Narrative:

We were given a vector to intercept the localizer on the ILS 16R CAT ii approach with 'maintain 1800' till established on the localizer'. When we intercepted the localizer we were outside the IAF of metre (a LOM) and the minimum altitude at that point was 1800' until intercepting glide slope which we had not yet done. The pilot flying started a descent immediately upon intercepting the localizer, the pilot not flying spoke up about 150' below our proper altitude and then approximately 300' below our proper altitude yelled, pull up, you are too low. We need 1800'. The pilot flying complied but was still not aware of what he had done wrong. After establishing correct altitude there was a short discussion on correct procedure. Contributing factors were possibly: the WX was poor at sacramento metropolitan. There had been missed apches and much discussion about whether to do a CAT I or CAT ii approach and who would be flying. The pilot flying was a reserve pilot and expected things a little differently. It was the 3RD leg on the 1ST day of the trip at the end of the month. I know for myself, I was tired. We had been flying approximately 5+ hours, been on duty approximately 9 hour 45 min, and our crew meals had not been boarded on the aircraft.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR LGT ALT DEVIATION DESCENT BELOW INTERCEPT ALT OUTSIDE THE IAF AT SMF.

Narrative: WE WERE GIVEN A VECTOR TO INTERCEPT THE LOC ON THE ILS 16R CAT II APCH WITH 'MAINTAIN 1800' TILL ESTABLISHED ON THE LOC'. WHEN WE INTERCEPTED THE LOC WE WERE OUTSIDE THE IAF OF METRE (A LOM) AND THE MINIMUM ALT AT THAT POINT WAS 1800' UNTIL INTERCEPTING GLIDE SLOPE WHICH WE HAD NOT YET DONE. THE PLT FLYING STARTED A DSCNT IMMEDIATELY UPON INTERCEPTING THE LOC, THE PLT NOT FLYING SPOKE UP ABOUT 150' BELOW OUR PROPER ALT AND THEN APPROX 300' BELOW OUR PROPER ALT YELLED, PULL UP, YOU ARE TOO LOW. WE NEED 1800'. THE PLT FLYING COMPLIED BUT WAS STILL NOT AWARE OF WHAT HE HAD DONE WRONG. AFTER ESTABLISHING CORRECT ALT THERE WAS A SHORT DISCUSSION ON CORRECT PROC. CONTRIBUTING FACTORS WERE POSSIBLY: THE WX WAS POOR AT SACRAMENTO METRO. THERE HAD BEEN MISSED APCHES AND MUCH DISCUSSION ABOUT WHETHER TO DO A CAT I OR CAT II APCH AND WHO WOULD BE FLYING. THE PLT FLYING WAS A RESERVE PLT AND EXPECTED THINGS A LITTLE DIFFERENTLY. IT WAS THE 3RD LEG ON THE 1ST DAY OF THE TRIP AT THE END OF THE MONTH. I KNOW FOR MYSELF, I WAS TIRED. WE HAD BEEN FLYING APPROX 5+ HRS, BEEN ON DUTY APPROX 9 HR 45 MIN, AND OUR CREW MEALS HAD NOT BEEN BOARDED ON THE ACFT.

Data retrieved from NASA's ASRS site as of August 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.