A BOEING 727 FLC TURNED IN THE OPPOSITE THE DIRECTION EXPECTED BY EGTT NEAR DPE; FO.

Narrative

WHILE ON RADAR VECTORS IN A GPS EQUIPPED ACFT WE WERE CLRED DIRECT TO DPE. I SELECTED DPE IN THE GPS; SELECTED GPS ON THE MODE SELECTOR; WENT TO AUX NAV ON THE AUTOPLT; AND SELECTED NAV ON THE FLT DIRECTOR. THE CDI NEEDLE ROTATED TO THE R AND THE ACFT THEN BEGAN A R TURN. I THEN LOOKED AT THE ENRTE CHART TO VERIFY USING RAW DATA. LONDON THEN CALLED US TO VERIFY WE WERE GOING DIRECT DPE AS WE SHOULD HAVE BEEN IN A L TURN. THEY HAD US CONTINUE THE R TURN TO AVOID A LOSS OF SEPARATION. AFTER LNDG WE TALKED ABOUT WHAT OCCURRED. WE THINK THAT BASED ON OUR POS FROM DPE WHILE ON VECTORS THE GPS SAW US AS PAST DPE AND DETERMINED THAT WE HAD TO TURN R TO GET BACK TO DPE. I'M NOT SURE WHAT LOGIC THE GPS USES. THIS WAS MY 2ND LEG USING THE GPS (1ST AS PF) AFTER VERY MINIMAL TRAINING. HOWEVER; I AM VERY EXPERIENCED IN FMS PROCS. I AM SURE THAT PROCEDURALLY I USED THE AUTOPLT AND GPS CORRECTLY (EVEN THOUGH THERE IS NO REAL PROC SPECIFIED FOR OUR GPS ACFT). THIS WAS ALSO MY 4TH LEG (2ND AS PF) AFTER BEING OFF WORK FOR 3 MONTHS FOR MEDICAL REASONS. IN THE HIGH WORKLOAD ENVIRONMENT I FELT A BIT BEHIND; BUT AFTER REVIEWING THE SIT I AM CERTAIN THAT THE CORRECT STEPS WERE EXECUTED AND THAT THIS WAS AN ELECTRONIC ANOMALY.