Narrative:

While getting vectors after the commo arrival to runway 11 at oak; we were given a descent from 5;000 ft to what we thought was 2;100 ft. That is what I understood and set into the altitude window; and that is what my first officer read back to ATC. My first officer and I thought that was kind of low for where we were in relation to the airport and discussed it. My first officer then asked ATC what the MVA was in this area and he replied 5;000 ft where we were previously; a small band of 4;000 ft; and then 3;000 ft. We assumed that he was descending us into an area ahead of that with an even lower MVA. As we passed through about 2;700 ft ATC said the altitude was supposed to be 3;100 ft. I immediately started a climb to 3;100 ft and then I got a single GPWS caution; 'caution; terrain.' I disconnected the autopilot and continued correcting the flight path of the aircraft. We leveled at 3;100 ft and continued to the airport without further problems.since we were both sure of the clearance to descend to 2;100 ft and the subsequent read back was not challenged by ATC there is not too much I can think of to improve that part of the error. I think we did the right thing as far as discussing the MVA and asking ATC about it. I think my mistake was assuming that ATC was descending us into an area that the MVA was lower than the area that we were currently in. If something does not feel right; get clarification until it does feel right.

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Original NASA ASRS Text

Title: Although questioning their altitude clearance on vectors for Runway 11 at OAK; the flight crew of a B737-300 continued their descent until receiving a GPWS terrain warning at which time they confirmed their cleared altitude as 3;100 FT not 2;100 FT as they had read back and entered in the altitude alert window.

Narrative: While getting vectors after the COMMO Arrival to Runway 11 at OAK; we were given a descent from 5;000 FT to what we thought was 2;100 FT. That is what I understood and set into the altitude window; and that is what my First Officer read back to ATC. My First Officer and I thought that was kind of low for where we were in relation to the airport and discussed it. My First Officer then asked ATC what the MVA was in this area and he replied 5;000 FT where we were previously; a small band of 4;000 FT; and then 3;000 FT. We assumed that he was descending us into an area ahead of that with an even lower MVA. As we passed through about 2;700 FT ATC said the altitude was supposed to be 3;100 FT. I immediately started a climb to 3;100 FT and then I got a single GPWS caution; 'Caution; terrain.' I disconnected the autopilot and continued correcting the flight path of the aircraft. We leveled at 3;100 FT and continued to the airport without further problems.Since we were both sure of the clearance to descend to 2;100 FT and the subsequent read back was not challenged by ATC there is not too much I can think of to improve that part of the error. I think we did the right thing as far as discussing the MVA and asking ATC about it. I think my mistake was assuming that ATC was descending us into an area that the MVA was lower than the area that we were currently in. If something does not feel right; get clarification until it does feel right.

Data retrieved from NASA's ASRS site as of April 2012 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.