LOSS OF PWR #1 ENG RESULTS IN SHUTDOWN AND RETURN TO DEP POINT.

Narrative

MORNING OF A CONTINUOUS DUTY OVERNIGHT AFTER GETTING TO HOTEL AT XA00 AND HAVING TO RPT TO WORK AT XE45 AM (4 HRS SLEEP). FO AND I PREPARED ACFT AND FLEW NORMAL UNEVENTFUL FLT FROM ABERDEEN; SD; TO WATERTOWN; SD. WE HAD PAX BOARDED AND STARTED #2 AND #1 ENG; #1 ENG ON START PRODUCED A BIT OF SMOKE ON START BUT ALL ENG GAUGES NORMAL. WE COMPLETED BEFORE TKOF AND RWY ITEMS CHKLIST AND PROCEEDED TO TKOF. ALL ENG GAUGES AND INDICATIONS APPEARED NORMAL AND REQUIRED TKOF PWR SETTING WAS MADE WITH EXCEPTION OF 1/2 INCH PWR LEVER SPLIT WHICH IS A BIT ABNORMAL BUT NOT TOO UNCOMMON. WE LIFTED OFF AND RETRACTED GEAR AND FLAPS WHEN AT ABOUT 500-600 FT AGL #1 (L) ENG MANUAL LIGHT CAME ON FOLLOWED SHORTLY BY LARGE PWR DROP ON #1 ENG. LOOKED AT ENG INSTS AND NOTED THAT #1 ENG TORQUE 25 PERCENT AND #1 PROP RPM 1125 AND DROPPING. IN THE EXCITEMENT/SLEEP DEPRIVED STATE; I CAN'T SAY IF #1 AUTO FEATHERED OR NOT; BUT I DO BELIEVE WE GOT PWR UP TRIM ON #2. DUE TO CLOSE PROX WITH GND AND VERY GOOD SE PERFORMANCE WE DIDN'T WASTE TIME; WE SECURED #1 ENG AND CAME BACK AND LANDED ON RWY 30 UNEVENTFULLY. AFTER DEPLANING AND WITH DISCUSSIONS WITH MAINT; PROB MAY HAVE BEEN AN ELECTRONIC CTL UNIT (ECU) FAILURE OR A TORQUE MOTOR FAILURE WHICH SHOULD/WOULD EXPLAIN A LOSS OF TORQUE/PWR; BUT HOPEFULLY NOT AS MUCH AS WE EXPERIENCED. ALSO; THE SIGNIFICANT LOSS OF PWR SHOULD ONLY HAVE OCCURRED ONCE WE SELECTED OUR ECU TKOF POS (TOP) SWITCH TO NORMAL WHICH WE DID NOT DO. NOW COMPANY GRILLING ME FOR DETAILS OF WHICH I DID NOT NOTICE OR WAS TOO HURRIED TO RECORD. CONCLUSION: DUE TO LACK OF SLEEP AND A HURRIED EVALUATION OF PROB; I MAY HAVE SHUT DOWN A GOOD ENG AT LEAST ONE THAT MAY HAVE BEEN ABLE TO RESTORE NEAR NORMAL PWR. ALSO; A LACK OF EXPERIENCE ON ACFT WITH THIS SORT OF MALFUNCTION MAY HAVE CAUSED ME TO MISDIAGNOSE THE SIT AND I JUST ASSUMED IT WAS AN ENG FAILURE. THIS ACFT IS ALLOWED TO MEL THE ECU; I'VE FLOWN WITH THIS MEL PROC BEFORE AND NOW WITH THIS EXPERIENCE I FEEL THE ECU SHOULD NOT BE DEFERRABLE.