Narrative:

Air carrier X makes initial contact with ZID. Reports level at FL330 and says 'deviating east.' controller says roger. Air carrier X light plane shows dfw.1.lit.J131.pxv..vhp..fwa.1.dtw direct. He is roughly 30 mi southeast of course and still diverging from the lit to pxv leg by approximately 20 degrees. Memphis center had not coordinated the deviation with us, so we were unaware of how he had been cleared to deviate. Severe thunderstorms were in the northern portion of the sector as part of a line stretching west/east approximately stl..cvg. 2 gaps were present in the WX, 1 over the sam VOR, the other over the oom VOR. Thunderstorm line was severe with tops in excess of FL410. Air carrier Y makes initial contact with ZID. Reports level at FL330 and requests FL290. Controller acknowledged callup and tells air carrier Y that he will have FL290 in about a min. Is on his flight plan which shows from points southwest of pxv, lex and cvg. This was an alternate routing put out due to severe WX over the abb arrival gate to cvg. Air carrier Y was roughly at air carrier X 7 O'clock position, 8 mi, same heading, same speed. When air carrier Y crossed the common boundary he was issued FL290. Less than a min later both d-side and r-side were alerted to the loss of separation between air carrier X and air carrier Y when simultaneously conflict alert and adep went off. Air carrier X had begun a steep turn to the left without alerting the controllers. The 8 mi in trail quickly eroded as air carrier X turned nearly 90 degrees left in 1 min. When questioned as to his action, air carrier X said that he was trying to find vhp and go directly to it. Air carrier X turn was in excess of 30 degrees too far to the west for pxv. The controller had only been on position 7 mins when the error occurred. Prevention: aircraft should be required to alert controllers prior to making turns in excess of 10 degrees after having been established on a heading for deviations. This would eliminate the verbage 'advise the controller when you are ready to turn back on course.' it would prevent deviating aircraft from changing course several times while deviating without ATC's acknowledgement. Currently, positive control near severe WX requires either all altitude separation (which is not usually practicable) or lots of excess verbage (which is already at a premium).

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Original NASA ASRS Text

Title: ACR X DEVIATING AROUND WX HAD LTSS FROM ACR Y. SYS ERROR.

Narrative: ACR X MAKES INITIAL CONTACT WITH ZID. RPTS LEVEL AT FL330 AND SAYS 'DEVIATING E.' CTLR SAYS ROGER. ACR X LIGHT PLANE SHOWS DFW.1.LIT.J131.PXV..VHP..FWA.1.DTW DIRECT. HE IS ROUGHLY 30 MI SE OF COURSE AND STILL DIVERGING FROM THE LIT TO PXV LEG BY APPROX 20 DEGS. MEMPHIS CTR HAD NOT COORDINATED THE DEV WITH US, SO WE WERE UNAWARE OF HOW HE HAD BEEN CLRED TO DEVIATE. SEVERE TSTMS WERE IN THE NORTHERN PORTION OF THE SECTOR AS PART OF A LINE STRETCHING W/E APPROX STL..CVG. 2 GAPS WERE PRESENT IN THE WX, 1 OVER THE SAM VOR, THE OTHER OVER THE OOM VOR. TSTM LINE WAS SEVERE WITH TOPS IN EXCESS OF FL410. ACR Y MAKES INITIAL CONTACT WITH ZID. RPTS LEVEL AT FL330 AND REQUESTS FL290. CTLR ACKNOWLEDGED CALLUP AND TELLS ACR Y THAT HE WILL HAVE FL290 IN ABOUT A MIN. IS ON HIS FLT PLAN WHICH SHOWS FROM POINTS SW OF PXV, LEX AND CVG. THIS WAS AN ALTERNATE ROUTING PUT OUT DUE TO SEVERE WX OVER THE ABB ARR GATE TO CVG. ACR Y WAS ROUGHLY AT ACR X 7 O'CLOCK POS, 8 MI, SAME HDG, SAME SPD. WHEN ACR Y CROSSED THE COMMON BOUNDARY HE WAS ISSUED FL290. LESS THAN A MIN LATER BOTH D-SIDE AND R-SIDE WERE ALERTED TO THE LOSS OF SEPARATION BTWN ACR X AND ACR Y WHEN SIMULTANEOUSLY CONFLICT ALERT AND ADEP WENT OFF. ACR X HAD BEGUN A STEEP TURN TO THE L WITHOUT ALERTING THE CTLRS. THE 8 MI IN TRAIL QUICKLY ERODED AS ACR X TURNED NEARLY 90 DEGS L IN 1 MIN. WHEN QUESTIONED AS TO HIS ACTION, ACR X SAID THAT HE WAS TRYING TO FIND VHP AND GO DIRECTLY TO IT. ACR X TURN WAS IN EXCESS OF 30 DEGS TOO FAR TO THE W FOR PXV. THE CTLR HAD ONLY BEEN ON POS 7 MINS WHEN THE ERROR OCCURRED. PREVENTION: ACFT SHOULD BE REQUIRED TO ALERT CTLRS PRIOR TO MAKING TURNS IN EXCESS OF 10 DEGS AFTER HAVING BEEN ESTABLISHED ON A HDG FOR DEVS. THIS WOULD ELIMINATE THE VERBAGE 'ADVISE THE CTLR WHEN YOU ARE READY TO TURN BACK ON COURSE.' IT WOULD PREVENT DEVIATING ACFT FROM CHANGING COURSE SEVERAL TIMES WHILE DEVIATING WITHOUT ATC'S ACKNOWLEDGEMENT. CURRENTLY, POSITIVE CTL NEAR SEVERE WX REQUIRES EITHER ALL ALT SEPARATION (WHICH IS NOT USUALLY PRACTICABLE) OR LOTS OF EXCESS VERBAGE (WHICH IS ALREADY AT A PREMIUM).

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.