Air carrier crew reported receiving a low altitude alert from ATC while descending below the vertical path on the initial approach in visual conditions to GJT. The Captain stopped the descent then continued on a visual approach to a safe landing.
Synopsis
Air carrier crew reported receiving a low altitude alert from ATC while descending below the vertical path on the initial approach in visual conditions to GJT. The Captain stopped the descent then continued on a visual approach to a safe landing.
Narrative
Prior to reaching CESBA and at an altitude of approximately 10;000 ft. we were given instruction from ATC - 'Cross CESBA at or above 8;500; cleared RNAV 29 approach'. I put 8500 on CESBA in the MCDU as instructed by the PF. The aircraft was in pink VNAV and went into FLCH and descended to 8;000. PF tried to correct the path with 8;500A but it didn't change the path the aircraft wanted to take. We were VMC. PF disconnected the autopilot. Reconfigured the plane on the path at 8;000 and I turned back on the Autopilot. ATC said they had a low altitude alert. I said yes we had descended to 8;000 but were VMC. They cleared us visual 29. We landed without incident. My understanding is that because we were below the original path; the plane logic couldn't honor the 8;500 so descended to 8;000. I think we stopped the descent at 8;000 as the PF pressed ALT at that point but I'm not exactly certain on that. I'm not exactly sure. I think that using a descent mode to descend to the 8;500 and then letting ALT capture at 8;500; and then putting the bottom altitude (FAP altitude) would have kept the aircraft on the path required by ATC.
Second reporter narrative
Given crossing at or above 8;500 at CESBA and cleared for approach. After CESBA aircraft started descending. VMC conditions; airport and terrain in sight. Turned off auto pilot; maintained current altitude and flew visual approach.Aircraft was descending below vertical path to next altitude.Maintain situational awareness and be ready to override automation as necessary.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.