A BELL 407 WITH TWO RPTS OF A VERTICAL VIBRATION IN CRUISE WAS INSPECTED A SECOND TIME AND FOUND THE ROTOR MAST CRACKED 60% AROUND THE CIRCUMFERENCE.

Narrative

WHILE CONDUCTING THE 21ST FLT OF THE DAY PART 135 OPERATION TO AN OFFSHORE LOCATION SOLO; WITH NO CARGO IN A BELL 407 TO PICK UP PERSONNEL; A SLIGHT VERTICAL VIBRATION WAS NOTED IN CRUISE FLT. THE VIBRATION WAS NOT SEVERE; HOWEVER SINCE IT HAD NOT BEEN NOTICED PREVIOUSLY THE DETERMINATION WAS TO MAKE A PRECAUTIONARY LNDG (19 MILES S; 174 DEG OF ZZZ TO INSPECT THE ACFT. DURING THE LNDG PHASE; BELOW TRANSLATIONAL LIFT THE VIBRATION WAS NOT NOTED. AFTER SHUTDOWN A THOROUGH INSPECTION OF THE ACFT WAS MADE. THE INSPECTION INCLUDED THE CTL TUBES; XMISSION MOUNTS; LNDG GEAR; ROTOR BLADES AND THE POSSIBILITY OF A BIRD STRIKE. NO DAMAGE OR DISCREPANCIES WERE NOTED. THE ACFT WAS RUN-UP AND HOVERED WITH NO VIBRATION AND THE DETERMINATION THAT THE ACFT WAS AIRWORTHY WAS MADE. THE ACFT WAS FLOWN BACK TO ZZZ. DURING THE RETURN FLT THE VIBRATION WAS NOTED AGAIN IN CRUISE FLT. THE ACFT WAS LANDED AND THE VIBRATION WAS RECORDED AS A DISCREPANCY IN THE ACFT LOGBOOK. DURING MAINT INSPECTION A CRACK WAS FOUND ON THE MAST OF THE ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A BELL 407 AND WAS BUILT IN 1996 AND IS OPERATED AND MAINTAINED IN A PART 135 OP. THE RPTR SAID A SLIGHT VERTICAL VIBRATION WAS NOTED IN CRUISE AND A PRECAUTIONARY LNDG WAS MADE. THE ACFT WAS INSPECTED; RUN UP AND TEST HOVERED WITH NO VIBRATION. THE RPTR STATED THE HELICOPTER WAS FLOWN TO THE BASE AND THE VIBRATION WAS AGAIN NOTED AND WRITTEN UP IN THE LOG. THE RPTR SAID AN INSPECTION WAS MADE AND THE ROTOR MAST WAS FOUND AT FIRST TO BE CRACKED 120 DEGS BUT THE CRACK CONTINUED UNDER THE PAINT TO A TOTAL OF 60 DEGS AROUND THE MAST CIRCUMFERENCE. THE RPTR STATED THIS WAS THE FIRST INCIDENT OF A CRACKED ROTOR MAST KNOWN TO OUR MAINT ORGANIZATION.