CONTINUED FLIGHT BY THE PILOT TO AN ALTITUDE THAT WAS NOT APPROPRIATE WITHOUT SUPPLEMENTAL OXYGEN, WHICH RESULTED IN A LOSS OF AIRCRAFT CONTROL DUE TO HYPOXIA AND SUBSEQUENTLY EXCEEDING THE DESIGN STRESS LIMITS OF THE AIRCRAFT. FACTORS RELATED TO THE ACCIDENT WERE: THE ADVERSE WEATHER CONDITIONS, AND CONTINUED FLIGHT BY THE PILOT INTO KNOWN ADVERSE WEATHER CONDITIONS.