Air carrier crew reported descending below an ATC assigned altitude while maneuvering prior to the non-precision published approach course; in mountainous terrain at night. ATC reported a low altitude alert to the crew and the crew climbed to the assigned altitude; then continued the approach to a safe landing.
Synopsis
Air carrier crew reported descending below an ATC assigned altitude while maneuvering prior to the non-precision published approach course; in mountainous terrain at night. ATC reported a low altitude alert to the crew and the crew climbed to the assigned altitude; then continued the approach to a safe landing.
Narrative
During descent into BZN we were communicating with Big Sky Approach. We were cleared direct to JOXIT intersection which was the IAF on the RNAV RNP Runway 30 Approach. We were instructed to cross JOXIT at or above 13000 ft. and cleared for the RNAV RNP 30 approach. We were in LNAV/VNAV PATH and I selected 11300 ft. which was the next lowest altitude after JOXIT on the RNAV RNP approach. I thought that I had set the crossing restriction in the FMC to 13000 ft. at JOXIT. Unfortunately; I failed to do so and the aircraft while following the descent guidance has us crossing JOXIT intersection at around 11800 ft. When we were at approximately 12300 ft; Big Sky Approach gave us a low altitude alert and told us the minimum vectoring altitude was 13000 and to climb immediately to 13000. I immediately disconnected the autopilot and climbed back up to 13000 ft. Once reestablished at 13000 ft. I reengaged the autopilot and we continued on the RNP approach to Runway 30. We landed in BZN without further incident.Suggestion: We were expecting a different runway at BZN and the change caused me to rush and miss making sure that I had manually entered a hard altitude at the IAF. I failed to verify when I entered the lower altitude without checking that the higher altitude was set correctly in the FMC. There isn't an altitude depicted on the Jeppesen chart; so the only way to trap the error is to make sure that the altitude is entered manually in the FMC. Due to the terrain in the area; it might be helpful to Crews to place that as the crossing restriction on the chart so that the FMC can account for this altitude automatically instead of having the Crew have to manually enter the altitude during a high workload time of the flight.
Second reporter narrative
When cleared for the approach; was given a crossing restriction at or above; to begin the approach. Aircraft began the descent mode early. As pilot monitoring I recognize that we were descending too early. As pilot monitoring I questioned why we were descending early. The Captain began to make adjustments to the descent and correct the deviation. I then queried ATC as to our original approach clearance; so we had a better understanding; and to clear up any miscommunication they instructed us to climb to meet the restriction we were given. Approach continued uneventful; and a normal stable approach.Suggestion: Make sure Approach clearance is understood completely and FMS is programmed correctly to safeguard any deviations
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.