Date: 202106
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: Challenger Jet Undifferentiated or Other Model
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ1
ATC / advisory: Center ZZZ1
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Time Pressure
Second reporter function: Handoff / Assist; Enroute
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Air Traffic Control; Person Air Traffic Control
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Environment - Non Weather Related; Human Factors; Procedure; Airspace Structure
Primary problem: Airspace Structure

Narrative: I was working a Radar Sector with a Radar Associate controller. I was informed prior to the event that I would be very busy and over numbers for the sector. I was working multiple; 8 plus IFR arrivals into ZZZ using the ILS approach. The volume and complexity of arrivals/departures/and overflights in the sector was high. After an initial descent; I had cleared Aircraft X to cross ZZZ1 at or above 14;000 ft. and cleared him for the ILS approach. I inadvertently issued this clearance in a 14;600 ft. MIA (Minimum IFR Altitude) area. Once my Associate Controller alerted me to the MSAW (Minimum Safe Altitude Warning) alert; the aircraft was already exiting the 14;600 ft. MIA; and as a result I did not issue a low altitude safety alert. The aircraft continued on the approach. ZZZ has shown a pattern of having very high peak demand in respect to arrivals. These spikes in demand are usually left up to the Radar Controller to take care of with little outside assistance from the Center TMU (Traffic Management Unit). This has resulted in numerous errors and other safety events as the Radar Controller is saturated. This high workload is also due in part to multiple transmitters for overflights and ZZZ arrivals and high terrain around ZZZ. I would suggest that preemptive procedures be developed for times that ZZZ is experiencing peak demand. Another possible long-term solution would be the stratification of Sectors into a high/low configuration so as to reduce the added complexity of overflight traffic. Frankly; with the volume ZZZ sees on a daily basis; especially in peak seasons; I am surprised it does not have an Approach Control.


Second narrative: I was working the Assistant Controller position dealing with communication with Tower and point outs with adjacent sectors. Aircraft X was level at 15;000 ft. above a 14;600 ft. MIA (Minimum IFR Altitude) area when the Radar Controller issued a crossing restriction and clearance to Aircraft X to cross the ZZZ VOR at or above 14;000 ft. and cleared the aircraft for the approach. The aircraft descended below the MIA to 14;000 ft. before the Radar Controller realized the low altitude. No low altitude alert was issued. Spacing from adjacent sectors for ZZZ arrivals. And rerouting aircraft not landing into the sector around the airspace assisting in lower work load on the Radar Controller. The job was doable.


Synopsis: Center Controllers reported the Radar Controller issued a descent clearance to an altitude below the Minimum IFR Altitude and did not notice it until the MSAW activated.