FO OF A CESSNA CITATION OVERSHOT ASSIGNED ALT ON 2 OCCASIONS DURING CLBOUT DUE TO THE ALT ALERT NOT SOUNDING AS DESIGNED AND MISPROGRAMMING THE AUTOPLT CAPTURE MODE.

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TWICE DURING THIS PARTICULAR CLBOUT DID WE CLB THROUGH AN ASSIGNED ALT -- FIRST 17000 FT WHILE STILL WITH CBM DEP CTL; AND SECOND; FL230 DURING THE HDOF TO ZTL. THERE WAS NO CONFLICT; NO TARGETS WERE SHOWING ON THE TCASII WITHIN 20 MI. UPON RECOGNITION OF DEV WE IMMEDIATELY RETURNED TO ASSIGNED ALT. THERE WAS NO INDICATION BY ATC THAT ANY CONFLICT OR PROB HAD OCCURRED. FROM POSTFLT ANALYSIS OF WHAT COULD HAVE CAUSED THIS TO HAPPEN TO 2 SEASONED PROFESSIONALS; WE CONCLUDED THAT THERE WERE 4 THINGS THAT CUMULATIVELY CONTRIBUTED TO US MISSING ASSIGNED ALTS. 1) UNFAMILIARITY OF PF WITH SUBTLE DIFFERENCES BTWN THE AUTOPLT/FLT DIRECTOR SYS IN THIS AIRPLANE AND THE ONE HE NORMALLY FLIES. THE PF ON THIS LEG OF A MULTI-LEG TRIP IS A 20 YR AIRLINE VETERAN; AN EXCELLENT PLT WITH NO BLEMISHES ON HIS RECORD. HE WAS FLYING WITH US THAT DAY AS A CONTRACT PLT; FILLING IN WHILE 2 OF OUR STAFF PLTS WERE AT FLT SAFETY FOR RECURRENT TRAINING. ON THE ACFT TYPE THAT HE FLIES FOR HIS AIRLINE; THE AUTOPLT SYS ALLOWS YOU TO KEEP USING THE THUMB TRIM WHEEL ALL THE WAY TO LEVELOFF WITHOUT AFFECTING THE 'ALT CAPTURE' MODE. ON OUR ACFT; HOWEVER; ONCE 'ALT SELECT' SWITCHES TO 'ALT CAPTURE' MODE (USUALLY AROUND 300-400 FT BEFORE REACHING SELECTED ALT; DEPENDING ON RATE); ANY USE OF THE THUMB TRIM WHEEL WILL DISCONNECT THE 'ALT CAPTURE' MODE (AND 'ALT SELECT' FOR THAT MATTER). OUT OF HIS HABIT OF WANTING TO PROVIDE THE SMOOTHEST POSSIBLE LEVELOFF; HE UNKNOWINGLY DISCONNECTED THE 'ALT CAPTURE' MODE. 2) WE DISCOVERED ON THE SUBSEQUENT CLB TO OUR FINAL ALT OF FL290 THAT THE ALT SELECT AURAL TONE WARNING; THAT IS SUPPOSED TO CHIME AT 1000 FT BEFORE REACHING SELECTED ALT; AND AT 300 FT DEV FROM SELECTED ALT; WAS NOT WORKING. IT HAD BEEN OPERATING NORMALLY ON THE PREVIOUS LEGS OF THIS FLT; BUT HAD APPARENTLY FAILED ON THIS CLBOUT (MURPHY'S LAW!). DURING THE REMAINDER OF THIS FLT WE VERIFIED THAT IT WAS NOT GIVING THE AURAL WARNING IT IS SUPPOSED TO; AND IT WAS WRITTEN UP AS A SQUAWK AT THE END OF THE FLT. MAINT SUBSEQUENTLY FOUND A CHAFED WIRE LEADING TO THE BELL (HORN) IN THE OVERHEAD; AND DETERMINED THAT TO BE THE CAUSE FOR THE FAILURE. 3) THIS WAS A RELATIVELY SHORT LEG; WITH RATHER HIGH WORKLOAD FOR THE PNF -- CLEANING UP AFTER DEP; OBTAINING WX AT DEST; AND PREPARING FOR ARR; HANDLING THE RADIO WORK; AND DOING SOME COMPANY REQUIRED PAPERWORK. 4) RADIO FREQS WERE VERY CONGESTED; CONSTANT 'NOISE' SO TO SPEAK; WHICH PROBABLY CONTRIBUTED TO OUR ATTN BEING SOMEWHAT DIVERTED. WE BOTH FEEL THAT THE FIRST 2 ITEMS WERE THE MAIN CAUSE FOR THE ALTDEVS; BUT THIS CLRLY DEMONSTRATES THE NEED FOR INCREASED VIGILANCE WHEN CREW MEMBERS ARE NOT USED TO THE SAME EQUIP; AND ESPECIALLY WHEN THEY ARE NEW TO EACH OTHER AS A CREW. WE ALSO FEEL THAT OUR SUBCONSCIOUS RELIANCE ON AUTOMATIC FEATURES; SUCH AS THE AUTOPLT AUTOMATIC LEVELOFF AND THE ALT ALERT WARNING BELL; CAN EASILY GET ONE IN TROUBLE WHEN THESE FEATURES DISCRETELY CHOOSE TO FAIL. SUPPLEMENTAL INFO FROM ACN 422950: ON ACFT I USUALLY FLY A B747-400. ONCE IN ALT CAPTURE MODE; YOU MUST SELECT VERT SPD BEFORE THE VERT SPD RATE WHEEL IS ACTIVE; PREVENTING PLT FROM DISENGAGING AUTO ALT CAPTURE (A 2 STEP METHOD). IN THE CITATION; THE VERT SPD RATE WHEEL IS ALWAYS ACTIVE; AND THEREFORE COULD BE BUMPED OR USED AND DISENGAGE AUTO ALT CAPTURE. THE VERT SPD WHEEL IS LOCATED ON REAR OF CTR PEDESTAL BTWN PLTS WHERE IT COULD BE BRUSHED OR BUMPED.