Piedmont Aviation Inc.'s chairman resigned yesterday and signed on to head up an effort by United Airlines pilots who are seeking to buy the carrier.

The resignation of William R. Howard, 65, who had headed Piedmont for 10 years, came as something of a surprise to the company's directors, who accepted it with regret, according to a spokesman. Howard was replaced by William G. McGee, an executive vice president of Piedmont.

Piedmont is being acquired by USAir Corp., pending Department of Transportation approval. Although the airline will continue to operate as a separate subsidiary for a time, the merger would change the nature of Howard's job.

Howard's move to the head of Airline Acquisition Corp., the vehicle set up by the pilots to pursue their offer for United, adds to the credibility of the pilots' efforts, airline industry observers said.

"He's very highly thought of, and he had a great record at Piedmont," said Louis A. Marckesano, an industry analyst with Janney Montgomery Scott Inc.

The pilots union also announced yesterday that Chemical Bank of New York has agreed to help finance the pilots' bid for United. Chemical has told the pilots that it is "highly confident" of its ability to raise the additional funds needed for the transaction, according to a letter from F.C. (Rick) Dubinsky, head of the Master Executive Council, which represents pilots at United.

Dubinsky wrote Allegis Corp. Chairman Frank Olson that the agreement with Chemical, which supplements an agreement with Salomon Brothers Inc., to raise $1.5 billion in junior securities for the offer, "completes the financial requirements necessary for the transaction we have proposed."

Dubinsky urged the board of Allegis, which is the parent company of United, to work with the pilots to complete the transaction, but warned that the pilots would pursue their goal with or without that cooperation.

"We really have no comment, except that the airline is not for sale," said Allegis spokesman Matt Gonring