Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 8.0
Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Aircraft; Airspace Structure; Equipment / Tooling
Aircraft X checked on my frequency and requested to change destination. I gave the clearance and took mental note that in about 20 minutes he would need a climb to 13000 feet. After about 20 minutes I issued the clearance to climb to 13000 feet with a new altimeter setting with no response. I tried two other transmitters with no acknowledgement. I then relayed through another close by aircraft who got no response. I then tried Aircraft X on guard frequency and to acknowledge with an ident with no response. I then told my Supervisor because I wanted to make sure there wasn't anything else I could try; and to make him aware of the situation. My supervisor had one more idea which we tried along with trying another relay aircraft and guard frequency again. About this time Aircraft X was heard talking on the frequency and was responding to other aircraft and then to me. He was level at 12000 feet and had flown in a 12500 foot MIA (Minimum IFR Altitude) for about 8 minutes and a 12900 foot MIA for about 2 minutes and then into a 13000 foot MIA before getting the clearance to climb to 13000.I'm not sure if this pilot was disconnected somehow or not; but if this was an radio frequency issue maybe there is something that could be done with that. If not; I don't have any recommendations other than me climbing this aircraft when I noticed the eventual need.
A Center Controller observed a NORDO aircraft fly below the minimum IFR Altitudes along their route.