Date: 201904
Local time of day: 1801-2400
Location: BUR.Airport, CA
Flight conditions: Mixed
Light: Night
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E SCT
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Training / Qualification; Distraction; Situational Awareness; Confusion; Communication Breakdown
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were on a flight to BUR; on descent on the JANNY 5 Arrival into BUR. At some point near JANNY intersection we were cleared to descend to 6;000 feet. Since this clearance was understood to be a 'descend to' (now) not 'descend via;' I commanded a new non-VNAV mode which we had been using. I believe the new mode was Level Change/Vertical Descent in FPM. It was not a VNAV descent mode. I verbalized this change. Shortly afterward; the Approach Controller asked if we would prefer the ILS or the RNAV Y to Runway 8 in BUR. We requested the RNAV Y Runway 8. He then cleared us for this approach; direct WABBT. RNAV Y links up with JANNY 5 at WABBT Intersection and we were cleared to this intersection. We selected WABBT direct; closed the discontinuity; reviewed WESKI as the FAP [Final Approach Point] with a corresponding 2;700 feet altitude; agreed and set that altitude into the MCP window. At that point I believe I made an error by not verifying and verbalizing VNAV PATH as the vertical mode. This set up the aircraft to continue a moderate rate; non-idle; descent on autopilot toward 2;700 feet. At about 4;900 feet and approaching WABBT; a GPWS Caution Obstacle Alert was heard. I immediately disengaged the autopilot and the autothrottles and pitched aircraft into a climb. Shortly afterwards; Approach Control asked us to verify altitude. We responded we were climbing to 6;000 feet. After WABBT; RNAV 8 Approach then started a series of stepdown altitudes and arc left turns. I asked my F/O [First Officer] for the next altitude so I could safely begin to descend again. He interpreted this as a verbal altitude whereas I wanted an MCP altitude window setting. I also commanded a direct enter toward the ZEMEP intersection but inadvertently forgot the autopilot was disengaged and would not execute the turn automatically so we briefly overflew this intersection before turning back on course to re-intercept the approach. We re-established the approach; configured; then flew the approach to Runway 8 which became visually in sight several miles out. We were handed off to Tower and made a normal landing at BUR. When we were cleared to 6;000 feet I should have chosen a VNAV mode rather than assuming I would be pulled off further STAR/approach restrictions for the remainder of the flight. I had an expectation bias that we would be receiving vectors but were re-cleared on a procedure that required VNAV. More importantly; I should have verbalized and verified VNAV PATH on procedure requiring its use. This was missed as we turned the discussion to the minimum altitude to set in the MCP window. Lastly; on any segment of the approach a missed approach should have been called and executed even when a long way from the MDA/DDA point of the approach.


Second narrative: During descent on the JANNY 5 Arrival/RNAV Runway 8 Approach; the aircraft descended below hard 6;000 feet MSL altitude. Causing a 'Terrain Caution'. We were cleared to descend to 6;000 feet MSL approximately 8 to 12 miles from waypoint WABBT. The Captain; as PF [Pilot Flying]; entered 6;000 feet in the altitude window and selected Level Change. We were subsequently cleared the RNAV Runway 8 Approach to BUR at WABBT. As it was our next point; the Captain closed the discontinuity in the flight plan and put 2;700 feet in the Alt Window for the FAP on the approach. I agreed with the altitude setting. Apparently; the Captain never selected VNAV; didn't verbalized the descent mode; and I never verified VNAV PATH. I looked out the window and less than a minute later I heard an aural 'OBSTACLE Caution;' looked over and saw a yellow obstacle on my display; told the Captain we are below path and to climb 'NOW!' He disconnected the autopilot; advanced the thrust levers and climbed back to altitude. The obstacle was about eight to ten miles away and more than 1;000 feet below; but we were over 1;000 feet below altitude. ATC called us as we were climbing through 5;700 feet to verify altitude. While maneuvering back to course we were laterally displaced for a few minutes by about a mile and slightly late on the turn to the strait-in portion of the approach. ATC queried if we were in our turn; to which I stated we were. At about the eight-mile final; Approach asked what happened around WABBT with our altitude. I told him we were still trying to figure that out; but probably a programming or automation error. Foremost; we must verify VNAV modes and closely monitor the descent profile; this would have prevented everything that occurred. Also; we should have verbalized the descent mode; then I we would have been more likely to verify and/or catch our error. When I realized that the Captain was not immediately correcting back to course after recovering our altitude; I should have either taken the aircraft; requested vectors to final and/or told the Captain to execute a missed approach. Since we were in VMC conditions and about ten miles from the field and were never really established on the approach; I didn't think about executing a missed approach. If ATC would have given us our clearance earlier; we may not have missed the VNAV PATH problem. Not that it was any fault of ATC; but the sequence and timing of the clearances made the mistake much more likely; by giving the 'descend to' instead of 'descend via' clearance and then clearing us an RNAV approach very close to WABBT. This made the pilot have to edit the flight plan; change altitude window; and change the VNAV profile (if not already descending via) in a very short time period.


Synopsis: B737 flight crew received a Ground Proximity Warning System Alert while on descent to destination airport.