MALFUNCTION OF ONE OR MORE FLIGHT INSTRUMENTS FOR AN UNDETERMINED REASON, FAILUE OF THE PILOT TO MAINTAIN CONTROL OF THE AIRCRAFT WITH PARTIAL PANEL INSTRUMENTS AFTER BECOMING SPATIALLY DISORIENTED, AND HIS EXCEEDING THE DESIGN STRESS LIMITS OF THE AIRCRAFT. THE WEATHER (LOW CEILING) AND MALFUNCTIONING DIRECTIONAL GYRO WERE RELATED FACTORS.