B737 CREW FLEW A VISUAL LDA APCH 640 FT LOWER THAN THEY SHOULD HAVE.

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APCHING FL200 WE RECEIVED CLRNC TO CROSS FAKES AT OR ABOVE FL190; DSND AND MAINTAIN 11000 FT. SLIGHTLY NONSTANDARD FOR THIS ARR. THIS IS ABOUT THE TIME WE WOULD NORMALLY BE PERFORMING THE 'APCH CHK' WHERE 29.92 WOULD BE RESET TO QNH (TODAY'S QNH WAS 29.28). RATHER THAN REPROGRAMMING THE FMS; THE PF UTILIZED VERT SPD MODE OF AUTOPLT TO MAKE THE XING RESTR; THEN RESET THE ALT TO 11000 FT. SOON AFTER THE XING RESTR; WE ENCOUNTERED SOME HVY RIME ICE BUT FOR ONLY A SHORT PERIOD OF TIME. WHILE DSNDING; I WAS A LITTLE SELF ABSORBED ABOUT RPTING THIS. THE ICING FOLLOWED BY A BREAKOUT INTO BEAUTIFUL CAVU WX CONDITIONS FURTHER DISTR ME INTO SOME UNTIMELY SIGHTSEEING IN SE ALASKA. ALTHOUGH THE WX WAS GOOD ENOUGH FOR A VISUAL APCH THE PF ELECTED TO FLY THE PROC FOR PRACTICE. THE FIRST HINT SOMETHING MAY HAVE BEEN WRONG WAS XING DIBOL INTXN WHERE THE PEAKS SEEMED TO LOOK A LITTLE CLOSE. THE 'LIGHT' NEVER CAME ON UNTIL THE MISSED APCH (3.3 NM FROM THE ARPT; 1000 FT MSL) WHERE THINGS DEFINITELY LOOKED WRONG AND THE 3 DEG VASI SHOWED US RED/RED. I SAW THE ALTIMETERS SET AT 29.92. AT THIS POINT ALMOST SIMULTANEOUSLY; THE PF CLBED BACK TO THE VASI AND THE FLT WAS COMPLETED UNEVENTFULLY. WE WERE BOTH WELL RESTED FOR THE FLT; IT WAS THE FIRST LEG OF THE THIRD DAY OF A 3 DAY TRIP. CAVU CONDITIONS DEFINITELY LED TO COMPLACENCY IN THIS CASE; COUPLED WITH MINOR DISTRS ABOUT THE POINT WE WOULD NORMALLY PERFORM THE 'APCH CHK.' I HAVE ANNOTATED ON OTHER APCH/ARR PLATES WHERE THIS CHK SHOULD BE PERFORMED; ESPECIALLY WHEN IT COINCIDES WITH A BUSY INTXN. I WILL ADD THIS NOTE TO FAKES INTXN NOW ALSO. ALL IN ALL; A VERY CHEAP LESSON THAT IT CAN HAPPEN TO ANY OF US.