Date: 201604
Local time of day: 1201-1800
Location: SAT.TRACON, TX
Light: Daylight
Aircraft: Small Transport; Low Wing; 2 Turboprop Eng
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C SAT
ATC / advisory: TRACON SAT
Second aircraft: Small Transport
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Communication Breakdown; Distraction; Workload; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Working APW (Approach West) there was a call for release off of 5C1 airport on RWY 35. I gave the release of runway heading to 4000 and a void time due to locking up altitudes on the final in IFR conditions for SAT and the need for ILS approaches. At the time I had a sequence planned and descended Aircraft X to 4000 but realized I needed to change that plan as Aircraft Y would also need to be fit in the sequence. At that time I descended Aircraft Y to make him next for the ILS and later based Aircraft X and told to expect vector across the localizer for spacing to ensure spacing as the departure off 5C1 had not tagged up and the void time had been reached. I then cleared Aircraft Y for the approach and he never turned. Upon noticing he didn't take the approach I went to turn back and cancelled that and instructed Aircraft Y to maintain 3000 and fly present heading with a traffic call for Aircraft X. I then told Aircraft X to fly heading 150 and join the localizer with a traffic call about the Aircraft Y; below. Then I noted that Aircraft Y was turning back to a 150 heading to join the localizer instead of flying present heading and then told him that was incorrect but he had turned too far so I continued a southbound turn to heading 180 to get him across the final and establish divergence from Aircraft X. At this point I saw that Aircraft X was about 2 miles NW of Aircraft Y and was descending out of 3800; I then told him do not descend and maintain 4000 for traffic and continued the turn of Aircraft Y to heading 220 to avoid. Aircraft X said he was instructed to maintain 3000 but I believe; without hearing playback; that the pilots both may have taken clearances for the other aircraft. At this point with Aircraft Y moving southwest bound and Aircraft X joining the localizer I cleared Aircraft X to give him the ILS approach once the situation was safe and vectored Aircraft Y for re-sequence.This is a consistent problem with Aircraft Y when the aircraft comes into the facility. The pilot has constant issues both on the airfield and in the air with following instructions. It seems that he obeys instructions if/when he feels he is ready instead of when instructed to do so by the controller. He was given his approach clearance and simply didn't follow the instructions; and then once again didn't follow instructions when he was told to fly present heading for re-sequencing. Part of this is a language barrier issue but there is also an issue with him delaying in the executing of instructions and it is consistent in our operation when Aircraft Y is present.




Synopsis: SAT Approach Controller reported a loss of separation due to two aircraft possibly taking each other's instructions. Reporter stated there is a language barrier with one of the pilots and this pilot is a problem every time they are in the area.