Narrative:

Training in progress on final a; east operation with triple ILS precision runway monitoring (prm) approaches in IFR weather.[aircraft X] was on the downwind descending to 3;000 feet. [Aircraft Y] was on the base leg northbound descending to 3;000 feet. We turned [aircraft X] northbound on a right base. Once [aircraft X] began the turn; we turned [aircraft Y] to a 030 degree heading to achieve desired spacing behind [aircraft X]. [Aircraft X] appeared to roll out on a 300 degree heading instead of a 360 degree heading. The trainee then turned [aircraft Y] left to a heading of 360; and then turned him left again to a heading of 330. [Aircraft X] continued on what appeared to be a 300 degree heading. I then took the frequency and turned [aircraft X] all the way right to a 080 heading to join the final. Separation was lost and decreased to 2 miles with no vertical. Once the aircraft were separated again I turned [aircraft Y] east to join the final 4 miles behind [aircraft X]. Visual separation was not attempted during this incident because the aircraft were in IMC conditions the entire time.the very nature of working finals dictates that aircraft are on converging downwind and base headings. The volume of traffic here assures that aircraft will be in proximity to other aircraft; and prompt compliance with ATC instructions is of paramount importance to ensure passenger and crew safety. There is no place for a turn in the final box that is off by 60 degrees.we could have slowed [aircraft X] prior to a northbound turn instead of combining the two instructions. The spacing between aircraft in this scenario made that unnecessary; and I feel that I made every effort to put these two aircraft on the final safely and efficiently while conducting on the job training (OJT). The trainee action in the scenario was not incorrect in my opinion. I depend on the aircrew for prompt compliance; and expect them to perform the instructions that I assign and they read back. That did not happen here. I cannot speculate as to why; as I am not qualified and current in [that type of aircraft].to prevent re-occurrence of this event; I recommend the aircrew does what they are instructed to do when they are instructed to do it.

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Original NASA ASRS Text

Title: Controller Training in progress. Trainee gave radar vectors for sequencing and separation to the final approach course. Aircraft turned at a much slower rate than Controllers anticipated. Instructor took over sector and issued vectors to both aircraft but it was too late to maintain required separation standards. Pilots also reported and did not realize that the loss of separation had occurred.

Narrative: Training in progress on final A; east operation with triple ILS Precision Runway Monitoring (PRM) approaches in IFR weather.[AIRCRAFT X] was on the downwind descending to 3;000 feet. [AIRCRAFT Y] was on the base leg northbound descending to 3;000 feet. We turned [AIRCRAFT X] northbound on a right base. Once [AIRCRAFT X] began the turn; we turned [AIRCRAFT Y] to a 030 degree heading to achieve desired spacing behind [AIRCRAFT X]. [AIRCRAFT X] appeared to roll out on a 300 degree heading instead of a 360 degree heading. The trainee then turned [AIRCRAFT Y] left to a heading of 360; and then turned him left again to a heading of 330. [AIRCRAFT X] continued on what appeared to be a 300 degree heading. I then took the frequency and turned [AIRCRAFT X] all the way right to a 080 heading to join the final. Separation was lost and decreased to 2 miles with no vertical. Once the aircraft were separated again I turned [AIRCRAFT Y] east to join the final 4 miles behind [AIRCRAFT X]. Visual separation was not attempted during this incident because the aircraft were in IMC conditions the entire time.The very nature of working finals dictates that aircraft are on converging downwind and base headings. The volume of traffic here assures that aircraft will be in proximity to other aircraft; and prompt compliance with ATC instructions is of paramount importance to ensure passenger and crew safety. There is no place for a turn in the final box that is off by 60 degrees.We could have slowed [AIRCRAFT X] prior to a northbound turn instead of combining the two instructions. The spacing between aircraft in this scenario made that unnecessary; and I feel that I made every effort to put these two aircraft on the final safely and efficiently while conducting On the Job Training (OJT). The trainee action in the scenario was not incorrect in my opinion. I depend on the aircrew for prompt compliance; and expect them to perform the instructions that I assign and they read back. That did not happen here. I cannot speculate as to why; as I am not qualified and current in [that type of aircraft].To prevent re-occurrence of this event; I recommend the aircrew does what they are instructed to do when they are instructed to do it.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.