Date: 201905
Local time of day: 1801-2400
Location: ZDV.ARTCC, CO
Light: Night
Aircraft: S-70/UH-60 Blackhawk/Seahawk/Pavehawk/Knighthawk
Operating under: Part 91
Flight plan: IFR
Flight phase: Cruise
Airspace: Class E ZDV
ATC / advisory: Center ZDV
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion; Situational Awareness
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Airspace Violation All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: I had just arrived to take over to begin the mid shift. I was receiving the briefing when I noticed Aircraft X in PUB Approach airspace at 8;000 ft MSL on what appeared to be an IFR flight plan but not handed off nor handing off to our sector. I questioned the departing Controller about the aircraft but he had no information. I assumed control of the sector and after issuing a vector to avoid a (in my opinion) more imminent situation further to the east; I called Colorado Springs Approach.COS confirmed the aircraft was IFR heading to PUB then back to ZZZ; but there was some uncertainty as to the hand-off status and to what the aircraft was doing as it appeared to be headed towards the downwind for 8R at PUB without a clearance to do so. Also; it is unclear why an IFR aircraft had been allowed to leave COS airspace without coordination with my sector. The aircraft was potentially NORDO or had been at one point. I eventually said 'radar contact' as the aircraft was in close proximity to restricted area R2601 and I hoped to talk to the aircraft and regain control of the situation. After a minute or two I called COS again; and was informed the aircraft had gone NORDO again briefly; but was coming to me. The aircraft checked in while I was on the land line. At that point the aircraft seemed to be headed towards R2601. The aircraft stated it was circling to avoid some traffic at PUB I believe; the communications were not always clear with this pilot.At this point; in an attempt to regain control of the situation; as I was unclear on what COS had cleared the aircraft to do; and what the aircraft was actually doing; I issued a turn to PUB and holding instructions. The aircraft was at 8;000 ft MSL; and in an area where the MIA (Minimum IFR Altitude) is 8;300 ft. After the read back I climbed the aircraft to 9;000 ft for terrain.In retrospect I should probably have climbed the aircraft earlier; but I chose to regain control of the aircraft's route of flight first.After the routing and climb; I issued NOTAMS and verified receipt of weather; cleared the aircraft for an approach to PUB. In the end the aircraft flew in the wrong direction for the initial approach fix; and when questioned stated they were having equipment issues and subsequently canceled IFR and returned VFR flight following to ZZZ without every descending into PUB.I give permission to share this report and any associated information with any appropriate parties; both within and without my facility.1) COS Approach must not allow an IFR aircraft into another facility's airspace without a hand off. NORDO or not; I noticed the aircraft already a few miles into my airspace during a position relief briefing. 2) I should have required COS Approach to climb the aircraft prior to transferring communications to me.




Synopsis: Denver Center Controller reported an aircraft was NORDO and below the MIA; aircraft was having equipment problems and then returned to departure airport.