MED EVAC HELO MADE PRECAUTIONARY LNDG DUE TO IMPROPERLY SECURED ENGINE COWLING THAT CAME LOOSE IN FLT.

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I AM A PLT ON A HELI AIR AMBULANCE SVC. WE HAD A SPARE HELI (SMT) ON CONTRACT WHILE DOING ENG MAINT AND MAIN ROTOR (M/R) TRACKING AND BALANCING ON OUR PRIMARY HELI (ALSO AN SMT). WHEN THE ENG WORK WAS COMPLETED; I PERFORMED THE PLT'S PREFLT INSP AND THEN RAN THE ENGS FOR FUNCTION/LEAK CHECKS. NO DISCREPANCIES WERE NOTED SO I THEN PERFORMED AN OPERATIONAL FLT CHECK; A POST-FLT INSP; AND THEN MADE SEVERAL SHORT FLTS TO TRACK AND BALANCE THE M/R SYSTEM. JUST PRIOR TO FINISHING OUR MAINT; THE NURSES AND PLT ARRIVED WITH THE SPARE ACFT IN ORDER TO SWITCH ALL OF OUR MEDICAL EQUIP OVER. AFTER THE LAST FLT FOR M/R BALANCING WAS OVER; THE PLTS AND NURSES BEGAN SWITCHING THE MEDICAL EQUIP AND THE TWO MECHS BEGAN REMOVING THE TRACK AND BALANCE GEAR. THIS REQUIRED OPENING THE XMISSION COWLINGS. ONCE THE ACCELEROMETERS AND WIRES WERE REMOVED; I INSPECTED THE XMISSION AREA BUT GOT CALLED AWAY AND DID NOT CLOSE THE COWLINGS. WHEN I RETURNED TO THE ACFT; THE COWLINGS WERE CLOSED AND I MADE WHAT I THOUGHT WAS A THOROUGH WALK-AROUND CHECK OF THE ENTIRE ACFT. I FOUND THAT THE R/H TRANS COWL WAS CLOSED BUT ONLY HAD ONE FASTENER SECURED. MYSELF AND ONE OF THE MECHS SECURED THIS PANEL. AT THIS POINT; I THOUGHT ALL WORK WAS COMPLETED AND THAT THE ACFT WAS COMPLETELY AIRWORTHY AND READY TO FLY. I THEN LEFT THE ACFT FOR APPROX FIVE MINUTES TO SIT AT A DESK AND GO OVER THE LOGBOOK WITH THE MECH. UPON RETURNING; I TOOK ANOTHER QUICK LOOK AT BOTH SIDES OF THE ACFT; THEN GOT IN TO FLY TO ABC FOR FUEL. AFTER APPROX FIVE MIN FLYING TIME THE DOOR AJAR CAUTION LIGHT ILLUMINATED AND SHORTLY AFTERWARD THE L/H XMISSION COWL FLEW UP AND STRUCK THE M/R BLADES. I LANDED AT THE NEAREST AVAILABLE SAFE LANDING AREA; WHICH WAS ON THE ARPT. IMMEDIATELY AFTER THE INCIDENT; I REPORTED IT TO OUR CHIEF PLT. THE FOLLOWING A.M. I MADE A TELEPHONE REPORT TO THE FSDO. ON FEB/TUE; I WAS INFORMED THAT A FLT VIOLATION WOULD BE FORTHCOMING. I HAVE COPIES OF WRITTEN STATEMENTS FROM MYSELF; BOTH MECHS; AND THE OTHER PLT REGARDING THIS INCIDENT. I REALIZE THAT AS PLT IN COMMAND I WAS FULLY RESPONSIBLE FOR ENSURING THAT THE ACFT WAS AIRWORTHY PRIOR TO TAKEOFF. I FEEL THAT THE FOLLOWING FACTORS CONTRIBUTED TO THIS INCIDENT: I LET MYSELF GET HURRIED BECAUSE OF THE IMPENDING MISSIONS. ONE OF THE MECHS HAD NEVER WORKED WITH US BEFORE. IT IS A COMMON (BUT UNWRITTEN) PRACTICE FOR US TO NEVER CLOSE UP A COWLING UNLESS IT IS TO BE COMPLETELY SECURED. I FEEL THAT THIS COWLING HAD EITHER BEEN RE-OPENED WHILE I WAS CHECKING THE LOGBOOK OR ELSE IT HAD BEEN ONLY PARTIALLY SECURED AND I DID NOT SEE THIS. THIS PARTICULAR COWL HAS FIVE LOCKING DEVICES. I FEEL THAT ONE OR MORE OF THEM MUST HAVE BEEN SECURED BECAUSE OF THE FACT THAT I FLEW FOR FIVE MINUTES BEFORE THE DOOR LIGHT ILLUMINATED. THE OTHER PLT HAD ALSO WALKED AROUND THE ACFT JUST PRIOR TO MY DEP AND DID NOT SEE ANYTHING WRONG. CORRECTIVE ACTIONS: MY COMPANY HAS ORDERED US TO OBTAIN A RED AOG SOCK TO PLACE OVER THE CYCLIC STICK BEFORE ANY COWLINGS ARE OPENED OR ANY WORK STARTED BY ANY PERSON OTHER THAN THE PLT-IN-COMMAND. I HAVE REINFORCED IN MYSELF THE NEED TO COMPLETELY RECHECK THE ACFT JUST PRIOR TO FLT; REGARDLESS OF HOW MANY TIMES IT WAS CHECKED PREVIOUSLY.