Narrative:

I was the PF. We were with oak approach control at 5000 ft MSL when approach directed a descent to 2100 ft MSL. The first officer repeated the altitude back to ATC and I heard and confirmed the clearance to 2100 ft MSL. Passing 3000 ft MSL I commented to the first officer that we appeared low (I could see the ground through breaks in the clouds). Passing 2800 ft MSL ATC issued an altitude alert and then directed an immediate climb to 3100 ft MSL. I disconnected the autoplt and initiated a climb back to 3100 ft MSL; descending as low as 2700 ft MSL during the recovery. No further comments were made by ATC. From our point of view the controller incorrectly stated 2100 ft MSL when he meant 3100 ft MSL. Runway 11 at oak was being used which is not the usual runway (runway 29 is more common).

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Original NASA ASRS Text

Title: B737 ON DSCNT FOR OAK WITH NCT WAS ISSUED IMMEDIATE CLB TO 3100 FT AFTER AN ALLEGED ATC DSCNT CLRNC TO 2100 FT VICE 3100 FT.

Narrative: I WAS THE PF. WE WERE WITH OAK APCH CTL AT 5000 FT MSL WHEN APCH DIRECTED A DSCNT TO 2100 FT MSL. THE FO REPEATED THE ALT BACK TO ATC AND I HEARD AND CONFIRMED THE CLRNC TO 2100 FT MSL. PASSING 3000 FT MSL I COMMENTED TO THE FO THAT WE APPEARED LOW (I COULD SEE THE GND THROUGH BREAKS IN THE CLOUDS). PASSING 2800 FT MSL ATC ISSUED AN ALT ALERT AND THEN DIRECTED AN IMMEDIATE CLB TO 3100 FT MSL. I DISCONNECTED THE AUTOPLT AND INITIATED A CLB BACK TO 3100 FT MSL; DSNDING AS LOW AS 2700 FT MSL DURING THE RECOVERY. NO FURTHER COMMENTS WERE MADE BY ATC. FROM OUR POINT OF VIEW THE CTLR INCORRECTLY STATED 2100 FT MSL WHEN HE MEANT 3100 FT MSL. RWY 11 AT OAK WAS BEING USED WHICH IS NOT THE USUAL RWY (RWY 29 IS MORE COMMON).

Data retrieved from NASA's ASRS site as of January 2009 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.