Narrative:

This altitude infraction occurred approximately 10-12 mi from the oak airport on the approach to runway 29. We were cleared for the visual approach but with the restr of maintaining an altitude of not less than 200 ft at a point of 6 mi from the airport. The added element from bay approach control was that the localizer and GS were both OTS. This is where the problem may be in my opinion. Visibilities in the entire bay area that day were almost unlimited. I was able to see the runway 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 altitude at 3500 ft and slowly descending at what appeared to me to be the same as what would be seen if we were established on the ILS to that runway (runway 29). Our attention was primarily outside for traffic and just good old sightseeing (which may have contributed to being a distraction). My last check on altitude and distance was performed at what I thought the FMS to read 6 mi, at which I noted with my continued descent 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 captain and myself in postflt discussion). Oak ground inquired as to our altitude over hayward airport and I mentioned about 2500 ft. To my best knowledge I believe we may have descended below 2000 ft prior to the 6 mi restr. At no time was there any traffic conflict and there was no traffic 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 runway would help correct further and future repeated occurrences.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.