Piedmont Aviation Inc. said yesterday it is buying Henson Aviation Inc., a commuter airline operating mainly out of the Baltimore-Washington International airport that carried 650,000 passengers last year.

Piedmont would not reveal the value of the sale, but it involves five stock transactions over four years, Piedmont representative Don McGuire said.

The deal, which requires Civil Aeronautics Board approval, would make Henson a wholly owned subsidiary of Piedmont.

McGuire said Piedmont will file a request for approval of the acquisition with the CAB before mid-July, and that approval is expected by late fall.

As soon as CAB approval is secured, the first sale of stock will take place and Henson will begin operating as a Piedmont subsidiary. The sales will then take place at yearly intervals until the end of the fourth year.

Henson, the largest commuter operator at BWI with 56 daily departures, will complement Piedmont's services perfectly, said William R. Howard, Piedmont president and chief executive. Piedmont already is scheduled to become the largest carrier at BWI on July 15, when it expands its service from 6 to 28 daily flights.

Henson Aviation, a private company, currently operates as part of the Allegheny Commuter system, with 16 planes serving Atlantic Seaboard cities from New York to Virginia and including Pittsburgh.

Because Henson's contract with USAir to serve as part of Allegheny Commuter will not expire until 1987, the company will need to terminate its commitment as part of Allegheny and work out a mutually agreeable transition date with USAir, said Richard Henson, president of Henson Aviation. Henson is meeting with USAir today.

Henson said his airline will then operate under the name Henson instead of Allegheny Commuter.

Henson, 73, founder of the airline in 1931, said he had been looking at acquisition possibilities since he turned 70 in an effort to ensure the security of his company.