Air carrier flight crew reported descending below a charted altitude during approach into a non-towered airport in mountainous terrain. ATC issued an altitude alert and flight climbed back to safe altitude.
Synopsis
Air carrier flight crew reported descending below a charted altitude during approach into a non-towered airport in mountainous terrain. ATC issued an altitude alert and flight climbed back to safe altitude.
Narrative
Montrose; Colorado. We briefed our arrival and approach after obtaining the weather in MTJ. We discussed the threats of MTJ being an uncontrolled airport surrounded by high terrain and usually lots of VFR traffic when the weather is nice. We also discussed that the PM would have to make position reports on CTAF. The weather was clear skies; light variable winds and more than 10 mile's visibility. Under those flight conditions; I briefed that I would fly a visual approach.In the decent; the Denver controller asked us what runway we would like to land. The PM replied runway 17. The controller then cleared us direct to MEYRS intersection and instructed us to cross MEYRS at 14;000ft then cleared approach. I programmed the FMS for the crossing instructions at MEYRS and selected the glide slope intersecting altitude of 8100 at WIDRU on the 17 ILS. I managed the decent and verified the FMA (Flight Mode Annunciator). In the decent I began to slow the aircraft below 250 kts in preparation of adding flaps.During the decent; I was spending most of my time looking outside the cockpit for other traffic and terrain. Over MEYERS we already had the field in sight and our plan was to request the visual prior to GEJYU and cancel IFR in the air just in case we couldn't reach Denver on the ground at MTJ.About the time we were going to request the visual approach prior to GEJYU; Denver center came on the radio and announced an altitude alert for us and to climb immediately due to terrain clearance. I immediately looked at my altitude and FMA and noticed that we were approximately 11;700 ft and descending in vertical speed mode at 800 fpm. The course from MEYRS to GEJYU is charted 12;200 ft. I have no idea how the aircraft reverted from a managed decent to a vertical speed decent and not honor the altitude. I was not alarmed because we were well clear of all terrain. I turned off the autopilot and leveled the decent. I then requested the PM to tell the controller that we have visual contact with the MTJ airport and to request the visual approach. The controller cleared us for the visual but gave us a heading to fly around some VFR traffic that just departed MTJ. I slowed my airspeed more and configured with flaps and gear. We kept our aircraft at higher altitude until the other VFR traffic flew past our left wing approximately 3 miles away going the opposite direction. When the traffic was no threat; we continued our visual approach to an uneventful landing. Cause: I was more concerned and distracted about the visual approach and threats outside the aircraft and failed to keep watch on the aircraft automation as it was descending. Suggestions: Stay more focused on the immediate task at hand. In this case; making sure the aircraft was going to level off at the proper altitude.
Second reporter narrative
Before descending into MJT we got the WX and briefed the approach. The CA was flying so it was his brief. The WX was clear VFR and the CA said We will plan ILS 17 and when we get the field in sight we will request the visual and cancel IFR in the air. When we got on with the Denver controller we were asked what runway we were requesting and we told him 17. The controller cleared us direct MYERS and to cross MYERS at 14;000 feet then we were cleared for the approach. The CA set 8;100 on the altitude select for the final approach fix altitude verified 14;000 hard altitude at MYERS and managed descent. At about MYERS we got the field in sight and the plan was to request visual and cancel IFR before flying all the way out to the next fix GEJYU. Just as I was about to request the visual; approach told us they had an altitude alert and for us to climb immediately; that the min altitude between MYERS and GEJYU was 12;200 feet. looked at our altitude and we were 11;700 and descending; the CA disengaged the auto pilot and leveled off the descent. The CA pointed out that the FMA reverted to VS mode but we heard no alert. We were visual and well clear of terrain. The CA then had me notify approach that we have the field in sight and request visual approach and to cancel IFR. Controller cleared us visual but then gave us a heading to fly for a bit to clear VFR traffic coming off the airport. After the traffic was clear he cleared us for the visual and canceled our IFR and cleared us to switch to advisory. The CA flew a visual to 17 and we landed without incident. Cause: WE verified the FMA (Flight Mode Annunciator) when cleared the approach and managed the descent then we both spent time looking out Side for traffic and I should have called for the visual as soon as we got the airfield in sight; I was too slow at calling for it. I also should have split my time better looking in and then back out.Suggestions: be more prompt and clear without intentions with ATC and divide attention more on a visual VFR environment.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.