AN ACR CL60 FLC SELECTED THE WRONG AUTOFLT SYS MODE DURING A GAR AND DSNDED WHEN THEY SHOULD HAVE CLBED. THE DSCNT WAS SUCH THAT THE GPWS SOUNDED A TERRAIN WARNING BEFORE THE FLC RECOVERED.

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AS THE APCH (IN IMC) PROGRESSED; SPACING ON THE PRECEDING ACFT APPARENTLY CONTINUED TO DIMINISH. ATL TWR HAD ISSUED INSTRUCTIONS DIRECTING US TO SLOW TO 144 KTS. DESPITE SLOWING TO OUR FINAL APCH SPD; THE FLT APPARENTLY CONTINUED TO OVERTAKE THE PRECEDING ACFT. THIS RESULTED IN ATL TWR CANCELING OUR APCH CLRNC. ATL TWR ISSUED A NEW CLRNC FOR US TO TURN R; HDG 180 DEGS; CLB AND MAINTAIN 5000 FT. THE TURN WAS INITIATED SUCCESSFULLY; HOWEVER THE CLB WAS NOT. WITH THE APCH CLRNC HAVING BEEN CANCELED AT AN ALT OF 2000 FT (1000 FT AGL); (AS I BEST REMEMBER) I ELECTED TO LEAVE THE AUTOPLT ENGAGED. I ADVANCED THE THRUST LEVERS AND ENGAGED THE VERT SPD MODE. OR SO I THOUGHT. SOMEHOW; I OR MY COPLT SELECTED SPD MODE INSTEAD. I HONESTLY CANNOT REMEMBER. IN FACT I WAS NOT AWARE OF THE AUTOPLT HAVING BEEN IMPROPERLY PROGRAMMED; UNTIL MY COPLT AND I DEBRIEFED THIS INCIDENT. ADVANCING THE THRUST LEVERS AND SELECTING SPD MODE ALONG WITH RESETTING THE SPD BUG TO 200 KTS WITH THE ACFT'S SPD AT JUST OVER 140 KTS; CAUSED THE ACFT TO DSND IN AN ATTEMPT TO ACHIEVE THE NEWLY BUGGED AIRSPD. A TIME LINE IS DIFFICULT TO RECALL HERE. HOWEVER; MY CO-PLT ALERTED ME TO THE ACFT'S DSCNT AS I NOTED THE DSCNT MYSELF. THE AUTOPLT WAS DISENGAGED AND A PITCH UP MANEUVER INITIATED. SIMULTANEOUSLY THE GPWS BEGAN TO ISSUE TERRAIN WARNINGS. THE ACFT WAS PITCHED UP 20 DEGS WHILE MAX THRUST WAS SET. I BELIEVE THE ACFT DSNDED TO WITHIN 600 FT OF THE SURFACE OR DSNDED TO AN ALT OF 1600 FT MSL BEFORE ESCAPE WAS POSITIVELY ESTABLISHED. THE CLB TO 5000 FT AND SUBSEQUENT APCH BEYOND THIS POINT WERE UNEVENTFUL. THERE ARE USUALLY A MULTITUDE OF CONTRIBUTING FACTORS: 1) ATC WAS TRYING TO ACCOMMODATE TOO LARGE A VOLUME OF ACFT GIVEN THE 400 FT CEILING. VISIBILITY BENEATH WAS ABOUT 4 MI AS I REMEMBER. THE REDUCED OR TIGHT SPACING BTWN ACFT IS WHAT LED TO THE CANCELED APCH CLRNC IN THE FIRST PLACE. I KNOW THAT ATL TWR WAS DOING NOTHING WRONG AND WISH TO POINT THIS FACT OUT. AT LEAST IN A REGULATORY SENSE. 2) CONTINUOUS DUTY OVERNIGHTS CONTRIBUTE TO FATIGUE. I WAS FATIGUED AND KNEW IT. THIS IS WHAT LED ME TO LEAVE THE AUTOPLT ENGAGED THINKING THIS TO BE THE SAFEST OPTION. I KNOW NOW THIS TO BE INCORRECT. I AM SURE FATIGUE CLOUDED MY JUDGEMENT. DESPITE A PLT'S BEST EFFORTS TO MANAGE THE FATIGUE ISSUES ASSOCIATED WITH CONTINUOS DUTY OVERNIGHTS; COMPANY POLICY IS TO SCHEDULE NO MORE THAN 3 CONTINUOUS DUTY OVERNIGHTS IN A ROW FOLLOWED BY 2 DAYS OFF. THIS IS A VAST IMPROVEMENT OVER PAST SCHEDULING PRACTICES. 4 AND SOMETIMES 5; ALTHOUGH 5 BEING RARE; WERE STANDARD FARE. IT WAS ONLY HARD BARGAINING BY UNION PLTS THAT BROUGHT ABOUT THIS CHANGE. FROM DUTY IN TO DUTY OUT 15 HRS 12 MINS DUTY TIME ELAPSED. ONLY 4 HRS WERE SPENT AT THE HOTEL. FOR MYSELF; I PERSONALLY SLEEP 4 1/2 - 5 HRS BTWN ASSIGNED CONTINUOUS DUTY OVERNIGHTS. DURING THE LAYOVER PORTION OF THE CONTINUOUS DUTY OVERNIGHT; I ATTEMPT TO GET 2 1/2 - 3 MORE HRS SLEEP. MY RATIONALE IS THAT I AM GETTING 8 HRS; OR THEREABOUTS; SLEEP IN A 24 HR PERIOD. BUT WHAT QUALITY IS THIS SLEEP? DESPITE MY BEST EFFORTS; THERE ARE TIMES I AM NOT RESTED. THIS WAS THE CASE DURING THIS PARTICULAR ASSIGNMENT. 3) DURING TRAINING ENTIRELY TOO MUCH EMPHASIS IS PLACED ON AUTOMATION. THE ASSUMPTION BEING THAT THE AUTOMATION REDUCES PLT WORKLOAD; THEREBY CREATING A SAFER ENVIRONMENT. TO A POINT; I AGREE. 4) MY OWN INEXPERIENCE WITH THIS TYPE AND PARTICULAR MODEL CONTRIBUTED TO THIS INCIDENT. AT THE TIME OF THIS INCIDENT I HAD LOGGED 79 HRS; 53 HRS AS PIC. AS LONG AS PLTS REMAIN ON DUTY IN EXCESS OF 12-14 HRS; COMPANIES SCHEDULE CONTINUOUS DUTY OVERNIGHTS AND REDUCED REST PERIODS; ATC CONTINUES TO ATTEMPT ACCOMMODATING HIGH VOLUMES OF TFC INTO OUR EXISTING ARPTS AND AUTOMATION ADVANCES IN THE COCKPITS TO THE POINT OF OVERWHELMING PLTS; WE WILL CONTINUE TO FLIRT WITH DISASTER. 5) WHAT I HAVE LEARNED FROM THIS INCIDENT IS THAT AUTOMATION IS SOMETIMES NOT THE ANSWER. SHOULD I FIND MYSELF IN SIMILAR CIRCUMSTANCES AND THAT IS HIGHLY LIKELY; IN AS MUCH AS HAVING ANAPCH CLRNC CANCELED; I WILL BE REVERTING BACK TO WHAT I KNOW HAS WORKED FOR ME AND SO MANY OTHER PLTS. OUR BASIC FLYING SKILLS. AT THE VERY LEAST I AM NOW MORE AWARE OF THE PITFALLS OF RELYING TOO HEAVILY ON TODAY'S AUTOMATED COCKPITS.