Passengers using Dulles International Airport and Baltimore/Washington International Airport would increase significantly if more flights were available there, according to preliminary findings of an area passenger survey last summer.

The findings were released yesterday by the Metropolitan Washington Council of Governments with the caveats that they were only preliminary and that a second survey already scheduled for next month would be required for a statistically valid sample.

According to Phillip S. Shapiro, the COG staff member who served as project director for the survey, the preliminary results suggest that, overall, about 55 percent of the region's air travelers preferred National Airport, and about 23 percent each preferred Dulles and BWI. However, the data suggested, National was used by 68 percent of those surveyed, BWI was used by 20 percent and Dulles was used by 11 percent.

If passengers could use the airports they preferred, COG projected from the survey that, during a two-week period, passengers at Dulles would double in number from 39,000 to 82,000 and, at BWI, they would increase 25 percent, from 70,000 to 87,000.

The survey found that two-thirds of the passengers using National preferred it over the other two airports. Almost 80 percent of the passengers using BWI said they preferred that airport.

No data were available on the proportion of area residents versus visitors participating in the survey.

The survey covered 13,000 passengers on 500 flights--300 at National, 100 at Dulles and 100 at BWI--between July 8 and July 22 last year. The results released yesterday were based on the responses of 11,007 passengers who boarded at the three airports; the other 2,000 passengers were transferring flights or on flights that stopped here. Overall, 25,000 survey forms were distributed.

The survey was conducted by COG with the Maryland Department of Transportation in cooperation with the Virginia Department of Aviation, the Federal Aviation Administration and the airlines using the area's airports. The preliminary findings were presented to COG's transportation planning board yesterday.

None of the raw data or survey tabulations was made public, although Shapiro said they would be sent to airlines for perusal before meetings scheduled for March 2 here on the possibility of shifting flights out of National to Dulles or BWI.

Among other survey findings:

* Business-related travel accounted for slightly more than half of the region's air trips. At National, 23 percent of the trips were government-related and 33 percent were nongovernment business-related.

* Half of those responding reported that they began their trip at a private residence; 20 percent left from a place of business.

* About 83 percent of National's users reported arriving there by auto or taxi; more than 90 percent of the BWI and Dulles users arrived at those airports by car.

* Less than 2 percent of those surveyed cited availability of better public transportation as the primary reason they used National, but almost 20 percent cited it as a secondary reason.