PROBABLE ACFT EQUIP PROB AUTOPLT PITCHED DOWN WHILE ON A COUPLED APCH.

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B727 CAT II COUPLED APCH AT DTW. ATIS CALLED 100 FT CEILING AND RVR 2000 FT. AUTOPLT MADE A HARD PITCH DOWN AT 250 FT. GPWS SINK RATE SOUNDED. I CLICKED OFF AUTOPLT AND STARTED GAR. RWY CAME IN SIGHT AND I ELECTED TO LAND AS SAFEST COURSE. I HAVE HAD REPEATED PROBS WITH B727 AUTOPLTS DURING COUPLED APCHS. COUPLED APCHS ARE RARELY MADE AND ALMOST NEVER DURING VFR TO CHK THE AUTOPLT. I SUGGEST AUTOPLT COUPLED APCH CHK EVERY 60 DAYS LIKE THE MD80. CALLBACK #1 CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS THE CAPT ON THIS APCH TO DTW RWY 3L; A CAT II AND III RWY. THE AUTOPLT WAS COUPLED AND TRACKING PERFECTLY DOWN TO 250 FT. THE FO WAS MANAGING THE AUTOPLT AND THE CAPT WAS PREPARED TO TAKE OVER AND LAND. WHEN THE AUTOPLT PITCHED DOWN; THE FO JAMMED THE THROTTLES FORWARD; PUNCHED OFF THE AUTOPLT AND PITCHED UP. AT PITCH DOWN; THE NOSE WENT FROM ABOUT 3 DEG NOSE UP TO ABOUT 10 DEG NOSE DOWN; 'BAM; FULL FORWARD.' THE CAPT BELIEVES THAT HE WOULD HAVE BEEN 'DEAD MEAT' IF THE AUTOPLT HAD NOT BEEN PUNCHED OFF. THE GPWS SOUNDED AND THE CAPT SAW 1200 FPM DSCNT AT 200 FT ALT. THE AUTOPLT MAY NOT HAVE DESENSITIZED WHEN PASSING THE OM; BUT THE CAPT DOES NOT KNOW WHAT ACR MAINT FOUND WRONG WITH THE AUTOPLT. THE WX INCLUDED A 9 KT TAILWIND COMPONENT. THE ACR'S LIMIT IS 10 KTS TAILWIND. THE ACR'S PLT UNION'S SAFETY OFFICER IS VERY PLEASED WITH THE RPTR'S SUGGESTION OF REQUIRING A FLT CHK OF THE AUTOPLT COUPLER AS IS DONE IN THE 2 MAN ACFT. CALLBACK #2 CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: A SECOND CALL BACK WAS MADE TO THE RPTING CAPT. THIS CAPT IS EXPERIENCED ON THE B727; HAVING 7000 HRS IN THE ACFT. WHEN ASKED; HE STATED THAT THE STABILIZER OUT OF TRIM LIGHT NEVER CAME ON; THE AUTOPLT WAS PERFORMING FLAWLESSLY DOWN TO 250 FT AGL WHEN IT MADE ITS PITCH OVER MANEUVER. THE FO WAS FLYING BUT THE FO'S RESPONSE WAS INADEQUATE DURING THIS EVENT. THE FO RESPONDED TO THE PITCH OVER WITH THE PROGRAMMED PWR UP AND DID NOT DISCONNECT THE AUTOPLT; THEREBY ALLOWING THE AUTOPLT TO MAKE THE PROGRAMMED 5 DEG PITCH UP MANEUVER. THIS 5 DEG PITCH UP IS FROM THE ACFT'S PITCH ANGLE WHEN THE PWR IS APPLIED. IN THIS CASE THE PITCH WAS A MINUS OR NEGATIVE 10 DEGS. (THE NORMAL DECK ANGLE DURING APCH IS 2-3 DEGS NOSE UP.) THE RESULTANT 5 DEG PITCH UP INSTITUTED BY THE AUTOPLT GAVE THE ACFT A PITCH DOWN DECK ANGLE OF MINUS 5 DEGS. THE FO HAD BEEN TRAINED TO DO WHAT HE DID; ADD THE PWR UP APPLICATION AND ALLOW THE AUTOPLT TO ROTATE THE ACFT. THE CAPT; REALIZING THAT THIS MANEUVER 'WASN'T GOING TO DO THE JOB;' DISCONNECTED THE AUTOPLT; GRABBED THE YOKE AND ROTATED TO THE REQUIRED MANUAL GAR DECK ANGLE OF ABOUT 8-10 DEGS. HE ESTIMATES THE ACFT GOT TO ABOUT 75 FT AGL. THE ACFT IS CERTIFIED TO HAVE THE AUTOPLT COUPLED TO 80 FT AGL BUT NOT MORE THAN 50 FT BELOW THE DIRECTIONAL HT FOR THE APCH. THE CAPT STATED THAT HE HADN'T MADE A COUPLED APCH IN THE B727 IN OVER A YEAR; AS MOST OF THE B727 GUYS JUST HAND FLY THIS NON GEE WHIZ ACFT. HE FEELS QUITE STRONGLY ABOUT THE NEED TO 'CERTIFY' THESE AUTOPLT APCH COUPLERS PERIODICALLY. WHEN ASKED; HE REPLIED STRONGLY ABOUT THE TRAINING SYNDROME AS PRESENTED BY THE ACR. HE WANTS MORE EMPHASIS PLACED UPON THE 5 DEG MODE OR DECK ANGLE CHANGE FROM AN UNUSUAL PITCH ATTITUDE; NOT THE STANDARD 2-3 DEG PITCH ATTITUDE. HE ALSO STATED THAT THE CREW MUST BE ALERT TO THE NEED FOR AN AUTOPLT DISCONNECT IN CASES SUCH AS THIS EVENT. IT SCARED THE 'BEJESUS' OUT OF HIM; SEEING THE APCH LIGHTS FROM SUCH A LOW ALT. HE DOESN'T FEEL THAT THIS WAS A WINDSHEAR OR GND TFC PROB. THE EVENT WAS WRITTEN UP IN THE LOGBOOK BUT HE DID NOT CHK BACK WITH MAINT TO SEE WHAT THEY FOUND; IF ANYTHING.