Light Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Developmental
Situational Awareness; Training / Qualification
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Airspace Structure; Human Factors
Aircraft X entered my airspace at 11;000 feet and 210 knots on a STAR. He was requesting a change to a parallel runway. I told the aircraft [to] expect the assigned runway due to straight in traffic on their requested runway. Then a moment later I instructed the aircraft to descend and maintain 9;000 feet. The pilot read back the altitude correctly. A few moments later I told the aircraft that I would try to position them so that the Tower may be able to move them over to their requested runway and to make the request with Tower once I issue them a frequency change. After that transmission I noticed the aircraft was descending through 8;600 feet above an 8;500 foot MVA (Minimum Vectoring Altitude). I quickly gave the aircraft a base turn heading 070 degrees - to turn them away from the mountains beside them - and maintain 8;500 feet. The pilot quickly complied but had already descended through the MVA to 8;200 feet. I did not issue a low altitude alert as there were more pressing transmissions to make; the short time of the event and the aircraft's close proximity to airport on a base turn. I then notified the Supervisor and requested the Tower issue a brasher warning after the aircraft had landed. This event was clearly a pilot deviation but I could ensure to improve my scan to catch the aircraft descending through their assigned altitude sooner.
S56 TRACON Controller reported an aircraft on vectors descended below the assigned altitude and below the Minimum Vectoring Altitude.