Every evening, a computer operator seated in a carpeted, sixth-floor room at Federal Aviation Administration headquarters on Independence Avenue types out a message advising what to expect at the nation's airports and along its air routes on the following day.

The message for yesterday, flashed to airlines and traffic control centers around the country, began: "The FAA will continue to operate the air traffic system under the 50 plan on Tuesday, August 25, 1981, for 22 major airports with the following exceptions . . . . "

For more than three weeks, since the nation's air controllers walked off the job, the emergency measures to keep planes flying have been coordinated and fine-tuned from Room 626, the operational command post known as Central Flow.

Equipped with illuminated maps, telephones and high-speed printers, the center was set up 10 years ago to coordinate traffic into congested airports. In the mid-1970s, conserving fuel by reducing time spent in holding patterns became an important function. Now, in addition to those normal duties, Room 626 is the nerve center for minute-to-minute decisions on flight delays and reroutings caused by the strike.

During some of the day there is time for banter and coffee around the consoles. But in the morning and evening, rush-hour volumes of airplanes converge on major airports and the half-dozen or so flow controllers quicken their pace, conducting transcontinental conference calls on their long-cord phones, calling up flight data on computer screens and answering inquiries from airlines.

Preparation for the so-called 50 plan for managing traffic during the strike began long before the walkout on Aug. 3. Airlines were asked to submit schedules canceling 50 percent of their flights leaving at peak hours from the country's larger airports. FAA planners in Jacksonville, Fla., modified those submissions as necessary to fit the national scheme, creating a 50-plan schedule now in use.

There are loopholes in the 50 plan, meaning traffic is higher than that in some cases. Flights using only secondary airports -- and some as large as Dulles International are considered secondary -- are not restricted at all. All told, the restrictions have allowed about 75 percent of the normal day's 14,000 commercial flights to continue operating, according to the FAA.

While scheduling for specific flights is handled in Jacksonville, Washington's complementary task is to accommodate the planes into a system whose capacity changes daily, as airport towers and the 20 regional "enroute" control centers around the country seek ways to make their small strike-reduced staffs the most effective.

For example, on Friday last week, landings at JFK International in New York were restricted far below scheduled rates to 25 per hour, due to staffing problems in the control tower. Drawing on computers in Jacksonville, Central Flow called up a schedule of arrivals at JFK and calculated what airborne delays could be expected during specific hours at the airport if nothing was done.

Planes scheduled to land between 4:30 p.m. and 8 p.m. would have to circle for 60 minutes if they took off an schedule, it was found. In response the center dispatched orders to airlines and control centers to hold those planes at their departure gates for 60 minutes so that they would be able to land immediately on reaching JFK's vicinity.

As it happened, at 4:20 p.m. the center decided to cancel the restrictions after New York complained that, combined with other control measures, too few planes were coming into JFK. In Room 626, a flow controller picked up his phone, punched keys on his console to connect him simultaneously to all of the 20 regional traffic centers and announced the cancellation. Written confirmation followed on teleprinters to the centers and the airlines.

But, in fact, the towers rarely cause delays. Most have sufficient staff to handle reduced traffic arriving under the 50 plan, the FAA says. Delays originate mainly at enroute centers because of requirements laid down since the strike to increase separation "in trail," the minimum distance at which an airliner can follow another.

In normal times, minimum separation for planes enroute is five miles. But due to the shortage of controllers, regional centers have lengthened these distances, both to lower the number of planes in the sky and keep those that are flying farther apart as a safety measure. Vertical separation has been increased as well.

The regional centers directing traffic in the Chicago and New York areas appear to have been the hardest hit by the strike and now form the narrowest bottlenecks in the system. Chicago, for instance, yesterday was requiring some planes entering its sector to be at least 50 miles apart.

The normal five-mile separation means that jets cruising at 500 mph could enter Chicago's zone on a particular corridor and altitude at a rate of 100 per hour. Fifty miles' separation at that speed, in contrast, means that only 10 jets could use that route in an hour.

At preferred traffic hours, the airlines in many cases still can schedule far more flights across the Chicago zone than the center can handle, creating a potential for fuel-wasting holding patterns at its fringes. The solution, again, is to delay planes on the ground so that they can reach Chicago airspace just as their "turn" to go in comes up.

Similarly, flights to New York from National Airport are late taking off not because the tower is too sparsely staffed but because New York's enroute air control center is accepting planes from the Washington area air control center at Leesburg at a rate of only 10 per hour. At peak hours that creates a backup at National that is cleared in the less busy hours that follow.

In setting up such flow measures, enroute centers do not normally phone each other directly. Instead, they contact Central Flow in Washington, which sets up a conference call. Washington listens in, making suggestions and vetoing measures it thinks would conflict with ones in force elsewhere in the country.

Horse-trading goes on constantly as the controllers devise a solution to a glitch in the system or try to assure that a plane flying from Dulles to the West Coast will not take off until it has guaranteed clearance across each of the regional zones it will have to pass through.

"Ninety-nine percent of all delays in the system now are taken at the departure gate," says Sam Rosenzweig, operations officer at Central Flow.