Date: 201603
Local time of day: 1201-1800
Location: NCT.TRACON, CA
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E NCT
ATC / advisory: TRACON NCT
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Situational Awareness; Workload; Time Pressure
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Approaching OAK from the east; we requested the RNAV Z Runway 12 from HIRMO intersection and were told that the request would be forwarded to the Final Controller. Upon check-in with the Final Controller; we mentioned that we were set up for the RNAV Z and were told to expect it. As we neared the HIRMO intersection; and still not cleared for the approach; we queried the Controller to see if we should track the RNAV approach track and were told to do so. It became clear; as we progressed a few more miles past HIRMO without a clearance for the approach; that spacing might not be working out with other aircraft on other approach transitions and others being vectored for the ILS.Then we were vectored off the HIRMO transition of the RNAV Z 12R and were assigned a heading to intercept the final approach course for the RNAV Z (different transition) and also assigned an altitude.I read back heading 250 descend and maintain 2500 feet. Shortly after the descent; we were told that our last assigned altitude was 3100 feet. We were in VMC conditions and there were no threats indicated on the terrain display. We climbed back to 3100 feet and then were subsequently cleared for the approach and landed. Looking back at the clearance having a heading of 250 and my reading back the altitude of 2500 feet; I might have mistakenly read back 2500 feet when that altitude was not issued.I believe that I felt rushed when we were vectored off of the planned approach near the initial fix. Knowing that we were being vectored to a different transition requiring reprogramming of the FMC; I might have mistakenly prioritized making the necessary changes in the FMC for the PF (Pilot Flying) ahead of the more important task at hand which was to make sure that the heading and altitude were correctly acknowledged. I remember thinking that 2500 feet seemed like it was assigned a few miles early; but the VFR conditions at the field and no terrain threats ahead kept me from querying the clearance as we quickly discussed the changes to the approach. Also; requesting the RNAV RNP approaches while ILS approaches are in use increases Controller workload and can complicate sequencing. This can lead to the RNAV approach clearance being obtained late or denied late in the arrival. I have learned that unless the RNAV Z is offered; that sticking with the ILS involves much less chance for last minute changes and resultant errors associated with being put 'in the Yellow' late in the approach environment.


Second narrative: I was the PF (Pilot Flying) as we descended out of the clouds. We were cleared the RNAV Z 12 OAK Approach but maintain 3100 feet. Passing HIRMO we were then vectored heading 250 and the Captain heard descend to 2500 feet. I turned the aircraft to heading 250 and the Captain said we were supposed to descend to 2500 feet. He read back; '[call sign]; turn left heading 250 and descend to 2500 feet. The Controller never corrected the radio call but when we were passing 2700 feet he said; 'Altitude Alert; maintain 3100 feet.' Our Captain said; 'ok we were cleared to 2500 feet but will climb back to 3100'. No further comment about this from ATC.Don't assume anything. I wasn't 100 percent sure about the new altitude assignment but was able to justify it as the vector would take us closer to the 1800 foot FAF altitude that should be assigned soon. The terrain looked clear and when the Captain read it back without a correction from ATC; I assumed it was right and I must have misheard it. Next time I should still say; let's confirm the altitude.


Synopsis: B737 flight crew reported they were cleared to 3100 ft on an approach to OAK; but misunderstood the clearance and descended to 2500 ft; prompting a low altitude call from ATC.