U.S. SAYS USAIR <U> MAY BUY 51 PCT OF PIEDMONT The Department of Transportation said it will allow USAIR Group to acquire up to 51 pct of Piedmont Aviation <PIE> voting stocks pending final approval of the proposed merger of the two airlines. The agency said the stock would have to be held in a voting trust controlled by an independent trustee. An agency spokesman said that if USAIR, which has proposed to buy all of Piedmont shares, controls more than 51 pct of the firm's outstanding stock, it will have one week to sell those excess shares. USAIR asked the Department of Transporation earlier this month to approve a voting trust. An agency spokesman said this is a device that airlines use to get majority control of a company it is trying to acquire while their application before the government is pending approval. The spokesman said the firm had asked the transporation agency permission to buy all of Piedmont's voting stock, the but agency decided to give approval for 51 pct of shares. The spokesman said agency action on the proposed merger application could take as long as six months.