MD80 FLT CREW PROGRAMS SOLDO RNAV SID FROM DFW MULTIPLE TIMES AND GETS DIFFERENT INITIAL DEP LEGS EACH TIME. CREW CANCELS THE RNAV SID AND REQUESTS THE DALLAS SID WHICH UTILIZES RADAR VECTORS VICE RNAV.

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FEEDBACK ON WAYPOINTS DROPPED FROM RNAV DEPS AT DFW. AT THE GATE; AFTER THE RTE WAS LOADED IN THE GPS; WE CHKED THE WAYPOINTS. RWY 35L WAS SELECTED FOR THE DEP. ON THE LEGS PAGE; THE 1100 FT POINT AFTER TKOF WAS NOT LISTED AND THE COURSE TO THE FIRST FIX WAS 358 DEGS. THE MAP SHOWED THE SAME DATA. I RE-LOADED THE SOLDO2; INCLUDING THE RWY; AND RECHKED THE LEGS PAGE. THIS TIME IT SHOWED A DEP OFF RWY 35L TO INTERCEPT THE 009 RADIAL. THE 009 RADIAL IS FOR A RWY 35C DEP. THE MAP MODE CONFIRMED THIS SETUP. I-RELOADED THE RNAV DEP AGAIN AND GOT A COURSE OF 354 DEGS TO THE FIRST FIX. ONCE AGAIN; THE MAP DISPLAY SHOWED THE COURSE WELL OFF THE R OF THE RWY 35L POSITION. THE 1100 FT FIX OFF THE END OF THE RWY DID NOT APPEAR IN ANY ATTEMPT. AFTER CHKING THE GPS PAGES FOR ERROR MESSAGES (THERE WERE NONE); I CONTACTED CLRNC DELIVERY AND CHANGED OUR DEP TO THE DAL8. EXCEPT FOR THE RNAV DEP; THE GPS APPEARED TO OPERATE NORMALLY. I MADE A LOGBOOK ENTRY AT DEST. NEITHER THE FO NOR I COULD ACCOUNT FOR THIS ANOMALY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR; WITH MORE THAN 10000 HOURS IN THE MD80; WAS VERY FORTHRIGHT IN HIS COMMENTS REGARDING THE FMS SYSTEM ON HIS COMPANY'S ACFT. HE FEELS THAT UNLIKE MORE MODERN ACFT IN WHICH THE FMS SYSTEMS ARE WELL INTEGRATED AND CONSISTENTLY RELIABLE; THE MD80 SYSTEMS ARE A 'MIX AND MATCH' OF ORIGINAL AND UPDATED SYSTEMS. HE STATES THERE ARE CONSISTENT ANOMALIES ABOUT WHICH EXPERIENCED USERS ARE AWARE AND FOR WHICH THEY TAKE PRECAUTIONS OR ARE ACCUSTOMED TO MAKING ACCOMMODATIONS. OF PARTICULAR INTEREST REGARDING THE RNAV DEP ANOMALIES; HE RELATED A RECURRING EXAMPLE WHICH MAY BE SUPPORTIVE OF ACN'S WHICH RPT 'WAYPOINTS DROPPING OR DISAPPEARING.' HE WAS ABLE TO DEMONSTRATE TO A COPLT THAT THIS DID; IN FACT; OCCUR. AFTER THE RTE IS INSTALLED IN THE FMS; IT IS HIS PRACTICE TO 'COPY' THAT RTE TO A SEPARATE 'FILE' IN THE FMS MEMORY. THE USE OF THIS 'STORED RTE' IS COMMONLY UTILIZED BY MANY PLTS FOR SUCH THINGS AS ENG OUT PROCS; ETC. THE RPTR USED THE 'RTE COPY' PROCESS TO DUPLICATE THE CLRED RTE BY WHICH THE ACFT WAS PROGRAMMED TO FLY. WHEN THEY ENCOUNTERED A LOSS OF WAYPOINTS HE WAS ABLE TO THEN PULL UP 'RTE TWO' AND VERIFIED THAT AT ONE TIME THE RTE DID IN FACT CONTAIN THE MISSING WAYPOINTS. IT IS THE RPTR'S OPINION THAT SUCH ANOMALIES; RARE IN MORE MODERN ACFT; ARE NOT A BIG SURPRISE TO EXPERIENCED MD80 PLTS. HE; ONCE AGAIN; FEELS SUCH PROBLEMS ARE LIKELY THE RESULT OF LESS PERFECTLY INTEGRATED COMPATIBILITY OF THE VARIOUS OLDER AND NEWER COMPONENTS OF WHICH THE MD80 SYSTEMS ARE CONFIGURED.