Sequence of Events
The flight crew had planned to conduct the POLLI FOUR PAPA arrival followed by the RNAV-Z approach. However, air traffic control (ATC) issued instructions for the POLLI FOUR BRAVO arrival and the VOR approach. The flight crew accepted the POLLI FOUR BRAVO arrival but continued preparing for the RNAV-Z approach instead of the associated VOR approach. This mismatch created a discontinuity in the programmed flight path between the arrival and the approach.
Flight Management Computer Input
During the approach setup, the flight crew did not select the entry for the approach in the flight management computer (FMC). Instead, they manually entered the waypoint SCBSG. Because SCBSG was manually entered, the altitude constraint of 7,500 ft was not included in the FMC's programmed flight path. The flight crew did not detect that this altitude constraint was missing.
Altitude Deviation
Prior to commencing the approach, the flight crew set the autopilot altitude selector to 3,900 ft. With the autopilot engaged, the aircraft descended through 7,500 ft before reaching waypoint SCBSG. The flight crew did not notice that the aircraft had descended below the 7,500 ft minimum sector altitude (MSA). The approach controller identified the deviation and alerted the flight crew.
Visual Approach Without Clearance
Once the crew had visual contact with the runway, they elected to conduct a manually flown visual approach. However, they did not advise ATC of this change nor did they receive clearance to discontinue the RNAV-Z approach and perform a visual approach. Subsequently, the aircraft descended below the standard profile, which led to it descending below the 5,300 ft sector minimum safe altitude (SMSA) before passing waypoint SCBSI.