AS50 HELICOPTER CREW HAD THE R SIDE XMISSION COWLING RIP-OFF THE ACFT INFLT AND THEN STRUCK THE MAIN ROTOR AND TAIL ROTOR.

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AT CRUISE OF 3000 FT MSL; 118 KIAS; PLT AND ONE MEDICAL CREW HEARD A SERIES OF THREE SLAPPING SOUNDS. PLT AND CREW MEMBER DISCUSSED POSSIBILITY OF BIRD STRIKE; BUT DECIDED THE NOISE HAD COME FROM UP AND TO THE R; DIRECTLY BEHIND THE CABIN. PRECAUTIONARY LNDG AT ZZZ1 REVEALED R SIDE XMISSION COWLING RIPPED OFF AND CONTACTED TAIL ROTOR. THIS FLT WAS SECOND FLT SINCE COWLING HAD BEEN OPENED LAST. ALL THREE CREWMEMBERS HAD COMPLETED VISUAL WALK AROUND OF HELICOPTER TO VERIFY ALL COWLINGS/COVERS LATCHED AND SECURED. ALL THREE CREW MEMBERS AGREE COWLING IN QUESTION WAS LOCKED AND SECURED. SUPPLEMENTAL INFO FROM ACN 579013: WHILE ENRTE TO ZZZ FOR AN AEROMEDICAL FLT 3 RAPID 'BANGING' NOISES WERE HEARD. DISCUSSED THE SIT WITH THE PLT. PLT DECIDED TO MAKE A PRECAUTIONARY LNDG AT ZZZ1 ARPT TO INVESTIGATE. INVESTIGATION REVEALED THE R SIDE XMISSION COWLING BLEW OFF STRIKING COMPONENTS OF THE ACFT INCLUDING THE TAIL ROTOR. ACFT WAS TAKEN OUT OF SVC AND DISPATCH/MGMNT WAS NOTIFIED. PRIOR TO THIS FLT I PERSONALLY GAVE A BRIEF VISUAL INSPECTION TO THE COWL LATCHES. I ALSO NOTED THE PLT PERFORMING A CHECK OF THE LATCHES AND OTHER COMPONENTS AS PER HIS USUAL PREFLT ROUTING. SUPPLEMENTAL INFO FROM ACN 579012: ENRTE TO ZZZ FOR A PATIENT TRANSPORT BACK TO THE MEDICAL CENTER. THE R SIDE COWLING CAME OFF AND HIT THE TAIL ROTOR OF THE ACFT. PREFLT WAS DONE BY ALL CREW MEMBERS PRIOR TO FLT. ALL COWLING COVERS CHECKED AND SHOWED TO BE SECURED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT WAS FLYING AN AEROSPATIALE AS-350.B2 HELICOPTER. PLT STATED THE COWL IS MADE OF FIBERGLASS WITH A FOAM CORE. FROM MAINT INSPECTION; THE SEPARATED COWL STRUCK THE MAIN ROTOR; IN ADDITION TO THE TAIL ROTOR. FROM THE APPEARANCE OF THE COWLING PIECE THAT REMAINED ATTACHED TO THE ACFT; THE COWLING APPEARED TO SPLIT IN THE MIDDLE. AT THAT POINT IT WOULD BE FREE TO DEPART THE ACFT; LATCHES OR NOT. AN ATTEMPT WAS MADE TO FIND THE SEPARATED COWLING. THIS INCLUDED SEARCHING BY AIR. THE COWLING WAS NEVER RECOVERED. THE PLT WAS CONTACTED BY AN INSPECTOR WITH FSDO OFFICE; TO REVIEW WHAT HAPPENED. PLT NOTES THAT IN CONVERSATION WITH ANOTHER PLT; HE ALSO HAD A SIMILAR INCIDENT IN THE SAME MODEL ACFT.