PWR TRAIN COWLING ON AN AS355 TWIN-STAR HELI ACFT CAME LOOSE IN FLT STRIKING THE MAIN ROTOR BLADE BEFORE FALLING OFF THE ACFT. THE HELI WAS LANDED WITHOUT FURTHER INCIDENT.

Narrative

DURING A MAINT FLT TO TRACK AND BAL THE MAIN ROTOR SYS ON AN AS355 F-1 HELI; THE R XMISSION COWLING OPENED IN FLT AND WAS STRUCK BY THE MAIN ROTOR SYS. MAINT PERSONNEL HAD REMOVED THE COWLING EARLIER AND REINSTALLED BEFORE THE FLT. I; AS THE PLT; LOOKED THE ACFT OVER AND DID NOT NOTICE THE SPECIFIC COWLING HATCHES BEING OPEN. THE ACFT HAD BEEN CRANKED AND GND RUN AT LEAST 3 TIMES BEFORE A FLT WAS MADE. AFTER THESE RUN-UPS AND ADJUSTMENTS BY MAINT PERSONNEL; ACFT WAS SAID TO BE READY FOR FLT TO CHK TRACK AND BAL IN FLT. ON FIRST FLT; ACFT WAS PICKED UP TO A HOVER; TKOF MADE; THE ACFT TRANSITIONED TO LEVEL FLT AT 500 FT MSL. ON APCH TO THE HELIPORT AT PATTERSON; LA; THERE WAS AN INITIAL SHUDDER. AT ABOUT 100 FT AGL THERE WAS ANOTHER SHUDDER. ACFT WAS LANDED WITHOUT FURTHER INCIDENT. ON EXITING ACFT; FOUND COWLING HAD COME OFF AND GONE THROUGH MAIN ROTOR SYS. THIS HAS HAPPENED BEFORE ON THIS TYPE ACFT. I FEEL ANOTHER SAFETY SYS OF SOME TYPE SUCH AS A CABLE ATTACHMENT TO PRECLUDE THE COWLING FROM BEING ABLE TO OPEN TO THE EXTENT OF HITTING ROTOR SYS SHOULD BE IMPLEMENTED.