Date: 201601
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Light: Daylight
Aircraft: Medium Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Light Transport
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion; Situational Awareness; Training / Qualification
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Contributing factors: Procedure; Airspace Structure; Human Factors
Primary problem: Human Factors

Narrative: Aircraft X was in level flight and lost cabin pressure and started a descent out of 24;000 feet without clearance. Aircraft Y was leaving 15;000 feet when Aircraft X began his descent. I was the Radar Assistant during this event. The Radar Controller stopped aircraft Y at 18;000 feet and then issued aircraft X 19;000 feet. I coordinated with [the adjacent Center] and sector and informed them we were turning aircraft Y to the right to accommodate aircraft X who needed a lower altitude. The Radar Controller turned aircraft Y to a heading of 240 degrees and the pilot turned to a heading of 140 degrees. We later learned that aircraft Y misheard the clearance when he turned to a 140 heading. At this time I coordinated with TRACON and requested 10;000 feet and informed them of the situation. While on the line; aircraft X was just nearing 19;000 feet and requested 10;000 feet again. The Radar Controller issued 10;000 feet. At this time we noticed aircraft Y had made a left turn (he heard heading 140 instead of 240) and the Radar Controller issued aircraft Y the descending traffic and instructed him to turn to a 220 degree heading for the traffic. Aircraft Y said he's making the turn now as aircraft X said he has the traffic in sight.When aircraft X requested lower than 19;000 feet we should have issued the traffic. In my opinion this would not have prevented the event because aircraft X needed to continue descending; but the aircraft would not have been as close especially considering aircraft Y turned to an incorrect heading.


Second narrative: Aircraft X was level at 24;000 feet northwest bound; enroute. Simultaneously aircraft Y; had departed the airport in a climb to 23;000 feet southeast bound. As the Radar Controller aircraft X advised me that the aircraft was experiencing a cabin pressure issue and needed to descend. I stopped aircraft Y's climb at 18;000 feet; then instructed aircraft X to descend and maintain 19;000 feet. Aircraft X read back the instructions; however; the pilot had already began descent prior to the issuance of the descent instructions. The pilot of Aircraft X then advised me that he required further descent to 10;000 feet. I then issued Aircraft Y a right turn to a heading of 240 degrees for emergency traffic; and aircraft X a descent clearance to 10;000 feet. It was then observed that aircraft Y was not flying the appropriate heading and was actually turning eastbound. I then instructed aircraft Y to fly a heading of 220 degrees; however the pilot's response was unintelligible. I again instructed the pilot to fly a heading of 220 degrees and verify making the turn. Traffic was issued to both aircraft and aircraft X reported aircraft Y in sight as they passed off each other's side with less than standard separation. Afterwards; aircraft Y advised that he misheard the initial heading and flew a heading of 140.As I treated this incident as an emergency; I was inclined to approve whatever the pilot in distress was requesting. Ensuring that the turning aircraft was in fact on the appropriate heading prior to the issuance of descent and issuing a turn to the pilot in distress would have solved the situation.


Synopsis: An aircraft was making an emergency descent. ATC issued a heading to opposite direction climbing traffic to avoid the descending traffic. The climbing traffic turned to the wrong heading causing a conflict with the emergency descending traffic.