What happened
During an arrival procedure, the flight crew was instructed by air traffic control to transition from the POLLI FOUR PAPA arrival to the POLLI FOUR BRAVO arrival, which required a VOR approach. While the crew accepted this new arrival, they continued preparing for an RNAV-Z approach instead of the mandated VOR procedure. This discrepancy created a mismatch between the programmed flight path and the actual arrival instructions.
To bridge this gap, the pilot manually entered the waypoint SCBSG into the Flight Management Computer (FMC) rather than selecting the specific approach procedure. This manual entry resulted in the omission of a 7,5/00 ft altitude constraint within the FMC's programmed path. Unaware of this omission, the crew set the autopilot altitude selector to 3,900 ft. Consequently, the aircraft descended through the 7,500 ft Minimum Sector Altitude (MSA) before reaching the waypoint. Although an approach controller identified the error and alerted the crew, the aircraft subsequently descended below the 5,300 ft Segment Minimum Safe Altitude (SMSA) near waypoint SCBSI after the crew transitioned to a manual visual approach without notifying air traffic control.
The investigation
The investigation focused on the discrepancy between the flight crew's programmed flight path and the ATC instructions. Investigators examined the process of waypoint entry in the FMC and how the altitude constraint was lost. The investigation also reviewed the crew's transition from instrument procedures to a visual approach and the communication protocols used with air traffic control during that phase of flight.