FLC OF MD80 RESETS THE FMS TO CHANGE FROM 20 MI TO 10 MI LEGS IN HOLDING PATTERN. THOUGH THEY FOLLOW CORRECT PROCS THE FMS DOES NOT CHANGE TO 10 MI LEGS.

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A CLASSIC AUTOMATION BITES YOU STORY. HOLDING AT MIGET INTXN ON J146; CLRED FOR 20 MI LEGS. CTLR TOLD US TO CHANGE LEG LENGTH TO 10 MI. I WENT TO HOLD PAGE ON FMS AND ENTERED 10 MI LEG LENGTH. IT ACCEPTED THE ENTRY AND THE EXECUTE KEY ILLUMINATED. I PUSHED THE EXECUTE KEY. I THEN CHANGED TO A LONGER RANGE DISPLAY ON THE NAV DISPLAY TO ENTER AN EXPECTED RTE FOLLOWING HOLDING. WE WERE VERY BUSY DECIDING HOW LONG WE COULD HOLD AND TALKING TO THE DISPATCHER TO SELECT AN ALTERNATE FOR POSSIBLE REFUELING. WHAT HAPPENED WAS THAT ALTHOUGH THE FMS ACCEPTED THE 10 MI LEG INPUT; IT DID NOT ACTUALLY FLY 10 MI LEGS BUT CONTINUED TO FLY 20 MI LEGS. I HAD ETG VOR TUNED TO DOUBLECHK THE FMS. MIGET INTXN IS 33 MI W ON J146. BY THE TIME WE FINISHED TALKING TO THE DISPATCHER AND I NOTICED THE DME; IT SAID 55; NOT 43; WHICH WOULD REPRESENT THE END OF THE OUTBOUND LEG OF A 10 MI LEG HOLDING PATTERN. I TURNED INBOUND IMMEDIATELY. NO OTHER ACFT WERE ON TCASII AND THE CTLR NEVER SAID ANYTHING TO US. HOWEVER; THE MISTAKE WAS MINE FOR NOT MONITORING THE FMS.