As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ONE OF THE MAGNETS CAME LOOSE FROM THE MAIN GEAR BOX YOKE, THERE ARE 2 MAGNETS IN THIS HALL EFFECT MAIN ROTOR RPM SENCING SYS WHEN ONE OF THE MAGNETS CAME SEPARATED FROM THE YOKE YOU LOOSE RPM INDICATION AND THE LOW ROTOR RPM WARNING HORN AND LIGHT THEN ACTIVATE. THIS FAILURE RESULTED IN AN AUTOROTATION AND THE TAILBOOM BEING CUT OFF BY THE MAIN ROTOR BLADES.