Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance
Human Factors
Aircraft X asked for an RNAV approach; I cleared him over the initial fix. The aircraft deviated right of course and began descending below the MIA as well as the altitudes published on the approach. I made several attempts to contact the aircraft; including a low altitude alert. I then asked Aircraft Y to switch to a UNICOM to see if Aircraft X was over there. He was; and he came back to me and informed me that he had intended on cancelling IFR and was proceeding visually to the field. I told him he needs to tell me what he's doing prior to doing it; and to not switch frequencies without ATC approval.
ZLC Center Controller reported that an aircraft changed radio frequency as well as deviated from the approach course and approach altitude before being given ATC approval.