Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 8
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
Airspace Structure; Human Factors; Procedure
I cleared Aircraft X to 11;000 feet which was the lowest I could clear them. I accidentally said 10;000 feet; but corrected myself and listed to be sure the pilot readback 11;000 feet. They reported weather and that she expected a visual approach and asked for a fix on the RNAV approach to set up for it. I didn't know the fix; but since 11;000 feet was the highest terrain in the area; I told them they could start heading toward the fix while I found it. They turned direct and began descending below 11;000 feet. I first noticed it at 10;500 feet. I asked them to say altitude. They reported 10;500 feet descending to I think 7;200 feet. The fix in question has an altitude of 7;200 feet if you are cleared for the RNAV approach. I gave them a low altitude alert; informed them that they were cleared only to 11;000 feet and told them to climb immediately to 11;000 feet. They said they were starting the climb; but had the field in sight and wanted to cancel IFR. I accepted the cancellation.This was pilot error. Perhaps I should have pursued a pilot deviation.
ZLC Center Controller reported they observed an aircraft descend below the assigned altitude and below the Minimum Vectoring Altitude.