PVT PLT USING GPS FOR IAP PROC VOR DME APCH RWY 27 AT PDK SUFFERS A HEADING TRACK POS DEV. MAKES MISSED APCH UPON HAVING ATL APCH CTLR CANCEL HIS APCH CLRNC.

Narrative

THIS PROB OCCURRED AT THE END OF A FLT IN MY 1984 BE58 BARON FROM NASHVILLE; TN; TO ATLANTA; GA; ON OCT/XX/96. I WAS GIVEN AN INTERCEPT VECTOR AND CLRED BY ATLANTA APCH FOR THE VOR/DME OR GPS RWY 27 APCH CIRCLE TO LAND RWY 2R AT PEACHTREE-DEKALB ARPT; ATLANTA; GA. I HAD ELECTED TO FLY THE GPS APCH AS LOADED IN THE DATABASE ON MY KLN 90B GPS WITH THE AUTOPLT ARMED FOR THE APCH. THE AUTOPLT COUPLED THE APCH AND I BEGAN A DSCNT FROM 3000 FT MSL TO 2500 FT MSL AS SPECIFIED BY THE APCH PROC. BEFORE REACHING 2500 FT; I NOTED THE MOVING MAP DISPLAY ON THE GPS SHOWED THE AIRPLANE S OF THE APCH COURSE WHICH WAS CONFIRMED BY CROSS REF TO THE SECOND VOR INDICATOR. AT THIS POINT; I WAS CONTACTED BY ATLANTA APCH WHO STATED HE SHOWED ME S OF THE APCH COURSE AND CANCELED MY APCH CLRNC. AS I BEGAN THE MISSED APCH PROC AND DISCONNECTED THE AUTOPLT; I ASKED FOR ANOTHER APCH AND RECEIVED VECTORS. I ALSO INFORMED THE CTLR I HAD FLOWN A GPS PROC. HE SUGGESTED MAKING THE SECOND APCH A VOR APCH AND I AGREED. AT THIS POINT; I ALSO ASKED HIM TO REPEAT THE PREVIOUS VECTOR. WHEN HE REPEATED IT; I REALIZED I HAD TURNED PAST IT TO THE L AND BEGAN AN IMMEDIATE TURN BACK TO THE VECTOR. THE CTLR SAID IT APPEARED I WAS TURNING TO 360 DEGS AND I INFORMED HIM I HAD TURNED BACK TO 120 DEGS AS REQUESTED. THE CTLR CLRED ME ONTO THE APCH COURSE AGAIN. I INTERCEPTED THE APPROPRIATE VOR RADIAL; FLEW THE APCH BY HAND ACCORDING TO THE PUBLISHED APCH; BROKE OUT WELL ABOVE THE PUBLISHED MINIMUM; AND LANDED ON RWY 2R WITHOUT INCIDENT. I AM STILL NOT SURE WHAT CAUSED THE PROB WITH THE ORIGINAL GPS APCH. I HAVE PRACTICED MANY SIMILAR APCHS WITH A FLT INSTRUCTOR WITHOUT PROBS SINCE HAVING THE GPS INSTALLED IN MAY 1996. THE DATABASE WAS ALSO CURRENT (THE GPS WILL NOT ALLOW THE APCH TO ACTIVATE IF THE DATABASE IS OUT OF DATE). I AM CURRENTLY REVIEWING THE GPS MANUAL AND INTEND TO FLY GPS APCHS WITH MY FLT INSTRUCTOR THIS WK TO TRY TO DUPLICATE THIS SIT AND TO MAKE SURE I UNDERSTAND THE PROCS NECESSARY TO PROPERLY COUPLE SUCH AN APCH TO THE AUTOPLT. IT IS POSSIBLE THAT I SHOULD HAVE HAD THE GPS ON OBS MODE INSTEAD OF LEG MODE AT THIS PARTICULAR SEGMENT OF THE APCH; I WILL TEST THIS POSSIBILITY DURING THE APCHS WITH MY FLT INSTRUCTOR. I SHOULD HAVE MADE SURE I WAS FIRMLY ESTABLISHED ON THE NEW MISSED APCH VECTOR BEFORE DISCUSSING THE NEW APCH PROC WITH THE CTLR. I ALLOWED THIS DISTR TO CAUSE ME TO FLY THROUGH THE VECTOR BEFORE I VERIFIED THE VECTOR AND CORRECTED MY HEADING. I REALIZE THE JUDICIOUS USE OF THE AUTOPLT USING THE HEADING MODE WOULD HAVE BEEN VERY HELPFUL AT THIS POINT IN MANAGING THE INCREASED WORKLOAD OF SETTING UP THE NEW APCH ON VOR 1 AND THE HSI. IN MY OPINION; NOTHING OCCURRED THAT COMPROMISED THE SAFETY OF THE FLT SINCE THE DEV FROM THE APCH COURSE WAS IDENTED BY BOTH THE PLT AND THE CTLR BEFORE A DSCNT TO THE FINAL MDA WAS INITIATED. HOWEVER; THE OCCURRENCE CERTAINLY DEMONSTRATES THE IMPORTANCE OF USING ALL AVAILABLE RESOURCES IN MONITORING AN INST APCH; PARTICULARLY ONE THAT CAN BE FLOWN USING EITHER VOR OR GPS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE NEXT WEEKEND THE RPTR WENT OUT WITH AN INSTRUCTOR PLT AND THEY PRACTICED APCHS; USING THE GPS. RPTR FOUND THAT HIS PROB HAD BEEN THAT HE HAD FAILED TO SWITCH FROM THE 'LEG' PHASE ON THE GPS TO THE 'OBS' POS. WHILE IN THE 'LEG' POS THE GPS WILL WANT TO TAKE THE ACFT TO THE NEXT FIX WITHIN THE DATABASE IF COUPLED TO THE AUTOPLT. IF THE FLT DIRECTOR IS ALSO ON IT CAN BE VERY CONFUSING TO THE PLT. IF RECEIVING VECTORS; ONE MUST SELECT 'OBS' TO OVERRIDE THIS DATABASE DEFAULT AND THEN THE ACFT WILL TAKE THE VECTORS NEEDED FOR THE APCH. THIS IS ONLY WHEN OFF THE FINAL APCH COURSE. ONCE ON THE FINAL APCH COURSE; THE 'LEG' POS IS ONCE AGAIN SELECTED.