Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2
Situational Awareness; Distraction; Human-Machine Interface
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Human Factors
Aircraft X checked on at 160 for visual approach. I instructed the pilot to maintain 080 pilot readback 050 and for unknown reason [I] missed the readback. At this time; the pilot decided to descend abnormally fast and at some point during their descent; Tech Operations was on the phone with someone else and was asking for another radio check to test [the frequency] and I had looked away to inform them the standbys had also stopped working. As they asked for the radio check and I was turning back toward the scope; I began asking the aircraft for a radio check and that is when I saw their altitude was around 075. I second guessed whether or not I had cleared them on a visual approach as I was asking for a radio check and decided I had not. I thought by that point they were still above the next MVA so [I] decided to clear them on the visual rather than have them climb again. After they were cleared on the visual and switched radios; they continued to descend to 050 by which time myself and the Supervisor had given multiple low altitude alerts on all Tower and Radar frequencies; as well as Guard. The aircraft landed without further incident. I would recommend that controllers watch aircraft level out at the altitude they are instructed to be at. Also; when an aircraft is in a rapid descent; to reiterate what altitude you need them at. Also I would issue the low altitude alert and climb them before asking if they had the airport in sight.
TRACON Controller reported missing an incorrect readback.