Narrative:

We departed dfw using the CLARE2 RNAV departure. Looking at the departure; when departing runway 17R it is about a 180 degree heading to trexx; then calls for a right turn after trexx to 098 degrees. About 4 miles before we got to trexx; the FMS began calling for a turn to intercept the 098 degree course. While I was hand flying the airplane; I started to follow the FMS. The FMS hesitated and went back to a south heading; and we intercepted the course. According to ATC; we were more than 2 miles off our course and caused a separation issue with another flight that departed after us on a different departure. After realizing that the FMS showed the turn; we turned back to the right to intercept the course. We feel that this happened because the FMS is programmed to cut corners at fixes. Based on ground speed it calculates the rate of turn; and turns to join the new course. The turn was somewhere around a 90 to 100 degree course change and we feel the FMS prompted us to turn early because of such a sharp turn. As I was beginning the turn; I was suspicious that the turn was not 'right.' I had glanced at the FMS just before it turned and it showed 4.9 miles to trexx. The first officer was busy looking for an MD80 in front of us and cross checking speed for the 220 KTS required by the departure. Keep in mind that we have not been to dallas in a long time; and the RNAV departures are new to us. There was a lot going on flying a departure that we had thoroughly briefed on the ground; but were flying for the first time. I think that there should be a warning to pilot crews about the tendency for the FMS to lead the turn. The universal FMS have an overfly option; but I do not believe there is that option on the honeywell. In addition; I should have used the autoplt sooner so that I could monitor the aircraft better.callback conversation with reporter revealed the following information: reporter advised his air carrier has since experienced at least four similar anomalies on various RNAV departures from dfw. The air carrier has issued a bulletin prohibiting their flight crews from accepting the RNAV departures on company aircraft equipped with honeywell FMS systems. Other company aircraft equipped with universal FMS system have not experienced the problem. He noted that his initial thought regarding 'flyover fixes' was not germane because; as charted; all fixes on the clare departure are flyby; thus directing the FMS to start the turn at a point prior to the turn fix. The lead distance; however; should have been no more than the equivalent of ten percent of their IAS; or about 2.2 NM. In his case; the turn started at 4.9 NM prior to trexx.

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Original NASA ASRS Text

Title: E145 CAPT RPTS FMS STARTED TURN TOWARDS DOWN COURSE FIX TOO EARLY ON CLARE DEP FROM DFW. LOSS OF SEPARATION WITH OTHER RWY DEP RESULTS.

Narrative: WE DEPARTED DFW USING THE CLARE2 RNAV DEP. LOOKING AT THE DEP; WHEN DEPARTING RWY 17R IT IS ABOUT A 180 DEG HDG TO TREXX; THEN CALLS FOR A R TURN AFTER TREXX TO 098 DEGS. ABOUT 4 MILES BEFORE WE GOT TO TREXX; THE FMS BEGAN CALLING FOR A TURN TO INTERCEPT THE 098 DEG COURSE. WHILE I WAS HAND FLYING THE AIRPLANE; I STARTED TO FOLLOW THE FMS. THE FMS HESITATED AND WENT BACK TO A S HDG; AND WE INTERCEPTED THE COURSE. ACCORDING TO ATC; WE WERE MORE THAN 2 MILES OFF OUR COURSE AND CAUSED A SEPARATION ISSUE WITH ANOTHER FLT THAT DEPARTED AFTER US ON A DIFFERENT DEP. AFTER REALIZING THAT THE FMS SHOWED THE TURN; WE TURNED BACK TO THE R TO INTERCEPT THE COURSE. WE FEEL THAT THIS HAPPENED BECAUSE THE FMS IS PROGRAMMED TO CUT CORNERS AT FIXES. BASED ON GND SPD IT CALCULATES THE RATE OF TURN; AND TURNS TO JOIN THE NEW COURSE. THE TURN WAS SOMEWHERE AROUND A 90 TO 100 DEG COURSE CHANGE AND WE FEEL THE FMS PROMPTED US TO TURN EARLY BECAUSE OF SUCH A SHARP TURN. AS I WAS BEGINNING THE TURN; I WAS SUSPICIOUS THAT THE TURN WAS NOT 'RIGHT.' I HAD GLANCED AT THE FMS JUST BEFORE IT TURNED AND IT SHOWED 4.9 MILES TO TREXX. THE FO WAS BUSY LOOKING FOR AN MD80 IN FRONT OF US AND CROSS CHKING SPD FOR THE 220 KTS REQUIRED BY THE DEP. KEEP IN MIND THAT WE HAVE NOT BEEN TO DALLAS IN A LONG TIME; AND THE RNAV DEPS ARE NEW TO US. THERE WAS A LOT GOING ON FLYING A DEP THAT WE HAD THOROUGHLY BRIEFED ON THE GND; BUT WERE FLYING FOR THE FIRST TIME. I THINK THAT THERE SHOULD BE A WARNING TO PLT CREWS ABOUT THE TENDENCY FOR THE FMS TO LEAD THE TURN. THE UNIVERSAL FMS HAVE AN OVERFLY OPTION; BUT I DO NOT BELIEVE THERE IS THAT OPTION ON THE HONEYWELL. IN ADDITION; I SHOULD HAVE USED THE AUTOPLT SOONER SO THAT I COULD MONITOR THE ACFT BETTER.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED HIS ACR HAS SINCE EXPERIENCED AT LEAST FOUR SIMILAR ANOMALIES ON VARIOUS RNAV DEPS FROM DFW. THE ACR HAS ISSUED A BULLETIN PROHIBITING THEIR FLT CREWS FROM ACCEPTING THE RNAV DEPS ON COMPANY ACFT EQUIPPED WITH HONEYWELL FMS SYSTEMS. OTHER COMPANY ACFT EQUIPPED WITH UNIVERSAL FMS SYSTEM HAVE NOT EXPERIENCED THE PROBLEM. HE NOTED THAT HIS INITIAL THOUGHT REGARDING 'FLYOVER FIXES' WAS NOT GERMANE BECAUSE; AS CHARTED; ALL FIXES ON THE CLARE DEP ARE FLYBY; THUS DIRECTING THE FMS TO START THE TURN AT A POINT PRIOR TO THE TURN FIX. THE LEAD DISTANCE; HOWEVER; SHOULD HAVE BEEN NO MORE THAN THE EQUIVALENT OF TEN PERCENT OF THEIR IAS; OR ABOUT 2.2 NM. IN HIS CASE; THE TURN STARTED AT 4.9 NM PRIOR TO TREXX.

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.