Narrative:

We were cleared for the PODDE3 to abi off runway 18L. I loaded the FMS. I checked all the points against the departure page and the flight plan. Later; the captain independently checked all points himself. When we contacted queue; the navigation display displayed runway 18L; and the CDU showed larrn in magenta. When we took the runway; the magenta line went straight south to larrn. The captain followed the navigation steering after takeoff. At about 2000 ft AGL; he engaged the autoplt. I checked to make sure that the aircraft 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 traffic. The aircraft started a 20 degree right turn. I felt that it was too early to be at larrn; and told the captain so. I looked at the navigation 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 south of TTT. I looked at the departure 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 right corner of the page. Larrn is at 11 DME.) I looked back at the navigation display; and there was a magenta line running from east to west across the top of the 10 mile display leading to gigem. There was another curved magenta line in the bottom right corner (northwest corner) of the display. It appeared to be coming from the vicinity of the airport and heading off to the north. The aircraft continued in a right hand turn through about a 250 degree heading; when gigem crossed the nose. The captain intervened with heading sel; and turned back to about 220 degree heading to rejoin the departure. I checked the TCAS; and the nearest traffic was 5 miles behind and 1700 ft below. A baffled ATC called; trying to figure out where we were going. We explained that the aircraft was not flying the RNAV departure; and we were manually correcting back to course. We were cleared direct to podde. The captain 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; checked; and flew the departure in accordance with all the correct procedures. Nevertheless; something burped in the system; and the RNAV departure 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 procedures. However; I discovered that if something goes wrong with the box on departure; there is no way to crosscheck it with raw data. The RNAV track is not defined by ground 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 departure. If we had been at an unfamiliar airport; IMC; flying a new RNAV departure; we could have been in big trouble. I will be a lot less confident of RNAV departures in the future.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of January 2009 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.