PLT OF A BEECH BARON; BE55; LOST CTL OF THE ACFT DURING AN IFR DSCNT WHEN THE CABIN COCKPIT DOOR UNEXPECTEDLY OPENED. THE RPTR LOST 700 FT BELOW ASSIGNED ALT BEFORE REGAINING CTL OF THE ACFT. A DIVERSION TO A NEARBY ARPT WAS MADE TO EXAMINE DOOR AND PROPERLY LATCH IT FOR CONTINUED FLT TO DEST.

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AFTER A CRUISE DSCNT WAS INITIATED (OUT OF 6000 FT FOR 5000 FT) THE COCKPIT DOOR BLEW OPEN. THE ACFT (BE55) AT THE TIME WAS UNDER APCH CTL; ON A RADAR VECTOR FOR AFW. THE FLT WAS ON AN IFR FLT PLAN; SINGLE PLT; FROM DWH TO FWA; WITH NO PAX. THE ACFT AT THE TIME WAS ALSO ON THE AUTOPLT. AS A RESULT OF THE DOOR OPENING; THE ACFT NOSED DOWN VIOLENTLY; CAUSING PARTIAL LOSS OF CTL. THE AUTOPLT WAS DISENGAGED; THROTTLE RETARDED; AND PITCH ADJUSTED IMMEDIATELY. RUDDER EFFECTIVENESS WAS NOTICEABLY AFFECTED BY THE DOOR OPENING AND REMAINING OPEN. AS A RESULT OF THE TEMPORARY LOSS OF CTL; THE ACFT DSNDED BELOW THE ASSIGNED ALT OF 5000 FT MSL. AT THE TIME OF THE DOOR BECOMING OPEN AND ACFT CTL WAS IN QUESTION; IMMEDIATE ACTION WAS INITIATED BY MYSELF TO INFORM THE ATC OF MY SIT; AND THAT I WAS DSNDING BELOW 5000 FT MSL; WITH NO ATC RESPONSE. IT WAS ONLY AFTER; THAT I STABILIZED THE ACFT AT 4300 FT MSL; THE ATC ASKED WHAT I WANTED AND WHAT WAS MY ALT. AT THIS TIME; ATC WAS FINALLY MADE AWARE OF MY SIT. A LNDG WAS MADE AT A NEARBY ARPT; THE DOOR EXAMINED; AND THE FLT WAS CONTINUED TO AFW. CAUSE: THE LATCH WAS IMPROPERLY SECURED. THE DOOR AT THE BEGINNING OF THE FLT WAS PROPERLY LATCHED AND FOUND TO BE SECURE. THOUGH THE LATCH MECHANISM HAD FAILED TO 'HOOK' AT THE TOP OF THE DOOR ALL OF THE WAY. THE ACFT HAD RECENTLY BEEN PAINTED AND NEW INTERIOR AND DOOR SEALS INSTALLED. ON EXAMINATION OF THE DOOR UPON LNDG; I FOUND THE TOP TO BE WORN AND TENDENCY NOT TO SHUT ALL OF THE WAY (WITHOUT SOME EFFORT); BECAUSE OF THE NEW SEAL.