A Frontier Airlines jet returned safely to Oakland International Airport when the pilot suffered an apparent heart attack 12 minutes after takeoff on a flight to Denver. Co-pilot Whitney Brandon turned back and made a normal landing at Oakland when Capt. Gerald Britton experienced severe chest pains. Britton was taken by ambulance to Alameda Hospital, where he was reported in stable condition following tests to determine the nature of his problem. Passengers continued on their way aboard other airliners.