MECHANIC REPORTS ABOUT A BRASILIA EMB-120 AIR TURNBACK; CAUSED BY PW-118 ENGINE AIR INLET 'S' DUCT SEPARATING FROM THE ENGINE INLET; RESULTING IN A SIGNIFICANT LOSS OF POWER ON THE #2 ENGINE.

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I MUST TESTIFY THAT IT WAS LATE. I USED AN ASSISTANT TO HELP ME PLACE THE INLET DUCT. WHEN IN POSITION; WE INSTALLED A V-BAND CLAMP -- THE MAIN SUPPORT HOLDING IT IN PLACE. IN SECURING THE REST OF THE DUCT WORK; I LOOSENED THE CLAMP TO REPOSITION; ALIGNING THE AFT PORTION WHICH; ONCE THE AFT PORTION WAS SECURED; I ENDED MY SHIFT. IN MY PASS DOWN I OPENED PAPERWORK FOR THE FRONT PORTION; THE CHIN COWL TO BE INSTALLED; WHICH IT WAS; BUT THE CLAMP I HAD LOOSENED DID NOT GET SAFETIED OR TIGHTENED; THUS RESULTING IN THE DUCT DROPPING DOWN AND CAUSING SIGNIFICANT LOSS OF PWR THAT THE CREW RETURNED TO BASE. AFTER SEVERAL PWR RUNS THE ACFT WAS ABOUT TO BE RETURNED TO SVC. AFTER A LOOK AT THE ENG; THAT IS WHEN IT WAS DISCOVERED. NEW CLAMP ON ORDER AND NO DAMAGE; NO PERSONNEL INJURED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE 'S' DUCT IS ATTACHED TO THE LOWER SIDE OF THE PW-118 ENGINE AT THE AIR INLET. THE DUCT IS SECURED TO THE ENGINE INLET BY A MORMON V-TYPE CLAMP THAT IS TIGHTENED AND THAN SAFETY WIRED WITH 040 INCH WIRE. REPORTER STATED THEY HAD A PROBLEM ALIGNING THE 'S' DUCT WITH THE ENGINE INLET BECAUSE THE ALIGNMENT DOWELS ON THE DUCT THAT HELP POSITION THE DUCT TO THE ENGINE INLET WERE MISSING. HE HAD EARLIER TIGHTENED THE MORMON CLAMP; BUT HAD TO LOOSEN THE CLAMP NUT TO GET ALIGNMENT WITH THE FORWARD AND AFT SECTIONS OF THE DUCT BELLOWS CLAMPS. REPORTER STATED HE SHOULD HAVE FLAGGED THE MORMON CLAMP AS A REMINDER TO RETIGHTEN THE NUT; BUT HE DID NOT AND FORGOT TO MAKE A SHIFT TURNOVER TO HAVE THE CLAMP RESECURED. AS A RESULT; THE #2 ENGINE AIR INLET DUCT DROPPED DOWN; INFLIGHT; REDUCING AIR INTAKE; AND REQUIRING AN AIR TURNBACK OF THE EMB-120 AIRCRAFT.