THE GUEST OBSERVER PLT OF AN SMA HAD TO TAKE OVER THE CTLS TO COMPLETE AN INST APCH.

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AN ACFT BROKER NEEDED TO FERRY AN SMA FROM ISP TO CMH. I WAS LED TO BELIEVE THAT THE BROKER WAS CURRENT IN THE ACFT AND HELD AN INST RATING. HE ASKED ME TO COME ALONG FOR MORAL SUPPORT; COMPANY; AND TO HELP HIM WITH THE RADIOS. I INFORMED HIM THAT I WAS NOT FAMILIAR WITH THE SMA SYS; ESPECIALLY THE AUTOPLT SYS. HE ASSURED ME HE WAS. HE WOULD BE DOING THE FLYING. WE DEPARTED ISP IN VMC CONDITIONS; FLEW FOR 4 HRS USING THE AUTOPLT SYS MOST OF THE TIME. UPON REACHING THE CMH AREA; WE WERE CLRED OUT OF 4000 IN IMC CONDITIONS. THE BROKER WAS STILL FLYING THE ACFT USING THE AUTOPLT. HE APPEARED TO BE NERVOUS IN THE IMC CONDITIONS. THE BROKER WAS ATTEMPTING TO SET A RATE OF DSCNT IN THE ALT PRESELECT WHEN THE AUTOPLT CAUSED THE ACFT TO PITCH DOWN ABRUPTLY. THIS CAUSED THE AIRSPD TO ACCELERATE IN THE YELLOW ARC; AND RESULTED IN A BRIEF 2000 FPM RATE OF DSCNT. I THEN GRABBED MY YOKE AND USED MANUAL FORCE TO OVERRIDE THE AUTOPLT AND STOP THE DIVE. I ASKED THE BROKER TO DISCONNECT THE AUTOPLT SYS. HE EVENTUALLY COMPLIED; BUT IT TOOK ABOUT 20-30 SECONDS FOR HIM TO DO SO; DURING WHICH TIME WE FLEW THROUGH THE LOC COURSE. WHEN THE AUTOPLT SYS WAS COMPLETELY DISCONNECTED; I TOOK OVER AND EXECUTED A MISSED APCH. AFTERWARDS I HAND FLEW THE ILS 28 AT CMH WITHOUT INCIDENT. I FEEL THAT THE LACK OF AN AUTOPLT DISCONNECT ON THE R SIDE IS A SAFETY PROB. I AND THE BROKER WERE NOT FAMILIAR ENOUGH WITH THE AUTOPLT SYS. I FEEL THAT THE BROKER SELECTED A VERY HIGH RATE OF DSCNT IN THE ALT PRESELECT OR HE SELECTED THE APCH MODE AND THE AUTOPLT WAS TRYING TO CAPTURE THE GS WHICH WAS FAR BELOW US. IN THE FUTURE; I WILL MAKE SURE TO CHK THE PLT QUALIFICATIONS BEFORE GETTING INTO AN ACFT THAT I AM NOT FAMILIAR WITH; WITH THAT PLT.