Narrative:

Aircraft X had to be vectored in trail. Aircraft X was level at 29;000 feet and slowed to 270 knots. About 10 miles southwest was aircraft Y level at 27;000 feet. Aircraft X was given a clearance to descend to 27;000 feet and fly heading 020. Approximately a 15 degree turn to the left. Pilot read back 27;000 feet and heading 020. About a minute later aircraft X was given a clearance to turn direct to a fix; which is a turn back to the right. Aircraft X did not turn back to the right but instead tracked way left; much further than the 020 heading that was issued. I then descended aircraft Y to 25;000 feet; and when I noticed that aircraft X did not turn to a 020 heading; I turned aircraft Y to a 010 heading. I then turned aircraft X to a 100 heading; but the pilot said she was responding to a RA and had to climb. Loss of separation occurred with 4.2 miles and 900 feet. I believe the pilot mistakenly flew the wrong heading. The plane's track looked as if it was trying to fly a 320 heading instead of a 020 heading; a very wide turn that affected the ability to turn back to the right. After asking the pilot what heading she was issued she said heading 020.I would recommend perhaps a redundancy system for heading inputs to make sure the correct heading is applied.

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Original NASA ASRS Text

Title: A controller was vectoring an aircraft for in trail spacing to the next facility. The controller issued a 020 degree heading. The controller observed the aircraft turning much farther left than the 020 heading towards other traffic. The controller descended the other traffic and attempted to vector the first aircraft back to the correct direction. During the turns and descents the deviating aircraft received a TCAS/RA alert and responded accordingly.

Narrative: Aircraft X had to be vectored in trail. Aircraft X was level at 29;000 feet and slowed to 270 knots. About 10 miles southwest was aircraft Y level at 27;000 feet. Aircraft X was given a clearance to descend to 27;000 feet and fly heading 020. Approximately a 15 degree turn to the left. Pilot read back 27;000 feet and heading 020. About a minute later aircraft X was given a clearance to turn direct to a fix; which is a turn back to the right. Aircraft X did not turn back to the right but instead tracked way left; much further than the 020 heading that was issued. I then descended aircraft Y to 25;000 feet; and when I noticed that aircraft X did not turn to a 020 heading; I turned aircraft Y to a 010 heading. I then turned aircraft X to a 100 heading; but the pilot said she was responding to a RA and had to climb. Loss of separation occurred with 4.2 miles and 900 feet. I believe the pilot mistakenly flew the wrong heading. The plane's track looked as if it was trying to fly a 320 heading instead of a 020 heading; a very wide turn that affected the ability to turn back to the right. After asking the pilot what heading she was issued she said heading 020.I would recommend perhaps a redundancy system for heading inputs to make sure the correct heading is applied.

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.