BEECH 58 PLT DSNDS BELOW ASSIGNED ALT WHILE DEALING WITH ACFT MALFUNCTION.

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DEPARTED 52F AT APPROX XA00; IFR TO DTS. ABOUT 10 MINS INTO THE FLT; THE AIRPLANE DEVELOPED A MECHANICAL PROB WHICH I DID NOT DEEM AN EMER; BUT DID REQUIRE AN IMMEDIATE RETURN TO 52F. I ADVISED DFW APCH OF MY SIT. HE ADVISED ADS WAS CLOSEST ARPT. I FELT I COULD MAKE 52F. HE GAVE ME A HDG BACK TO 52F AT 3000 FT MSL. ABOUT 3 MINS FROM 52F THE SIT REQUIRED A DSCNT TO MAINTAIN AIRSPD IN SAFE RANGE. AT THIS MOMENT; THE CTLR HANDED ME OFF TO ANOTHER CTLR. BY THE TIME THE FREQ CLRED FOR ME TO SPEAK; I HAD THE PROB UNDER CTL BUT HAD DSNDED TO 2500 FT MSL. THE CTLR ASKED IF I HAD BEEN CLRED TO 2500 FT. I ADVISED 'NO.' I WAS VMC AT THIS TIME BUT STILL ON IFR PLAN. I FELT THE UNDECLARED EMER ALLOWED ME TO DEVIATE AS NECESSARY; HOWEVER; I FELT THE NEED TO SEND THIS IN TO PROTECT MYSELF. I LANDED SAFELY AT 52F AND THE CTL PROB WAS FIXED. WE WERE ON OUR WAY AT XC00 AGAIN TO DTS WITHOUT INCIDENT. IN RETROSPECT; I SHOULD HAVE DECLARED AN EMER. THE SIT CERTAINLY WARRANTED SUCH A CALL. I WAS RELUCTANT TO DO SO.