A DC8 FREIGHTER FO DSNDS BELOW THE ASSIGNED XING ALT NEAR HWD ARPT ON A VISUAL APCH TO OAK; CA.

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THIS ALT INFRACTION OCCURRED APPROX 10-12 MI FROM THE OAK ARPT ON THE APCH TO RWY 29. WE WERE CLRED FOR THE VISUAL APCH BUT WITH THE RESTR OF MAINTAINING AN ALT OF NOT LESS THAN 200 FT AT A POINT OF 6 MI FROM THE ARPT. THE ADDED ELEMENT FROM BAY APCH CTL WAS THAT THE LOC AND GS WERE BOTH OTS. THIS IS WHERE THE PROB MAY BE IN MY OPINION. VISIBILITIES IN THE ENTIRE BAY AREA THAT DAY WERE ALMOST UNLIMITED. I WAS ABLE TO SEE THE RWY AT OAK FROM OVER LIVERMORE; WHICH IS SOME 30 MI PERHAPS. I ROLLED ONTO ABOUT A 12 MI FINAL USING VISUAL CUES BACKED UP WITH THE FMS AND VOR (TUNED TO OAK). I NOTED MY ALT AT 3500 FT AND SLOWLY DSNDING AT WHAT APPEARED TO ME TO BE THE SAME AS WHAT WOULD BE SEEN IF WE WERE ESTABLISHED ON THE ILS TO THAT RWY (RWY 29). OUR ATTN WAS PRIMARILY OUTSIDE FOR TFC AND JUST GOOD OLD SIGHTSEEING (WHICH MAY HAVE CONTRIBUTED TO BEING A DISTR). MY LAST CHK ON ALT AND DISTANCE WAS PERFORMED AT WHAT I THOUGHT THE FMS TO READ 6 MI; AT WHICH I NOTED WITH MY CONTINUED DSCNT RATE TO MATCH NICELY WITH WHAT APPEARED TO BE ON GS VISUALLY. WE LATER DISCUSSED THAT WE MAY HAVE MISREAD THE DISTANCE. I MAY HAVE READ 6 INSTEAD OF 8 (THE CAPT AND MYSELF IN POSTFLT DISCUSSION). OAK GND INQUIRED AS TO OUR ALT OVER HAYWARD ARPT AND I MENTIONED ABOUT 2500 FT. TO MY BEST KNOWLEDGE I BELIEVE WE MAY HAVE DSNDED BELOW 2000 FT PRIOR TO THE 6 MI RESTR. AT NO TIME WAS THERE ANY TFC CONFLICT AND THERE WAS NO TFC SEEN IN THE PATTERN AT HAYWARD AS WE PASSED OVER. ATC DID NOT ASK ANY FURTHER QUESTIONS OR MADE NO OTHER REQUESTS! RECOMMENDATION: PERHAPS A VASI ON THAT RWY WOULD HELP CORRECT FURTHER AND FUTURE REPEATED OCCURRENCES.