Small Transport
Air Traffic Control Fully Certified
Workload; Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
Aircraft X was one of 3 aircraft in the airspace; the other 2 were already sequenced onto the ILS for Runway 20R and talking to Tower (which was also me). I descended Aircraft X to 4200 ft.; and for clarity reinforced the altitude by saying 'four point two' the ready back I heard was 'four thousand two hundred' after a short time I turned Aircraft X to intercept the localizer and he was at 4200 ft. I went back to the aircraft on final gave one exiting instructions off the Runway and cleared the other one to land. Then I was going to go clear Aircraft X for the approach; when I noticed he was at 3600 ft. as I was about to issue a clearance and gave him a low altitude alert. When I listened to the tapes I heard a correct read back on the altitude; and I am making an assumption but when I turned the aircraft he did not take the turn nearly as soon as I expected; so it kind of appeared that maybe his autopilot got set to an altitude of 1;600 ft. instead of a heading of 160. But this is just an assumption based on how the plane tracked on the radar. When you are descending an aircraft right to the MVA be very vigilant in your scan that they don't descend below the MVA. I believe that this was a pilot error but maybe I could have caught it sooner.
FAI Controller reported Small Transport descended below MVA.