Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 146
Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
We were assigned the RNAV (GPS) Runway 18 into Corpus Christi. ATC gave us direct to CAVLA intersection; told us to cross CAVLA at 2100 ft and cleared us for the approach. The Pilot Flying set 1700 ft into the altitude window and verified that LNAV and VNAV were active. As we descended towards CAVLA; the Pilot Flying entered a speed at CAVLA of 200 knots. It took a long while for VNAV to compute the new input. As we were about to cross CAVLA; VNAV disconnected. I pointed this out to the Pilot Flying and we dipped slightly under 2100 ft. He reestablished the aircraft at 2100 ft and then set the altitude selector to zeros and started a Vertical Speed of around 1000 ft fpm descent. The next fix on the approach was PEERS and the charted altitude is 1700 ft. Although I knew he had already set zero in the altitude window; when I looked at the ND (Navigation Display) I noticed that the range to altitude line was right at PEERS intersection. Forgetting for a moment that he had set zeros; I thought that we were on profile to cross PEERS at 1700 ft; but we were actually on a glidepath that was putting us well below the correct altitude. When I figured this out I mentioned to the Pilot Flying that we should set 1700 ft in the altitude selector as we were no longer in VNAV; but by that time we were already flying below 1700 ft. We were around 1400 ft when the Pilot Flying corrected the altitude and climbed back up to 1700 ft. We were in VMC conditions with the airport in sight. The flight landed without further incident.
Air carrier flight crew descended below cleared altitude while on an RNAV approach to CRP.