FO OF FA50 ADVISES CAPT ENGAGED APCH MODE OF AUTOFLT SYS PREMATURELY RESULTING IN ALTDEV WHEN GS WAS CAPTURED PRIOR TO BEING CLRED TO A LOWER ALT.

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WE WERE CLRED TO DSND TO 2400 FT AND GIVEN A HDG TO INTERCEPT THE LOC. THE CAPT SELECTED 'APCH' ON THE FLT DIRECTOR. I SUGGESTED HE ONLY USE HDG AND NAV; SINCE WE WERE NOT CLRED FOR THE APCH; ONLY TO INTERCEPT THE LOC AND; THEREFORE; SHOULD NOT DSND BELOW 2400 FT. HE FOLLOWED MY SUGGESTION; HOWEVER; WHILE I WAS LOOKING OUTSIDE THE ACFT FOR TFC; HE AGAIN HIT 'APCH' AND WE WENT RIGHT THROUGH THE 2400 FT. AS HE WAS DSNDING I REALIZED WHAT WAS GOING TO HAPPEN AND CALLED OUT 'ALT;' BUT HE CONTINUED BELOW 2400 FT AND THEN CAUGHT IT AND RETURNED TO THE CORRECT ALT. THE CAPT WAS EXTREMELY INEXPERIENCED AND HAD NOT FLOWN A DA50 IN OVER 10 YRS -- ONLY THE SIMULATOR ON A RECURRENT TRAINING SESSION. HE HAD NOT FLOWN A HIGH PERFORMANCE JET IN OVER 10 YRS EITHER. HE WAS TOTALLY UNFAMILIAR WITH THE ACFT'S AVIONICS AND FMS. FOR THE ENTIRE FLT; I FELT THAT THE CAPT WAS WAY BEHIND THE AIRPLANE AND SHOULD NOT HAVE BEEN DISPATCHED AS THE PIC. HE LATER ADMITTED THAT HE WAS VERY UNCOMFORTABLE WITH THE AIRPLANE AND ALSO IN THE LOW LEVEL FLT ENVIRONMENT. HOWEVER; HE NEEDED THE MONEY AND HE WAS TECHNICALLY 'CURRENT' HAVING JUST COMPLETED A REFRESHER COURSE WHICH HE PAID FOR HIMSELF. PERHAPS SOME OF THE PART 91 REGS NEED TO BE TIGHTENED UP OR INDIVIDUALS NEED TO BE AWARE OF WHAT THEY ARE HIRING IN TERMS OF QUALIFICATIONS. THIS WAS A NEW ACFT OWNER AND A VERY OLD FALCON 50 -- TOGETHER WITH A VERY UNSEASONED CAPT; THIS WAS A POTENTIALLY DANGEROUS SIT.