Date: 201911
Local time of day: 1201-1800
Location: ZSE.ARTCC, WA
Aircraft: S-70/UH-60 Blackhawk/Seahawk/Pavehawk/Knighthawk
Operator: Military
Flight plan: IFR
Mission: Tactical
Flight phase: Cruise
Airspace: Class E ZSE
ATC / advisory: Center ZSE
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Airspace Structure
Primary problem: Aircraft

Narrative: [Aircraft] was filed on an airway. I climbed them to 11000 ft. when they checked in. I lost radar contact with the aircraft which is a common occurrence due to our radar coverage. I told him radar contact lost and to expect identification soon. He didn't respond. I tried him on my other transmitter. I didn't get anything back. It is also fairly common to not be able to hear aircraft at that altitude in the area. I didn't think much of it. Then a few minutes later; I was shipping an airplane who told me they were hearing an [Military] aircraft on guard who said they couldn't maintain altitude and needed vectors to the closest airport. I asked the pilot if the call sign was [Aircraft] and they said it was. I told the aircraft to remain on my frequency and immediately began to try and relay through them. I relayed that I had lost [Aircraft] on radar and said where the nearest airport was reference their last position before I lost radar.I kept trying to relay through various aircraft; assuming [Aircraft] was in an extreme urgency situation. A few minutes later I saw [Aircraft] come back up on my radar below the MIA in the area. I relayed to an airplane where another nearby airport was. I had an airplane relay to me that the aircraft wanted vectors toward their destination. In lieu of radar-identifying the aircraft through a relay and given the situation I made the judgment to believe that the radar hit correlated with the aircraft in question. I relayed to have them fly a heading toward their destination as requested. At this point; it appeared they began to climb again. I relayed to have them maintain an altitude at the MIA; which they were back at. I then tried to talk directly to the aircraft. This worked momentarily; but then I couldn't get them to read anything back. I relayed through another plane that they were radar contact again and to maintain their current heading and altitude. I then relayed a frequency change to Approach. I did not ever ask why they couldn't maintain altitude; nor did I try to relay a safety alert. I believed the pilots were in a very; very stressful situation and knew that they were in an unsafe situation. Resultantly; my goal was to give them whatever assistance I could to get them pointed toward their airport of choice.




Synopsis: Center Controller reported a helicopter in an area of no radar coverage and intermittent radio coverage experienced difficulty and descended below the Minimum IFR Altitude.