A SOCATA TB20 ON CLB AT 5000 FT DECLARED AN EMER DUE TO LOSS OF ENG THROTTLE CTL. RETURNED TO DEP ARPT. FOUND COTTER LOCKING PIN IN THROTTLE LINKAGE MISSING.

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PRIOR TO FLT; ANNUAL PERFORMED. LYCOMING CRANKSHAFT AIRWORTHINESS DIRECTIVE PERFORMED WITH REINSTALLATION OF ENG. 2 PRIOR FLTS CONDUCTED EARLIER THAT DAY -- ZZZX DIRECT ZZZY DIRECT ZZZ WITH NO ABNORMALITY. VFR FLT ZZZ DIRECT ZZZY PLANNED; CRUISE AT 5500 FT. ON CLBOUT FROM ZZZ; HDG 060 DEGS; 12.5 NM FROM ARPT; I ADJUSTED THE THROTTLE TO COMPENSATE FOR MANIFOLD PRESSURE DROP FROM ALT; 5000 FT; MANIFOLD PRESSURE SUDDENLY DROPPED TO 2.3 INCHES FROM 25 INCHES; THROTTLE CTL MOVED FORWARD AND BACKWARD -- NO EFFECT; ENG WAS RUNNING; BUT CREATING LOW PWR. I CALLED ZZZ TWR; WHICH ACKNOWLEDGED; AND I DECLARED AN EMER; STATING THROTTLE BROKEN; ENG RUNNING AND THE NEED TO RETURN. I CONSIDERED CHANGING TO ARPT BUT DECIDED AGAINST IT; WHEN I WAS 5 MI AND 7 FROM ZZZ. AIRSPD MAINTAINED 80-90 KTS. ARRIVED NE AREA OF FIELD 1000 FT AGL. LOWERED GEAR; PARTIAL FLAPS; LNDG ASSURED; FULL FLAPS; MIXTURE; FUEL OFF; MADE UNEVENTFUL TOUCHDOWN RWY 25. ROLLED OFF ONTO TXWY E AND STOPPED. FIRE CREWS PRESENT; ASKED IF OK; AND WE WERE. LATER; I SPOKE WITH MECH ABOUT WHAT HAPPENED. WE WENT TO PLANE THE NEXT DAY; DISCOVERED THROTTLE LOCKING COTTER PIN NOT PRESENT; RESULTING IN DISCONNECT. MIX CTL COTTER PIN ALSO NOT INSTALLED. MECH HAD GONE OVER ALL CONNECTION 3 TIMES PRIOR TO SIGNOFF; HAD AN ADDITIONAL MECH LOOK OVER ENG INSTALL AS WELL. DEFECT NOT FOUND. MECH DOES REMEMBER HE WAS INTERRUPTED WHEN HE WAS INITIALLY RECONNECTING THE THROTTLE CABLE WHEN REINSTALLING THE ENG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE AIRPLANE WAS JUST OUT OF ANNUAL INSPECTION AND THIS WAS THE THIRD FLT. THE RETURN TO THE FIELD AND LNDG WAS UNEVENTFUL AND THE AIRPLANE INCURRED NO DAMAGE. ENG CHK OF THE THROTTLE LINKAGE REVEALED A MISSING LOCKING COTTER PIN THAT WAS OVERLOOKED WHEN THE ENG WAS INSTALLED DURING THE ANNUAL INSPECTION.