Date: 201511
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Night
Aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Ambulance
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Other / Unknown
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Human Factors

Narrative: Aircraft X was at 36000 feet. Frequencies were somewhat congested and a Radar Assist with trainer had just plugged in to assist. Aircraft X experienced what I can only assume was rapid decompression. The first time Aircraft X called; the radio was very scratchy and unreadable; but upon reviewing the tapes he advised of either needing an emergency descent; or that he was performing one. I did not hear this transmission due to other aircraft on frequency and the fact that it was very difficult to read. Aircraft X called at least twice more advising of their intentions; but I still could not discern what aircraft was calling me due to the poor quality of the transmission. Perplexed; my best guess was that the aircraft calling was requesting a popup IFR clearance in the area. I asked the aircraft to repeat their call sign and intentions again. At this point; the call sign was a bit more clear and my Radar Assist pointed out the fact that it was indeed Aircraft X who had turned 180 degrees back and descended ~20;000 feet without a clearance to do either. Since Aircraft X; [which] I had been working; had safely climbed through any traffic conflictions and was on course; I failed to correlate that fact that the call sign being used by the nearly unreadable pilot was the same call sign as the aircraft I thought to be at cruise altitude. Still convinced that the aircraft calling was requesting an IFR popup; I radar identified them unnecessarily and asked if they were 'VFR' since they were under 18000 feet. The pilot indicated that they were in VFR conditions and requested to return to their departure airport. I instructed the pilot to maintain VFR and issued an altimeter setting. It was only at this time that I started to realize exactly what had occurred. I immediately inquired with the pilot if they were experiencing an emergency or required assistance. Their response was that they had performed an emergency descent; but no longer needed assistance or were an emergency. My Radar Assist coordinated verbally with TRACON that the aircraft was VFR and returning with no assistance required and effected a manual handoff. I issued a communication change to TRACON. One minute later; Aircraft X checked back on asking for 'an altitude'; presumably to pick up another IFR clearance as the weather required it. I issued another communication change to TRACON. The rapid sequence of unusual events left me very confused even after the dust had settled. My initial impression was that Aircraft X had just acquired on the radar because they had a flight plan on file and had squawked the code associated with it and auto acquired. I also considered the possibility that the aircraft calling me should have been on a different center frequency so I quickly looked at surrounding sectors searching for a call sign to no avail. In reality; Aircraft X had departed IFR; I took the handoff and climbed him to his requested altitude. As soon as Aircraft X was clear of any conflictions; I 'forgot' about the aircraft.There was no need to expend mental energy on the aircraft anymore in my estimation; and the next time I would address the aircraft would be when it approached the adjacent sector boundary and needed to be handed off. I've found this to be a very effective working practice as it allows for attention to be focused on more important tasks. In this instance; those items included a large military recovery from multiple MOAs; sequencing; and airport area traffic. I'd like to make it very clear that I'm not making excuses nor am I blaming this incident on high workload or combined sectors. I only mention this to help understand how a controller could possibly not recognize the call sign of an aircraft for which he is responsible for. There was another west bound IFR aircraft behind Aircraft X at 15;000 feet when they began their turn; but thankfully were far enough behind not to be a factor by the time Aircraft X descended through their altitude (10-15miles of separation existed).




Synopsis: ARTCC Controller reported an aircraft calling with an unreadable request; but the Controlled believed the aircraft was in distress. Aircraft was at 3600 feet descending and reversing course. Controller improperly issued aircraft a VFR clearance and sends them to TRACON even though the aircraft was IFR and wishing to remain as such.