Air Traffic Control Fully Certified
Communication Breakdown; Situational Awareness
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented; Flight Crew Landed As Precaution
Airspace Structure; Aircraft
Aircraft X was IFR when he requested a destination change. I issued the clearance 'Cleared to the ... airport via direct; descend at pilot's discretion; maintain one zero thousand; ... altimeter' The pilot stumbled a bit on his readback and added something at the end that was unreadable; but I figured it was the reason he was diverting; so I asked him to say again the last part. He responded that he had a left alternator issue. I asked if he needed any assistance; and he said 'no'.I then tried to communicate with the pilot about NOTAMs and have him report receipt of weather; but got no response. I later observed the aircraft descend below the MIA (Minimum IFR Altitude); down to 9500 feet. Since he had reported an alternator issue; I could only imagine that he'd lost radio contact and had the field in sight and was commencing approach. He later got back on radio and said he had been off the radio listening to weather. I restated one zero thousand as assigned altitude and he climbed back up. Upon review of the radio tape; the pilot did not read back the assigned altitude when I initially cleared him to his destination.I would question when a pilot doesn't readback an altitude clearance. Even though the only thing technically required of them to read back is a hold short clearance; the language says pilots 'should' readback altitude assignments; and I as a controller can solicit a readback.
A Center Controller observed a diverting aircraft with an alternator problem descend without a clearance to an altitude below the Minimum IFR Altitude.