B747-400 CREW DSNDED BELOW THE PUBLISHED ALT WHILE FLYING A VNAV AUTO COUPLED APCH AT DTW RWY 22R.

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WE WERE APCHING DTW FOR AN APCH TO RWY 22R AND DECIDED TO PRACTICE A VNAV APCH. WE SELECTED THE VNAV APCH TO RWY 22R. NUJNT WAS THE KEY WAYPOINT. ATC VECTORED US TO INTERCEPT THE LOC BTWN FEPBU AND JOYDD AT 4000 FT. WE WERE CLRED FOR THE APCH JUST OUTSIDE JOYDD. PER SOPA; WE HAVE A CHKLIST TO ACCOMPLISH WHEN 'CLRED FOR THE APCH.' THIS CHKLIST WAS DEVISED BECAUSE CREW MEMBERS WERE LACKING IN THE BASIC UNDERSTANDING OF HOW TO ACCOMPLISH A VNAV APCH AND THE CHKLIST WAS A SERIES OF ACTIONS THAT WOULD ENSURE THAT THE ACFT WOULD DSND ON THE GLIDE PATH AND NOT CRUISE BY THE ARPT IN VNAV ALT. WE ACCOMPLISHED THE CHKLIST AND THE ACFT DSNDED BELOW 4000 FT AT JOYDD. THIS IS OK WITH ATC APPROVAL; BUT WE DID NOT HAVE THAT. I BELIEVE WE WERE WITHIN 200 FT OF 4000 FT AT XING; BUT I AM NOT SURE. WE WERE SO BUSY TRYING TO MAKE SURE THE CHKLIST WAS ACCOMPLISHED THAT OUR ATTN WAS DIVERTED. THIS IS NOT THE FIRST INCIDENT WHERE THIS HAS OCCURRED. THE SAME THING HAPPENED AT JFK ON THE VOR APCH TO RWY 13L/R. I HAD TO DISCONNECT THE AUTOPLT AND HAND FLY TO STOP DSCNT BELOW 1500 FT. CAUSES: THE MAIN REASON FOR THESE SITS IS INADEQUATE TRAINING. I PRACTICED 'DIVE AND DRIVE APCHS' FOR YRS PRIOR TO MY EMPLOYMENT AS A PROFESSIONAL THROUGH HUNDREDS OF APCHS. I HAVE REFINED MY SKILLS. THE VNAV APCH IS A 'NEW BREED OF CAT.' MOST OF THE TRAINING HAS BEEN BY BULLETIN WITH VERY LITTLE SIMULATOR TIME. THERE HAS NEVER BEEN A GND SCHOOL WITH AN INSTRUCTOR WHO EXPLAINED THE NUANCES OF THE VNAV APCH. IN 90% OF THE SIMULATOR VNAV APCHS; THE APCH HAS FAILED. I HAD A RECENCY OF EXPERIENCE 1 YR AGO IN WHICH WE NEVER HAD A SUCCESSFUL VNAV APCH. SOLUTIONS. I KNOW IT COSTS MONEY; BUT WE NEED MORE THOROUGH TRAINING SO THAT WE ARE NOT CAUGHT UP IN ACCOMPLISHING CHKLISTS DESIGNED TO MAKE UP FOR OUR LACK OF KNOWLEDGE.