Date: 201807
Local time of day: 0601-1200
Location: BUR.Airport, CA
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach; Initial Approach
Airspace: Class C BUR
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Human-Machine Interface
Second reporter function: Captain; Pilot Not Flying
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was Pilot Flying (PF); autopilot was engaged; weather was exceptionally clear and radio traffic was pretty light. I briefed the approach RNAV (RNP) Y Runway 8 prior to descent. During the STAR; we were given a few altitude and airspeed restrictions and (maybe) a vector before being cleared direct to WABBT and cleared the RNAV (RNP) Y Runway 8 into BUR. After getting approach clearance; I went through my normal routine; pressed the VNAV button; and verbalized that I was setting 3;000 feet in the altitude window. I then proceeded to monitor the approach and spent a lot of time looking outside for traffic and looking for visual clues to find the runway. After passing WABBT; I checked for the next altitude crossing restriction (5;100 feet at NIBYI) and noticed that we were still descending below that altitude. I also noticed the terrain (3;741 feet) would be a factor if we continued descending. After assessing the situation; I realized the autopilot was still in Level Change with 210 knots selected. I clicked off the autopilot and shallowed the descent rate to ensure terrain clearance and very shortly after; the SoCal Controller advised us of our altitude deviation and then the low altitude alert. The Pilot Monitoring (PM) informed the controller that we had visual contact with the terrain and the runway and requested the visual approach. The rest of the flight was uneventful. We did not receive an EGPWS alert and; because we had visual contact with the terrain; we never felt terrain separation was a factor. I verbalized but we both did NOT verify and therefore we later had to intervene. I pushed the VNAV button but did not verify that VNAV PATH was engaged and that mistake led to our deviation. We could have (and should have) prevented this incident using better crew coordination and adhering to standard operating practices.


Second narrative: We were cleared the RNP Y Runway 8 into BUR; to start the approach from WABBT. I was looking outside at the airport and looking out for general aviation traffic when I realized that we appeared lower than normal. At about that same time; SoCal called us with a low altitude alert. SoCal called several times (three I think); with more concern in his voice each time. I saw we were in Level Change and the altitude alerter was set at 3000 ft. VNAV was not engaged. The First Officer reduced the descent. I informed SoCal that we had the airport and terrain in sight. We were then cleared the visual approach. The landing in BUR was uneventful. We had no GPWS alerts on any portion of the flight.


Synopsis: B737-700 flight crew reported descending below charted altitude on the RNAV (RNP) Y arrival to Runway 8 at BUR. Lack of FMC mode awareness was cited as contributing.