PA-24 Comanche
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 18
Situational Awareness; Distraction
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Weather; Human Factors
I don't know if anything like weather contributed to the aircraft altitude change. I was working Sector X as the D-side and the R-side said it looks like that guy is descending. MSAW was flashing; I waited an update or two and checked to see if the altimeters had a big change. Before I got to check out the altimeters; I had another call and when I looked back; the aircraft was still descending and below the MIA of 095 MSL. I called Sector Y and told them the aircraft looks like they are descending and released control of the aircraft.The Controller on Sector Y called me back to let me know the aircraft did not know why they could not hold altitude; wanted to reverse course; and that they were in VMC. The Sector Y Controller also let me know that he had [provided priority handling] for the aircraft. Sector Y called me back to let me know the pilot's intentions were to circle around ZZZ1 until the problem was discovered. Before the aircraft got back on my frequency or near ZZZ1; he said that it is good and wanted to continue to ZZZ2. So; Sector Y OK'd the data block back and the aircraft went on to ZZZ2. The pilot did say it might have been a downdraft as heading back westbound he noticed it happen again but was better prepared for it.
I observed Aircraft X descending below 10;000 and verified the altitude. The pilot advised that he was climbing back to 10;000 and stated he didn't know why he was losing altitude. I issued a low altitude alert and to maintain 10;000. Pilot stated he was making a 180; I then advised him of the location of ZZZ1. The pilot wanted to continue eastbound and troubleshoot the aircraft in the vicinity of ZZZ1. I provided priority handling and coordinated with Sector Z. The pilot then stated that he was ready to continue back westbound for ZZZ2.
Center controllers reported an aircraft having trouble maintaining altitude and descending below the MIA.