ST. PAUL, MN, USA
N181LL
BEECH B90
THE AIRPLANE WAS BEING OPERATED AS AN AIR AMBULANCE WHEN IT SUSTAINED SUBSTANTIAL DAMAGE DURING AN UNCOMMANDED HARD PITCHOVER. THE AIRPLANE WAS OPERATING IN VISUAL CONDITIONS ABOVE A SOLID CLOUD LAYER USING INSTRUMENT FLIGHT RULES. THE AIRPLANE RECOVERED FROM THE EXCURSION AND CONTINUED ON TO ITS DESTINATION. THE ATP RATED PILOT, THREE MEDICAL ATTENDANTS AND ONE PATIENT WERE NOT INJURED. TESTING OF THE PILOT'S WHEEL TRIM SWITCH SHOWED THE SWITCH WOULD STICK IN THE CLOSED OR ACTUATED POSITION AND NOT RETURN TO THE 'CENTER OFF' POSITION. THIS OCCURRED DURING TEMPERATURE CYCLING FROM ROOM TEMPERATURE TO BELOW FREEZING AND BACK AGAIN. THE CABIN HEATER HAD FAILED WITH AN OUTSIDE AIR TEMPERATURE OF -26 DEGS CELSIUS AND THE CABIN WAS COLD.
MECHANICAL FAILURE AND STICKING OF THE PILOTS THUMB OPERATED TRIM SWITCH CAUSING NOSE DOWN PITCH COMMAND. THE FAILURE OF THE CABIN HEATER AND THE LOW CABIN TEMPERATURE ARE CONTRIBUTING FACTORS.
Source: NTSB Aviation Accident Database
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