAC ALGAE--Voted to hire an independent consultant to determine the cause of toxic blue-green algae in the Potomac and to find ways to correct the problem. The board also delayed for three months the adoption of less stringent winter standards for sewage discharge into the river.

LORTON REFORMATORY--Voted to ask the District for access to prison security records and for expedient notification of county police when the prisoner count at Lorton raises the possibility someone is missing. The board will also ask D.C. Mayor Marion Barry to consider calling in the D.C. National Guard or finding alternative staffing to supplement the present shortage of security at Lorton.

METRO--Formally requested that the Metro board disclose all information on meetings with potential developers of areas near Fairfax Metrorail stations. The board also required that citizen members of Metro task forces disclose financial statements if they have property interests near the station sites. The board also formally supported continuing present levels of county bus service rather than cutting routes served by new subway lines.

LOCAL TRAFFIC--Directed a staff study of local traffic patterns, specifically truck and "cut-through" traffic on residential streets. The goal is to identify the county's trouble spots and to develop solutions and safety measures in coordination with local civic associations and the department of highways and transportation.

TYSONS CORNER--Approved plans by Tycon Beltway Ltd. for three 200-foot office buildings bounded by Tysons Corner Shopping Center, the Beltway and the new Marriott hotel. The semicircle complex has been designed by Philip Johnson, and, according to the Planning Commission report, "will become the most notable modern architectural development in the commonwealth."

BURKE CENTRE--Approved plans for 36 town houses, a Hechinger hardware and garden center, a restaurant, a bank and other commercial spaces and parking in Burke Centre, near Burke Centre Parkway and Oak Leather Drive.

COUNTY FESTIVAL--Adopted the final report of the Fairfax County Festival Committee, which called for continuation of the festival, a commitment of county resources until the event becomes self-supporting and the establishment of a private, nonprofit corporation to run the festival. The committee recommended that the festival be held at George Mason University through 1985.