Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 11
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Procedure; Human Factors; ATC Equipment / Nav Facility / Buildings; Airspace Structure
This is a very large sector that when worked on its own has 6 different VHF frequencies spread apart for coverage. In between these frequencies is a fairly large gap where aircraft below about 7;000 feet may have a brief period of no communication with ATC depending on how the Controller handles the frequency change. I received a handoff from Approach Control on Aircraft X which was level at 4;000 feet. An experienced Controller's first thoughts would be that this aircraft could not only go NORDO (No Radio) but also that the Minimum IFR Altitude (MIA) gradually increases to 5;100 feet to the western part of this sector. These thoughts came to my mind; and to be proactive; I issued a climb to 5;000 or 6;000 feet (don't recall which). Immediately after checking in; I realized this aircraft wouldn't need to be at 6;000 feet for a while and by then I'd be in communication with this aircraft on another frequency.Aircraft X requested to remain at 4;000 feet for better winds and flight conditions. I saw where the aircraft was landing; which was a few miles west of my sector; and measured from there to where I'd need the aircraft to be level at 5;000 feet. I instructed the aircraft to cross 120 miles east of the destination airport level at 5;000 feet and also to eventually check in on a different frequency when I was no longer coming in on the current frequency. Approximately 10 to 15 minutes later; MSAW (Minimum Safe Altitude Warning) was going off on this flight and flashing 4200 feet. It took a second for me to realize I made a mistake and overlooked a section of the MIA chart that was 4;200 feet. When MSAW flashed; the aircraft was beyond my frequency reach and we were unable to communicate.The aircraft was also just barely clipping the corner of this particular MIA section and then it was back to at or below 4;000 feet. Instead of trying to relay through an aircraft; I allowed the aircraft to fly through this corner since it was a 200 feet. difference we were talking about and it was such a brief time period. I was confident with my knowledge that although not ideal; this aircraft wasn't in danger. The Aircraft went on to meet my previous requirements of crossing at 5;000 feet at 120 miles from destination and checked in the next frequency and no further issues arose.I frequently skip a frequency that would likely work in between the check in frequency and the frequency at the western part of the sector. It's never been an issue because we very seldom allow an aircraft to make it that far west at 4;000 feet to begin with. I was confident I had been proactive and covered my bases when indeed I had not. I used a type of crossing restriction; at least for this purpose; that I never use and one that I had never used prior and I overlooked an important section of the MIA. I need to start using this intermediate frequency even if for a short time period. Pay more attention to detail when issuing a restriction of this kind. This was completely my fault.
ZKC Center Controller reported an aircraft flew below a Minimum IFR Altitude that the Controller overlooked.