Air carrier flight crew reported they erroneously thought they were cleared for a visual approach and descended below the minimum vectoring altitude.
Synopsis
Air carrier flight crew reported they erroneously thought they were cleared for a visual approach and descended below the minimum vectoring altitude.
Narrative
We were on the approach phase with the controller as we were tracking the RNAV for Runway XR and maintaining 8000 ft. Approach then asked us if we had the runway in sight and confusion began at that point. I stated affirmative and we both thought we heard we were cleared for the visual approach. As it was during the night; the pilot flying (PF) stayed on the RNAV profile; set the bottom altitude and selected APPCH mode. At approximately 7600 ft. the controller stated a low altitude alert and instructed us to climb to 8000 ft. We then got vectored around to re-attempt the approach.As we were deviating for some weather; Center had determined that we needed to set up for Runway Y instead. As we were below 18000 ft. I changed the arrival information on the Multipurpose Control Display Unit (MCDU); checked the waypoints and re-briefed the approach. Once we got handed over to Approach Control; they gave us the option to choose a landing runway so we chose XR as it was favoring the winds. We were told to expect it. So I re-loaded the MCDU to the RNAV X Runway XR. At this point we were closer to the airport than we thought and we felt overworked/saturated by all the quick approach and runway changes. It is possible we misinterpreted ATC's clearance while dealing in the high-workload environment. Make time; slow down and ask for delayed vectors if needed. If there is any doubt just ask your crew member to verify with ATC the proper instructions/clearance. All while keeping good crew coordination and communication which we had in this flight.
Second reporter narrative
On arrival with Approach Control we were tracking XR RNAV Approach and maintaining 8000 ft. Approach asked us if we had the runway; that's when the confusion started; my First Officer (FO) stated yes and we both thought we heard cleared for the visual. As it was night we stayed with the RNAV approach set the bottom altitude and selected APP. Approximately 7700 to 7600 ft. the controller gave us a low altitude alert and told us to climb back to 8000 ft. At that point he vectored us around to shoot the approach a second time without incident.Center changed our runway to Runway Y which didn't favor the crosswind; my First Officer loaded the new approach we checked the waypoints and quickly briefed the plate. As we switched to Approach Control we were asked what runway we preferred XR was our response we were told to expect it. Once again my FO loaded the approach checked the way points and since we had just briefed it we felt comfortable with the approach. We both felt over worked at this point; we were 2 hours behind schedule and the frequency was busy; thunderstorms and turbulence were also affecting our concentration. It is possible that we may have misinterpeted his clearance. Slow down; take a breath; if you have a question in your mind it's probably your spidey sense trying get you to listen. Get a clarification and wait for confirmation; ask for a vector to slow things down and good crew communication is paramount which we had.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.