Date: 201912
Local time of day: 1801-2400
Location: OAK.Airport, CA
Flight conditions: IMC
Light: Night
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 135
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C OAK
ATC / advisory: Tower OAK
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Confusion; Workload; Physiological - Other; Situational Awareness
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Aircraft Terrain Warning
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Chart Or Publication; Human Factors
Primary problem: Human Factors

Narrative: During the initial leg of the RNAV Y 28L into OAK; we were descending between JUPAP and HEDER - MDA is 2;800 MSL- and heard one 'terrain' audio warning from the GPWS; with no other visual or aural warnings. I was the pilot flying; and I immediately disconnected the autopilot in anticipation of a GPWS-generated climb. As I did so; there were no other warnings so I leveled off about 200 feet high on that segment; and stayed on course until HEDER; at which point I started a descent to the next altitude of 1;900 feet as published. I remained in control of the airplane with the autopilot off; as I was somewhat spooked about the GPWS and we were still in IMC. As a result; we ended up somewhat high on the descent; and I descended somewhat faster than normal - probably 1500 FPM.Because of the closely spaced step-downs; we were still somewhat high inside the FAF and descending in a rush to get to MDA. In all this; and due to letting myself get distracted on the approach it now looks like I descended below the LNAV step-down altitude of 1;080 feet at TENIE; which I had actually briefed before but apparently ignored at that moment. My intent was to be at the 1;000 feet altitude by about 3 miles from the MAP; but I allowed the descent to stay steep and ended up going below the MDA. Tower called and advised of low altitude and confirmed altimeter setting. The Controller advised us to be ready for a possible go-around; but didn't call for one. We leveled off at about 500 feet. I think; but by the time we had passed the 1;080 feet step-down we had the airport in sight and then continued to landing with no further issues. My first mistake was not adequately briefing the step-downs to include the 1;080 feet segment at TENIE; especially in light of the fact that the approach is pretty tight and there's not room to wait on descents. The First Officer and I should have really reviewed not only the step-downs; but also the expected rates of descent to meet them. My next mistake was to accept an unstable approach while still out on the intermediate step-down. Even a couple of hundred feet of altitude makes the next segments difficult to meet; as I look at the chart now not under the pressure of the moment. A go-around; although not critical at that moment; should have been nearing the top of my list of alternatives after having difficulty getting down to the next MDA. I think that the self-induced distraction of stress about the GPWS aural warning also contributed to this event; in that my mind was already occupied with duties of training the new First Officer with me; and this was the last leg of a fairly long day. I am not normally prone to fatigue; but the extra elements of acting as a training captain and also monitoring the First Officer's performance probably contributed to task-saturation. Lastly; although I really appreciate that ATC was monitoring the approach; when he announced the low altitude alert; there was a few seconds of disconnect between me and the First Officer as he answered the call and responded with the altimeter setting number. In that critical time; it would have helped for ATC (OAK Tower) to also announce the MDA. I was in a fairly high rate of descent at that point (about 1000 feet/min?); so I wasn't sure if the cause of the altitude alert was simply a rate of descent warning or if we were really low. This is not a criticism of ATC; since it was ultimately my responsibility to ensure the safety of the flight; but it did add to the confusion of the moment. Going forward; I will pay greater attention to the briefing of the approach and especially the segment lengths to consider. Ultimately; I could also have elected to conduct an ILS to Runway 30; which would have been easier. This was an eye-opener for me.




Synopsis: Air carrier Captain reported an altitude deviation during approach that resulted in a low altitude alert followed by a go-around.