General Dynamics will write off $420 million.

St. Louis-based General Dynamics Corp. said the writeoff from the $680-million purchase price of Cessna Aircraft Co. will be taken in the fourth quarter of 1986 and represents about $9.80 for each share of its outstanding common stock. The defense contractor acquired Cessna last year. Since then, General Dynamics suspended production of its piston aircraft product line and reduced the Cessna work force in what the firm called a “general aviation market which has been and continues to be severely depressed.” General Dynamics’ earned $273.6 million in the first nine months of 1986.