Date: 202012
Local time of day: 1801-2400
Location: ZDV.ARTCC, CO
Flight conditions: VMC
Light: Night
Aircraft: Small Transport; Low Wing; 2 Turbojet Eng
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZDV
ATC / advisory: Center ZDV
Second aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Aircraft component: Landing Gear
Component problem: Malfunctioning
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Time Pressure; Workload
Anomaly: Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Aircraft; Airspace Structure; Human Factors; Procedure
Primary problem: Human Factors

Narrative: Aircraft X took off RIL climbing to 15;000 feet; at around 10;000 feet; Aircraft X informed me he was having a landing gear issue; could not climb further and needed to return to RIL right away. I sent him direct to RIL. He had good sight of the terrain and once he got the visual he was cleared the visual approach. Aircraft Y was another aircraft going into RIL north of the airport level at 14;000 feet with no approach clearance. Aircraft X appeared to be circling to the runway so I did not clear Aircraft Y into the airport. Aircraft X then popped back on the frequency saying they cannot land at RIL and need to go to [another airport]. Aircraft X at this point was 8;000 feet climbing eastbound still in priority handling status. I re-radar identified Aircraft X and had them climb to 13;000 feet to try to maintain positive Minimum IFR Altitudes. They affirmed they had good sight of the terrain. I called the traffic to both Aircraft Y and Aircraft X hoping to apply visual separation. Neither saw one another so they both remained 13;000 feet (Aircraft X) and 14;000 feet (Aircraft Y).Once able; I vectored Aircraft X south to get him away from high Minimum IFR Altitudes and the RIL approach corridor. I asked Aircraft Y 'if I turn you direct to RIL; do you think you can get a visual with the airport?' Aircraft Y responded he could. I then said 'Aircraft Y; cleared direct RIL' I then went back to Aircraft X and asked fuel remaining; souls on board; what altitude they could climb to; and then explained why 15;000 feet was not feasible. I offered GJT etc. While discussing options with Aircraft X; I noticed Aircraft Y leave 14;000 feet and begin their descent while still 3 to 4 miles from Aircraft X. I gave a traffic alert to Aircraft Y and told them to climb back to 14;000 feet immediately. I then gave the traffic alert to Aircraft X. Aircraft Y climbed back up; I gained the 5 miles and climbed Aircraft X up and gave Aircraft Y the visual approach to RIL. I issued Aircraft Y advisories.In this session I had two separate situations. A pilot deviation; an arrival rush; a handful of ornery pilots on the ground at RIL clogging the frequency looking for releases off the ground. Had RIL sector been separated out through our split frequency procedures; this may not have occurred. There was a lot of frequency congestion between the sector's two frequencies which blocked transmissions to the priority handling aircraft. We used to do this procedure and have been recently barred from doing it again. This procedure could have helped!




Synopsis: ZDV Center Controller reported an aircraft that had deviated from their assigned altitude and descended towards another aircraft that was in a priority handling condition resulting in a loss of separation.