B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP)
Workload; Communication Breakdown; Situational Awareness; Fatigue
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Procedure; Human Factors
It was the fourth of five legs on the last day of our trip. We were both a little tired and looking forward to getting home as we descended via the EAGLZ 1 into ONT. The airspace was quiet and SoCal clearly wanted to help us expedite but ended up using far more words than were necessary as he mentioned a Visual Approach more than once. We were told to maintain 8000 ft and proceed direct to either TAKOE or PETIS to acquire the airport for the visual. We didn't have either point in our route and the FO (PF) selected the next point instead. Seeing the error; I went heads down to type in TAKOE.  At the same time the FO thought we were cleared for the approach and changed the altitude in the AFCS and started a descent. I did not hear him announce this. ATC queried us as we passed 7300 ft and told us to climb to 7700 ft. We were subsequently cleared for the visual and landed uneventfully on 26R. We clearly had a breakdown in [altitude awareness] and it was completely our fault. Contributing factors were slight fatigue; expectation bias as ATC mentioned a visual approach multiple times and the feeling of being rushed because of our altitude and proximity to the runway.Communication is the key. We both are very familiar with this airport but neither had flown the EAGLZ 1. A more thorough arrival briefing would have highlighted the fact the arrival does not connect to the runway points. When ATC mentioned the visual so many more times than normal we should have recognized and announced we were in the yellow as we continued to get closer to the airport and above all; we must always announce changes to the [autoflight system] and make sure both pilots are on the same page.
As the PF; I should have delayed any descent from our last assigned altitude until I could verify the clearance with my Captain. I allowed myself to feel I needed to rush into a steep descent while in proximity to terrain without allowing the Captain to finish programming our route so his attention could be fully on monitoring our flight path. I allowed an ATC routing instruction to distract me from properly verifying our vertical clearance. There did not seem to be hardly any other air traffic nearby; so a request for vectors to manage the descent would have been easy to get.
B737 flight crew reported an ATC low altitude alert due to an altitude deviation on arrival into ONT. Fatigue and workload were cited as contributing factors.