MD80 FLT CREW HAS INEXPLICABLE LOSS OF INITIAL WAYPOINT WHILE FLYING THE PODDE RNAV SID FROM RWY 18L AT DFW. TRACK DEV RESULTS.

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WE WERE CLRED FOR THE PODDE3 TO ABI OFF RWY 18L. I LOADED THE FMS. I CHKED ALL THE POINTS AGAINST THE DEP PAGE AND THE FLT PLAN. LATER; THE CAPT INDEPENDENTLY CHKED ALL POINTS HIMSELF. WHEN WE CONTACTED QUEUE; THE NAV DISPLAY DISPLAYED RWY 18L; AND THE CDU SHOWED LARRN IN MAGENTA. WHEN WE TOOK THE RWY; THE MAGENTA LINE WENT STRAIGHT S TO LARRN. THE CAPT FOLLOWED THE NAV STEERING AFTER TKOF. AT ABOUT 2000 FT AGL; HE ENGAGED THE AUTOPLT. I CHKED TO MAKE SURE THAT THE ACFT WAS TRACKING THE MAGENTA LINE; AND COMMENTED TO MYSELF THAT WE HAD NO PROBLEM MAKING THE 5000 FT RESTRICTION AT LARRN. AT THAT POINT I STARTED LOOKING FOR TFC. THE ACFT STARTED A 20 DEG R TURN. I FELT THAT IT WAS TOO EARLY TO BE AT LARRN; AND TOLD THE CAPT SO. I LOOKED AT THE NAV DISPLAY; AND LARRN WAS NO LONGER DISPLAYED. I LOOKED AT THE CDU; AND IT HAD GIGEM AS THE POINT I WAS GOING TO. I SWITCHED TO RAW DATA; AND SAW WE WERE 6.1 DME S OF TTT. I LOOKED AT THE DEP PLATE SEEKING A DME FOR LARRN; BUT DID NOT SEE IT DEPICTED ON THE SCHEMATIC. (I LATER FOUND THE DME IN THE FINE PRINT IN THE BOTTOM R CORNER OF THE PAGE. LARRN IS AT 11 DME.) I LOOKED BACK AT THE NAV DISPLAY; AND THERE WAS A MAGENTA LINE RUNNING FROM E TO W ACROSS THE TOP OF THE 10 MILE DISPLAY LEADING TO GIGEM. THERE WAS ANOTHER CURVED MAGENTA LINE IN THE BOTTOM R CORNER (NW CORNER) OF THE DISPLAY. IT APPEARED TO BE COMING FROM THE VICINITY OF THE ARPT AND HEADING OFF TO THE N. THE ACFT CONTINUED IN A R HAND TURN THROUGH ABOUT A 250 DEG HDG; WHEN GIGEM CROSSED THE NOSE. THE CAPT INTERVENED WITH HDG SEL; AND TURNED BACK TO ABOUT 220 DEG HDG TO REJOIN THE DEP. I CHKED THE TCAS; AND THE NEAREST TFC WAS 5 MILES BEHIND AND 1700 FT BELOW. A BAFFLED ATC CALLED; TRYING TO FIGURE OUT WHERE WE WERE GOING. WE EXPLAINED THAT THE ACFT WAS NOT FLYING THE RNAV DEP; AND WE WERE MANUALLY CORRECTING BACK TO COURSE. WE WERE CLRED DIRECT TO PODDE. THE CAPT HAS 9500 HOURS IN THE MD80 AND I HAVE 10500. I PRIDE MYSELF AS HAVING A GOOD SITUATIONAL AWARENESS AND BEING FAIRLY ADEPT WITH 'THE BOX.' WE LOADED; CHKED; AND FLEW THE DEP IN ACCORDANCE WITH ALL THE CORRECT PROCS. NEVERTHELESS; SOMETHING BURPED IN THE SYSTEM; AND THE RNAV DEP DID NOT WORK AS DESIGNED. WE SPENT A LONG TIME ANALYZING WHAT MAY HAVE HAPPENED; AND COULD COME UP WITH NO ANSWER. I HAVE ALWAYS BEEN A STRONG PROPONENT OF RNAV PROCS. HOWEVER; I DISCOVERED THAT IF SOMETHING GOES WRONG WITH THE BOX ON DEP; THERE IS NO WAY TO CROSSCHECK IT WITH RAW DATA. THE RNAV TRACK IS NOT DEFINED BY GND NAVAIDS; DME'S; OR RADIALS. EVEN THOUGH WE WERE FLYING AT OUR OWN HOME STATION IN VMC CONDITIONS; WE STILL ENDED UP OFF COURSE BECAUSE WE COULDN'T INSTANTLY REVERT TO RAW DATA TO RESUME THE DEP. IF WE HAD BEEN AT AN UNFAMILIAR ARPT; IMC; FLYING A NEW RNAV DEP; WE COULD HAVE BEEN IN BIG TROUBLE. I WILL BE A LOT LESS CONFIDENT OF RNAV DEPS IN THE FUTURE.