AN MD80 CREW WHILE FLYING THE PDZ ARR TO LAX EXPERIENCED AN FMS NAV ERROR.

Narrative

LOCATION: PDZ 4 ARR; LNDG RWY 25L. AT TEJAY; APCH CALLED TFC AHEAD OVER ARNES. WHILE LOOKING FOR THE TFC; I NOTICED WE HAD TURNED DIRECT HUNDA FROM TEJAY. I DIRECTED THE FO TO COME R AND I PUT ARNES BACK IN. FO TURNED R TO ARNES AND WISELY WENT TO RAW DATA TO INTERCEPT THE LOC. UNFORTUNATELY; THE TURN TO ARNES SET UP FOR AN OVERSHOOT OF THE RWY 25L LOC. THE FO CONTINUED THE L TURN TO REINTERCEPT. APCH ASKED US TO CONFIRM REINTERCEPTING. NO LOSS OF SEPARATION WAS NOTED. APPARENTLY THE FO HAD SELECTED THE ILS RWY 25L ON THE ARR PAGE OF THE GFMS SOMETIME ON THE ARR. THIS CAUSED THE DELETION OF KEY ARR POINTS. IT RE-EMPHASIZES THE NEED TO VERBALIZE ALL CHANGES TO THE PREDEP ROUTING SO THAT BOTH CREW MEMBERS CONCUR WITH THE NEW ROUTING. THE FO WAS NOT AWARE OF THE CORRUPTION OF THE ARR RTE CAUSED BY THE SELECTION OF THE ILS. THE SHORT TERM SOLUTION IS NOT TO SELECT THE ILS. BUT IT HIGHLIGHTS WHAT I BELIEVE IS A PROB. ALL 4 LNDG RTES ON THE PDZ 4 GO OVER ARNES. SO WHY WOULD THE SELECTION OF ANY OF THEM CAUSE THIS POINT TO BE DELETED. THE SAME HOLDS TRUE FOR ARRS INTO DFW. SELECTION OF THE ILS CAN DELETE ALL ARR POINTS INSIDE THE FEEDER FIX. EVEN IF YOU ARE NOT GOING TO FLY OVER ALL OF THEM -- WHAT HARM IS DONE BY NOT AUTOMATICALLY DELETING THEM?