Controller and Pilot reported descent below MVA which resulted in MSAW alert and CFIT incident.
Synopsis
Controller and Pilot reported descent below MVA which resulted in MSAW alert and CFIT incident.
Narrative
Aircraft X was enroute to ZZZ; a satellite airport of ZZZ1 airspace. When Aircraft X entered the airspace; they were direct to ZZZ airport descending to 150. Inbound to ZZZ1 at the same time was Aircraft Y and Aircraft Z. Two of three runways at ZZZ1 were closed meaning all traffic at ZZZ1 was using the same runway; Runway XX. Aircraft A was doing survey work 8 miles south of ZZZ and 10 miles west of ZZZ1. First control instruction to Aircraft X was to issue a turn to heading 070; and a descent to 9 thousand. Aircraft X readback this instruction properly. Aircraft Z was well ahead of Aircraft X cleared for a visual approach to ZZZ1. Aircraft Y was being vectored for a visual approach based on needed spacing behind Aircraft Z. To ensure separation between Aircraft X and Aircraft Y; I turned Aircraft X back to the northeast on a 040 heading. I did not change his altitude assignment. At this point Aircraft X was around 115 still descending to 090. I attempted to issue a new heading for Aircraft Y and they did not respond to my instruction. I then realized I needed to cancel Aircraft Z's approach due to compression with a VFR Aircraft B ahead of Aircraft Z on final. The overall traffic situation was complex enough that I did not feel I had time to coordinate with ZZZ1 Controller for a plan between the Aircraft B and Aircraft Z; so I just turned out the aircraft of which I still had control. I attempted multiple times to reach Aircraft Y but they stopped responding to instructions. With Aircraft Y NORDO; I started issuing more instructions to Aircraft X; Aircraft Z; and Aircraft A to get them clear of the flight path of Aircraft Y in case Aircraft Y turned or descended. Aircraft A was a VFR survey aircraft working near the finals of both ZZZ and ZZZ1. All of my previous instructions had been based on avoiding Aircraft A while providing for a stabilized approach. My Supervisor was able to reach Aircraft Y on guard frequency XXX.X and I was able to resume communications with Aircraft Y. I vectored Aircraft Z away from Aircraft A and to follow Aircraft Y; who was now cleared for a visual at ZZZ1. I then turned Aircraft X back towards ZZZ and started to point out ZZZ to Aircraft X so as to clear Aircraft X for the visual. My Supervisor pointed out that Aircraft X had descended below the MVA. I am not sure at which point Aircraft X first descended below the assigned 090; but I had never issued a lower altitude. Admittedly I was more focused on the situation regarding Aircraft Y; Aircraft Z; and Aircraft A. I issued a low altitude alert to Aircraft X and climbed them back to 9 thousand. Lowest point on Aircraft X deviated altitude was 071 in an 082 MVA. The three aircraft that were attempting to land got visual approaches and landed without incident. Aircraft A was able to resume their operation shortly afterwards. A brasher warning for Aircraft X was issued. This was a conflagration of multiple small issues leading to me missing Aircraft X's mistake. The fact all traffic at ZZZ1 was using the same runway despite aircraft characteristics added complexity and risk. The wind condition necessitating that ZZZ was active a runway perpendicular to the active runway at ZZZ1 added complexity and risk. Aircraft A's aerial survey work near the finals for ZZZ and ZZZ1 added complexity and risk. That Aircraft Y responded to two radio transmissions and then stopped responding to instructions; adding complexity and risk. Finally that Aircraft X might misremember being assigned 7 thousand when I had assigned him a 070 heading; was the pilot error that caused the unsafe altitude. Ultimately this chain of events would be very unlikely to ever reoccur. If something similar did occur the only things I could do to improve my performance is an increased vigilance. I could have reissued the 9 thousand altitude at various times during this sequence. I chose not to do so as I did not want to risk Aircraft X mistakenly believing they could descend below9 thousand. The fact that they did ultimately make that mistake does not change my mind that constantly reissuing the same altitude just adds frequency congestion and increases the chance of misunderstanding.
Second reporter narrative
ZZZ Approach gave us a descent into ZZZ and wanted us to do a box approach requiring several heading changes. I thought that I heard a descend to 7000 ft. so I set and was descending to that altitude. Approach did not question me about my altitude until I had leveled off at 7000. I had the airport in sight from a long way out and let Approach know that. After a heading assignment to the west it was evident that our altitude was too low for terrain a few miles in front of us which we had in sight the entire time. As I was about to call and question the altitude assigned; Approach called and asked me to climb to 9000 ft. which we complied with immediately. At no time were we in harm's way in terms of terrain or other traffic as we were visual and the airport was in sight. At their request I called and spoke to Person A at ZZZ TRACON after landing who was very pleasant and respectful. I'm not sure why I heard 7000. Possibly due to having the airport in sight and thinking that I would be cleared lower? Expectation bias? ZZZ did not seem busy. There was an Aircraft Y flight that had gone lost comms for a few minutes which likely also distracted the Controller.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.