PILOT'S BAD JUDGEMENT IN FLYING IN WEATHER CONDITIONS BEYOND THE CAPABILITIES OF HIS AIRCRAFT. THE PILOT OVERSTRESSED THE AIRCRAFT AND SHEARED THE LEFT WING BY MAKING AN ABRUPT TURN IN TURBULENT AIR ABOVE THE DESIGN MANEUVERING SPEED (VA). THE ACCUMULATION OF ICE, FLYING IN IMC CONDITIONS AND A PROBLEM WITH THE ATTITUDE INDICATOR WERE CONTRIBUTING FACTORS.