AN FMC EQUIPPED; NON GLASS B737-300 DEVIATED FROM AN SDF ARR ROUTING WHEN A WAYPOINT DROPPED FROM THE RTE OF FLT EITHER BY MACHINE MALFUNCTION OR CREW ERROR.

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WHILE FLYING DIRECT TO YOCKY INTXN TO FLY THE DARBY THREE ARR INTO SDF; THE FO WENT TO CHANGE THE RWY IN THE CDU FROM RWY 35R TO RWY 35L AT SDF. I WATCHED AS HE DID THIS AND HE CLOSED THE DISCONTINUITY ON THE LEGS PAGE. WHAT I DID NOT SEE WAS A POINT (UNCKL) ON THE ARR DROPPED OUT DURING THIS PROCESS. WE BOTH HAD CHKED THE RTE OF FLT AND THE ARR PRIOR TO CHANGING THE RWY AND UNCKL HAD BEEN ON THE LEGS PAGE. SINCE UNCKL HAD DROPPED OUT; THE ACFT STARTED GOING FROM YOCKY DIRECT TO DARBY. ATC CALLED US ON OUR ERROR AND WE MADE THE CORRECTION. ATC TOLD US THAT SEVERAL OF OUR ACFT HAVE MADE THIS SAME MISTAKE. SO MY QUESTION IS; WAS OUR ERROR TOTALLY SELF-INDUCED OR IS THERE AN ANOMALY IN THE FMS THAT WOULD CAUSE A POINT TO DROP OUT WHEN WE MADE THE RWY CHANGE? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ACFT HE WAS FLYING AT THE TIME WAS NOT GLASS PANEL EQUIPPED. HE REMEMBERS THAT SEVERAL YEARS AGO WHEN ANOTHER ARPT BEGAN ARRIVALS SIMILAR TO THIS; HIS ACR EXPERIENCED A NUMBER OF INCIDENTS IN WHICH WAYPOINTS DROPPED OUT OF FLT RTES. THE CAUSE OF THOSE PREVIOUS INCIDENTS WAS DETERMINED TO BE SOFTWARE. ATC'S STATEMENT ABOUT SEVERAL OF HIS ACR'S ACFT HAVING THE SAME DEV AS THEY JUST EXPERIENCED CAUSED HIM TO FILE THE RPT. ALTHOUGH HE COULD NOT SAY WITH ABSOLUTE CERTAINTY THAT HIS FO DID NOT MAKE AN INCORRECT SELECTION; HE WAS CERTAIN THAT THE LAST LOOK HE HAD AT THE CDU; THE WAYPOINT IN QUESTION WAS INCLUDED. IT COULD HAVE BEEN HUMAN ERROR; OR AS PREVIOUSLY EXPERIENCED; IT COULD BE SOFTWARE.