Narrative:

Our clearance was the podde 3 to abi; taking off on runway 36R with gvine as the first fix. We had correctly loaded it into the #2 FMS (#1 was MEL'd with a CDU data bus failure) and verified the waypoints against commercial chart as per the F-4 message. The first officer made the takeoff on runway 36R and I selected navigation after the gear was retracted. As I selected the yaw damper and the autoplt was engaged; I noticed the course line missing from the 1110 ft point to gvine. At acceleration altitude we were handed off to departure and after 200 KTS was set and the flaps retracted/climb checks were done; I noticed the line to the 1110 ft point had disappeared and a new course line was now depicted .5 miles to our left (west); coming from a point somewhere behind us and going to gvine. The first officer commented that something was wrong; and the autoplt had begun to turn the aircraft west towards the new course line. It appeared to me that we were still tracking towards gvine but at that point the controller called and stated that we had turned early; that there was no conflict with any traffic; and that we were now cleared direct to podde. We selected direct to podde; and the first officer turned to follow the course guidance. The asked us some questions about the reason for the early turn and; since we were not sure exactly what had happened at that point; I stated that the course segment from the 1110 ft point to gvine had dropped out. The controller said that he and his supervisor had discussed it and that there had been no conflict and that probably nothing would come of it. We were handed off to ZFW and as we flew towards podde; the first officer commented that the FMS didn't seem to be guiding autoplt towards podde very well. The aircraft seemed to wander .2 to .3 miles to either side of the course. About this time we got a 'position uncertain' message on the FMS; and; as I went to the data page to check our position; the message went out and the aircraft seemed now to be accurately tracking to podde. It made the correct turn to course at podde and by this time; I was in the 'green world;' backing up the FMS with conventional navigation. As we continued west; with everything working normally; I took a minute to call back to departure to tell him about our one MEL'd FMS; and that we had had a position problem with our remaining FMS after we had talked to him; and that it was probably related to the turn problem we had had near gvine. No further problems were encountered and the remainder of the flight was uneventful. Perhaps we should consider not flying an RNAV departure with only one FMS working. Supplemental information acn 687396: when you execute a GPS approach the GPS raim capability is verified to be sure it can see enough satellites to triangulate the aircraft position. No such verification is required for the RNAV departure.

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Original NASA ASRS Text

Title: DISPATCHED WITH ONE FMS INOPERATIVE; E145 CREW EXPERIENCE TRACK DEVIATION ON PODDE RNAV SID FROM DFW WHEN REMAINING FMS MALFUNCTIONS.

Narrative: OUR CLRNC WAS THE PODDE 3 TO ABI; TAKING OFF ON RWY 36R WITH GVINE AS THE FIRST FIX. WE HAD CORRECTLY LOADED IT INTO THE #2 FMS (#1 WAS MEL'D WITH A CDU DATA BUS FAILURE) AND VERIFIED THE WAYPOINTS AGAINST COMMERCIAL CHART AS PER THE F-4 MSG. THE FO MADE THE TKOF ON RWY 36R AND I SELECTED NAV AFTER THE GEAR WAS RETRACTED. AS I SELECTED THE YAW DAMPER AND THE AUTOPLT WAS ENGAGED; I NOTICED THE COURSE LINE MISSING FROM THE 1110 FT POINT TO GVINE. AT ACCELERATION ALT WE WERE HANDED OFF TO DEP AND AFTER 200 KTS WAS SET AND THE FLAPS RETRACTED/CLB CHKS WERE DONE; I NOTICED THE LINE TO THE 1110 FT POINT HAD DISAPPEARED AND A NEW COURSE LINE WAS NOW DEPICTED .5 MILES TO OUR L (WEST); COMING FROM A POINT SOMEWHERE BEHIND US AND GOING TO GVINE. THE FO COMMENTED THAT SOMETHING WAS WRONG; AND THE AUTOPLT HAD BEGUN TO TURN THE ACFT W TOWARDS THE NEW COURSE LINE. IT APPEARED TO ME THAT WE WERE STILL TRACKING TOWARDS GVINE BUT AT THAT POINT THE CTLR CALLED AND STATED THAT WE HAD TURNED EARLY; THAT THERE WAS NO CONFLICT WITH ANY TFC; AND THAT WE WERE NOW CLRED DIRECT TO PODDE. WE SELECTED DIRECT TO PODDE; AND THE FO TURNED TO FOLLOW THE COURSE GUIDANCE. THE ASKED US SOME QUESTIONS ABOUT THE REASON FOR THE EARLY TURN AND; SINCE WE WERE NOT SURE EXACTLY WHAT HAD HAPPENED AT THAT POINT; I STATED THAT THE COURSE SEGMENT FROM THE 1110 FT POINT TO GVINE HAD DROPPED OUT. THE CTLR SAID THAT HE AND HIS SUPVR HAD DISCUSSED IT AND THAT THERE HAD BEEN NO CONFLICT AND THAT PROBABLY NOTHING WOULD COME OF IT. WE WERE HANDED OFF TO ZFW AND AS WE FLEW TOWARDS PODDE; THE FO COMMENTED THAT THE FMS DIDN'T SEEM TO BE GUIDING AUTOPLT TOWARDS PODDE VERY WELL. THE ACFT SEEMED TO WANDER .2 TO .3 MILES TO EITHER SIDE OF THE COURSE. ABOUT THIS TIME WE GOT A 'POSITION UNCERTAIN' MSG ON THE FMS; AND; AS I WENT TO THE DATA PAGE TO CHK OUR POSITION; THE MSG WENT OUT AND THE ACFT SEEMED NOW TO BE ACCURATELY TRACKING TO PODDE. IT MADE THE CORRECT TURN TO COURSE AT PODDE AND BY THIS TIME; I WAS IN THE 'GREEN WORLD;' BACKING UP THE FMS WITH CONVENTIONAL NAVIGATION. AS WE CONTINUED W; WITH EVERYTHING WORKING NORMALLY; I TOOK A MINUTE TO CALL BACK TO DEP TO TELL HIM ABOUT OUR ONE MEL'D FMS; AND THAT WE HAD HAD A POS PROB WITH OUR REMAINING FMS AFTER WE HAD TALKED TO HIM; AND THAT IT WAS PROBABLY RELATED TO THE TURN PROB WE HAD HAD NEAR GVINE. NO FURTHER PROBS WERE ENCOUNTERED AND THE REMAINDER OF THE FLT WAS UNEVENTFUL. PERHAPS WE SHOULD CONSIDER NOT FLYING AN RNAV DEP WITH ONLY ONE FMS WORKING. SUPPLEMENTAL INFO ACN 687396: WHEN YOU EXECUTE A GPS APCH THE GPS RAIM CAPABILITY IS VERIFIED TO BE SURE IT CAN SEE ENOUGH SATELLITES TO TRIANGULATE THE ACFT POSITION. NO SUCH VERIFICATION IS REQUIRED FOR THE RNAV DEP.

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.