Narrative:

Day #3 of a 3-DAY pairing. The first officer was the PF. The captain was the PNF. The location of the event was in the vicinity of the oak 100 departure radial at 35 DME. The routing clearance was the panoche arrival at 7000 ft MSL. The controller issued a clearance direct mitoe (a fix on the ILS 29 approach), and directed a descent to 4000 ft. The PNF read back the clearance to include 4000 ft and set 4000 ft in the MCP altitude window. The readback was unchallenged by the controller. The PF acknowledged the clearance and verified that 4000 ft was set in the MCP window. While approaching 5000 ft in VMC, both pilots recognized and commented on the lower than normal proximity to the ground below. The PF initiated a leveloff with the intent to level at 5000 ft. From this point forward, the PF was flying the aircraft using visual reference to the outside environment to ensure terrain clearance. Actual leveloff occurred around 4800 ft. The controller asked us to verify our altitude. The PNF said we were descending to 4000 ft as instructed, but we have leveled off at 5000 ft because of the terrain below. The GPWS issued a terrain alert. The PF visually confirmed that contact with the ground was not an issue and initiated a controled climb. The controller directed a climb to 6000 ft, immediately followed by maintain 5000 ft. The PF corrected the altitude to 5000 ft. Both pilots were certain the altitude clearance was to 4000 ft. The remainder of the approach and landing were uneventful. After landing, we contacted norcal approach. The supervisor said they had checked the tapes and that they had assigned 6000 ft. We had read back 4000 ft, and they didn't catch it. This error chain began with the incorrect readback of the altitude assignment. The PNF, the PF, and the controller all heard the clearance and subsequent readback but no one caught the error. The chain ended when both pilots noticed the lower than normal proximity to the ground and initiated a leveloff. The GPWS added another layer of safety with the 'terrain, terrain' alert. The controller query was the final item that got everyone back in the loop, but did not happen until the aircraft was nearly 1000 ft below the assigned altitude. In this event, crew duty day was not an issue, both pilots were highly experienced, the flight was running ahead of schedule, all operational procedures were adhered to, there were no extraneous cockpit distrs, there was no expectation of receiving 4000 ft as an altitude assignment, the aircraft was on course, on airspeed in a controled descent when this situation developed. Human error by all 3 layers of redundancy (PNF, PF, and ATC) was the cause. Recognizing that human error can occur through these redundancies, and maintaining vigilance toward all ATC clearance assignments will help prevent events like this from happening in the future.

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Original NASA ASRS Text

Title: B737-700 CREW HAD AN EGPWS 'TERRAIN, TERRAIN' WARNING GOING INTO OAK WHEN NCT MISSED AN INCORRECT DSCNT ALT CLRNC READBACK.

Narrative: DAY #3 OF A 3-DAY PAIRING. THE FO WAS THE PF. THE CAPT WAS THE PNF. THE LOCATION OF THE EVENT WAS IN THE VICINITY OF THE OAK 100 DEP RADIAL AT 35 DME. THE ROUTING CLRNC WAS THE PANOCHE ARR AT 7000 FT MSL. THE CTLR ISSUED A CLRNC DIRECT MITOE (A FIX ON THE ILS 29 APCH), AND DIRECTED A DSCNT TO 4000 FT. THE PNF READ BACK THE CLRNC TO INCLUDE 4000 FT AND SET 4000 FT IN THE MCP ALT WINDOW. THE READBACK WAS UNCHALLENGED BY THE CTLR. THE PF ACKNOWLEDGED THE CLRNC AND VERIFIED THAT 4000 FT WAS SET IN THE MCP WINDOW. WHILE APCHING 5000 FT IN VMC, BOTH PLTS RECOGNIZED AND COMMENTED ON THE LOWER THAN NORMAL PROX TO THE GND BELOW. THE PF INITIATED A LEVELOFF WITH THE INTENT TO LEVEL AT 5000 FT. FROM THIS POINT FORWARD, THE PF WAS FLYING THE ACFT USING VISUAL REF TO THE OUTSIDE ENVIRONMENT TO ENSURE TERRAIN CLRNC. ACTUAL LEVELOFF OCCURRED AROUND 4800 FT. THE CTLR ASKED US TO VERIFY OUR ALT. THE PNF SAID WE WERE DSNDING TO 4000 FT AS INSTRUCTED, BUT WE HAVE LEVELED OFF AT 5000 FT BECAUSE OF THE TERRAIN BELOW. THE GPWS ISSUED A TERRAIN ALERT. THE PF VISUALLY CONFIRMED THAT CONTACT WITH THE GND WAS NOT AN ISSUE AND INITIATED A CTLED CLB. THE CTLR DIRECTED A CLB TO 6000 FT, IMMEDIATELY FOLLOWED BY MAINTAIN 5000 FT. THE PF CORRECTED THE ALT TO 5000 FT. BOTH PLTS WERE CERTAIN THE ALT CLRNC WAS TO 4000 FT. THE REMAINDER OF THE APCH AND LNDG WERE UNEVENTFUL. AFTER LNDG, WE CONTACTED NORCAL APCH. THE SUPVR SAID THEY HAD CHKED THE TAPES AND THAT THEY HAD ASSIGNED 6000 FT. WE HAD READ BACK 4000 FT, AND THEY DIDN'T CATCH IT. THIS ERROR CHAIN BEGAN WITH THE INCORRECT READBACK OF THE ALT ASSIGNMENT. THE PNF, THE PF, AND THE CTLR ALL HEARD THE CLRNC AND SUBSEQUENT READBACK BUT NO ONE CAUGHT THE ERROR. THE CHAIN ENDED WHEN BOTH PLTS NOTICED THE LOWER THAN NORMAL PROX TO THE GND AND INITIATED A LEVELOFF. THE GPWS ADDED ANOTHER LAYER OF SAFETY WITH THE 'TERRAIN, TERRAIN' ALERT. THE CTLR QUERY WAS THE FINAL ITEM THAT GOT EVERYONE BACK IN THE LOOP, BUT DID NOT HAPPEN UNTIL THE ACFT WAS NEARLY 1000 FT BELOW THE ASSIGNED ALT. IN THIS EVENT, CREW DUTY DAY WAS NOT AN ISSUE, BOTH PLTS WERE HIGHLY EXPERIENCED, THE FLT WAS RUNNING AHEAD OF SCHEDULE, ALL OPERATIONAL PROCS WERE ADHERED TO, THERE WERE NO EXTRANEOUS COCKPIT DISTRS, THERE WAS NO EXPECTATION OF RECEIVING 4000 FT AS AN ALT ASSIGNMENT, THE ACFT WAS ON COURSE, ON AIRSPD IN A CTLED DSCNT WHEN THIS SIT DEVELOPED. HUMAN ERROR BY ALL 3 LAYERS OF REDUNDANCY (PNF, PF, AND ATC) WAS THE CAUSE. RECOGNIZING THAT HUMAN ERROR CAN OCCUR THROUGH THESE REDUNDANCIES, AND MAINTAINING VIGILANCE TOWARD ALL ATC CLRNC ASSIGNMENTS WILL HELP PREVENT EVENTS LIKE THIS FROM HAPPENING IN THE FUTURE.

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.