As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON TAKEOFF OF AN INTER-FACILITY TRANSPORT, THE AIR CONDITIONING 90 DEGREE GEARBOX SEIZED CAUSING SHAFT TO SHEAR IN GEARBOX AND COUPLING TO SHEAR ON DRIVESHAFT. COUPLING SHEAR CAUSED SEVERE VIBRATIONS FORCING PILOT TO RETURN TO TAKEOFF POINT AND LANDED WITHOUT INCIDENT. PROBABLE CAUSE SHAFT SHEAR STRENGTH TOO HIGH. LOWER SHEAR STRENGTH, BELIEVE THIS WAS AN ISOLLATED OCCURANCE.