SF340 FLT CREW HAS AUTOCOARSEN EVENT AT ROTATION FROM TRI. FULL TORQUE IS REGAINED IN THE LEFT ENGINE WHEN THE POWER LEVER IS MOVED FORWARD; HOWEVER THE FLT CREW ELECTS TO RETURN TO TRI.

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DURING DEP FROM RWY 23 AT TRI THE FO APPLIED THE BRAKES AND ADVANCED THE PWR LEVERS TO APPROX 80. THE AUTOCOARSEN HIGH LIGHT ILLUMINATED AND I CALLED 'ARMED.' THE FO CALLED 'SET PWR.' I SELECTED THE CTOT TO APR (AUTO PWR RESERVE) AND VERIFIED 100% TORQUE. AT V1 I CALLED 'V1' AND AT VR THE FO PITCH UP FOR A NORMAL CLB ATTITUDE. AT POSITIVE RATE OF CLB I RETRACTED THE GEAR. AT FRA I WAS GOING TO BEGIN THE AFTER TKOF FLOW AND NOTICED THE L APR LIGHT OUT; L TORQUE LOW AND THE R ENG TORQUE AT APPROX 107; AND BOTH NG AND ITT INDICATIONS NORMAL. I THEN OBSERVED THE L PROP THROUGH THE WINDOW AND IT APPEARED TO BE FEATHERED; BUT STILL ROTATING. WE HAD NO ABNORMAL INDICATIONS ON THE CWP EXCEPT FOR THE AIR CONDITION LIGHT ASSOCIATED WITH THE BLEEDS OFF TKOF. WE TREATED THE INCIDENT AS AN ENG FAILURE AND I PROCEEDED TO COMPLETE THE MEMORY ITEMS. WHEN I TOUCHED THE L PWR LEVER THE L ENG ACCELERATED TO 100% TORQUE AND ALL OTHER ENG INDICATIONS SHOWING NORMAL. I OBSERVED BOTH ENGS TO BE PRODUCING NORMAL PWR AND I THOUGHT IT WAS AN UNCOMMANDED AUTOCOARSEN EVENT. I THEN COMPLETED AN AFTER TKOF CHK. I THEN NOTIFIED ATC THAT WE WOULD LIKE TO RETURN TO TRI. WE LANDED AT TRI WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPTAIN FEELS THAT THE FO MUST HAVE BUMPED THE LEFT POWER LEVER AS HE WAS REMOVING HIS HAND AT V1. THE WINDS WERE VERY GUSTY WITH TURBULENCE AT ROTATION. THE POWER LEVER MUST HAVE BEEN RIGHT ON THE EDGE OF THE AUTOCOARSEN RANGE AS THE ENGINE RETURNED TO NORMAL TORQUE WHEN THE LEVER WAS TOUCHED.