Date: 201701
Local time of day: 1201-1800
Location: ZMA.ARTCC, FL
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Climb
Airspace: Class A ZMA
ATC / advisory: Center ZMA
Second aircraft: Super King Air 200
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion; Distraction; Situational Awareness
Second reporter function: Enroute; Handoff / Assist; Instructor
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance
Contributing factors: Procedure; Human Factors; Staffing; Airspace Structure
Primary problem: Procedure

Narrative: Working sector 26 had just combined back with sector 6. Aircraft Y was level at FL240 over TADPO and about to land at MTH which is close to TADPO. Aircraft X was coming from Havana over TADPO at FL200. My D-side told me I had control from Havana to turn and climb Aircraft X. I climbed Aircraft X to FL230 which the pilot read back. I needed to start Aircraft Y down and keep Aircraft X climbing. Around TADPO I told Aircraft X to fly heading 310. Apparently he read back climbing to FL310 which I missed. A little later I observed Aircraft X hadn't turned and I told him again to turn. In the read back Aircraft X acknowledged the turn but also told me he was responding to an RA. I thought the RA was because he was under Aircraft Y now and maybe he had climbed to fast and the RA was forcing him down. I was able to turn him away from Aircraft Y and started Aircraft Y down to get my separation back.The sector was really busy and probably should not have been recombined but we had no staffing. Also there was a lot of frequency congestion adding to the problem. I missed the read back when I told Aircraft X to turn and he took it as a climb clearance.


Second narrative: At the time of the event I was training a developmental on the D-side. Traffic volume was very high and the sector was red after having Sector 6 had been combined on to it. Aircraft Y was in level flight north bound with Aircraft X overtaking from MUHA airspace. To the best of my knowledge the R-side issued a climb to flight level 220 and the data lock reflected this. This altitude should've kept the aircraft vertically separated; the trainee and I were constantly on and off line making manual handoffs with MUHA ARTCC. I honestly do not remember hearing a read back from Aircraft X as I don't remember if we were on or off line. Apparently the R-side issued a turn to Aircraft X to fly heading 310 but the aircraft took it as a climb and not a heading. This resulted in Aircraft X climbing through the altitude of Aircraft Y.Sector 06/26 is a disastrous sector. The sector is frequently overwhelmed with traffic; warning areas; and foreign pilots. The sector was combined too soon making of the sector unworkable safely. Volume and frequency congestion is a direct factor in this event occurring. They have been trying to redesign this sector for years now but have never implemented anything that works. Sector 06 and 26 should've remained split apart and it could have helped this particular situation.


Synopsis: ZMA Controllers reported that an aircraft took a heading as a climb instruction and became a conflict for another aircraft. Controllers reported the sector was combined and busy.