Narrative:

Due to developing thunderstorms, aircraft were deviating throughout the area. Air carrier X at FL310, was traveling wbound on J6 toward bwg and air carrier Y at FL310, was traveling northbound on J89 toward iiu. Air carrier X had begun to deviation to the south around WX and had been cleared to do so by the radar controller. At some point, while I was not monitoring the frequency due to other demands, the radar controller also cleared air carrier Y to deviation left or right of course as necessary. Upon hearing the controller descend air carrier X to FL290, I returned my focus to the radar scope at which time I observed that the 2 aircraft were head-on approximately 15-20 mi apart as air carrier Y had made a 90 degree plus turn. I suggested to the radar controller that he could climb air carrier Y to FL330, which he did. He also asked air carrier Y to turn left if feasible. The pilot replied that he could make a left turn; however, I never observed a turn before separation was lost. Air carrier X was eventually descended to FL270, and was asked to report leaving FL290 but vertical separation was not achieved before lateral separation was lost. Because the pilot of an aircraft has the final say as to where or when he will turn the aircraft for WX (or any reason for that matter), the only recommendation I could make for this type of situation is: when an aircraft advises that deviations will be necessary, the controller should ask the pilot which direction he will deviation, check for potential traffic and then assign a specific clearance to deviation. For example, '...cleared to deviation 20 degree right or left of course. Advise center if further deviations will be necessary.' in other words, tie down deviations and don't make assumptions based on previous aircraft deviations.

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Original NASA ASRS Text

Title: ACR X HAD LTSS FROM ACR Y. SYS ERROR. ACR Y FLC NON ADHERENCE TO ATC CLRNC FAILED TO CLB OR TURN. PLTDEV.

Narrative: DUE TO DEVELOPING TSTMS, ACFT WERE DEVIATING THROUGHOUT THE AREA. ACR X AT FL310, WAS TRAVELING WBOUND ON J6 TOWARD BWG AND ACR Y AT FL310, WAS TRAVELING NBOUND ON J89 TOWARD IIU. ACR X HAD BEGUN TO DEV TO THE S AROUND WX AND HAD BEEN CLRED TO DO SO BY THE RADAR CTLR. AT SOME POINT, WHILE I WAS NOT MONITORING THE FREQ DUE TO OTHER DEMANDS, THE RADAR CTLR ALSO CLRED ACR Y TO DEV L OR R OF COURSE AS NECESSARY. UPON HEARING THE CTLR DSND ACR X TO FL290, I RETURNED MY FOCUS TO THE RADAR SCOPE AT WHICH TIME I OBSERVED THAT THE 2 ACFT WERE HEAD-ON APPROX 15-20 MI APART AS ACR Y HAD MADE A 90 DEG PLUS TURN. I SUGGESTED TO THE RADAR CTLR THAT HE COULD CLB ACR Y TO FL330, WHICH HE DID. HE ALSO ASKED ACR Y TO TURN L IF FEASIBLE. THE PLT REPLIED THAT HE COULD MAKE A L TURN; HOWEVER, I NEVER OBSERVED A TURN BEFORE SEPARATION WAS LOST. ACR X WAS EVENTUALLY DSNDED TO FL270, AND WAS ASKED TO RPT LEAVING FL290 BUT VERT SEPARATION WAS NOT ACHIEVED BEFORE LATERAL SEPARATION WAS LOST. BECAUSE THE PLT OF AN ACFT HAS THE FINAL SAY AS TO WHERE OR WHEN HE WILL TURN THE ACFT FOR WX (OR ANY REASON FOR THAT MATTER), THE ONLY RECOMMENDATION I COULD MAKE FOR THIS TYPE OF SITUATION IS: WHEN AN ACFT ADVISES THAT DEVS WILL BE NECESSARY, THE CTLR SHOULD ASK THE PLT WHICH DIRECTION HE WILL DEV, CHK FOR POTENTIAL TFC AND THEN ASSIGN A SPECIFIC CLRNC TO DEV. FOR EXAMPLE, '...CLRED TO DEV 20 DEG R OR L OF COURSE. ADVISE CENTER IF FURTHER DEVS WILL BE NECESSARY.' IN OTHER WORDS, TIE DOWN DEVS AND DON'T MAKE ASSUMPTIONS BASED ON PREVIOUS ACFT DEVS.

Data retrieved from NASA's ASRS site as of July 2007 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.