What was found
AC LANDED FOR SHUTDOWN AFTER OPERATING FOR APPROX 3 FLIGHT HOURS, AC WAS SHAKING DURING SHUTDOWN. DURING POST FLIGHT INSP IT WAS FOUND THAT UPPER LT MOUNT HARD POINT PN 2050318211P ON TAILBOOM (PN 20503280071), WAS FRACTURED, HAD TORN THRU FWD TAILBOOM BLKHD (PN 205030807123S) ON AIRFRAME WAS ALSO CRACKED IN 2 PLACES RADIATING FROM BOOM MOUNT HOLE TO EDGE OF FITTING. AC RECORDS DO NOT REFLECT ANY INSTANCE OF HARD LANDING, OVER TORQUE, OR OTHER DAMAGE HISTORY. AC HAS APPROX 8400 HOURS TT, RECORDS SHOW THAT TAILBOOM WAS INSTALLED APPROX 5000 HOURS AGO. SUSPECTED, WHEN UPPER FITTING FAILED INSIDE TAILBOOM, BOOM ASSY SAGGED, PLACING EXCESSIVE TWIST ON LWR LT AIRFRAME MOUNT CAUSING IT TO CRACK IN 2 PLACES. (K)