MARTHA'S VINEYARD, MA, USA
N71MA
SMITH AEROSTAR 601
DURING AN ILS WITH THE AUTOPILOT COUPLER ON, A HIGH SINK RATE DEVELOPED TOWARD THE END OF THE APPROACH. THE PILOT UNCOUPLED THE AUTOPILOT AND BEGAN A MISSED APPROACH, BUT THE AIRCRAFT HIT THE TOP OF A TREE. HE DIVERTED AND LANDED AT PROVIDENCE, RI WITHOUT FURTHER INCIDENT. AFTER LANDING, SUBSTANTIAL DAMAGE OF THE RIGHT FLAP AND RIGHT PROPELLER WAS FOUND. A BENCH CHECK OF THE BENDIX FCS-810 AUTOPILOT REVEALED THAT THE TRANSISTORS IN THE GLIDE SLOPE WASH-OUT CIRCUIT WERE FAULTY. THEY WERE INTERMITTENTLY APPLYING A POSITIVE VOLTAGE THAT RESULTED IN A FALSE PITCH GYRO SIGNAL AND A DOWNWARD HARDOVER.
Source: NTSB Aviation Accident Database
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