FLT CREW OF MD80 MISTAKENLY IDENT EDF AS HOU AND INITIATE VISUAL APCH. APCH ABANDONED WHEN MISTAKE REALIZED.

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FLT WAS CLRED FROM A R BASE FOR THE VISUAL TO RWY 30L AT HOUSTON HOBBY. THE HOBBY VOR (HUB) WAS OTS AND THE ACFT HAD A 2 NM POS INS ERROR. WE STARTED TO FLY TO ELLINGTON BEFORE EXECUTING A MISSED APCH BECAUSE BOTH THE CAPT AND FO THOUGHT IT DIDN'T LOOK RIGHT. WE ASKED FOR POS CONFIRMATION AND APCH VECTORS FROM ATC. THE LNDG WAS UNEVENTFUL. 1) TO PREVENT OTHER AIRCREWS FROM MISTAKING ELLINGTON FOR HOUSTON HOBBY WHILE LNDG RWY 30L; PLEASE PUT A CAUTIONARY NOTE IN OUR APCH ARPT PAGES. I DISCUSSED THIS IN GREAT DETAIL WITH HOU TWR AND THEY SAID ELLINGTON IS MISTAKEN FOR HOU RWY 30L ON A REGULAR BASIS. 2) THE S80 INS ACFT HAS AN FMC THAT DOES NOT ALLOW THE PLT TO VERIFY THE REASONABLENESS OF A NAVIGATIONAL FIX BEFORE EXECUTING THE FIX. THE ILS; DME; IUPU; RWY 20L DID NOT AGREE WITH THE FMC INFO. WHEN A FIX IS ENTERED INTO THE FMC; THE FMC SHOWS THE COURSE TO THE POINT; BUT NOT THE DISTANCE. IT IS THEREFORE NOT POSSIBLE TO CONTINUOUSLY MONITOR THE COURSE AND DISTANCE TO A FIX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT STRESSED THE ISSUE HE FEELS EXISTS WITH THE FMS ON THESE ACFT. HE STATES IT DIFFERS IN FUNCTION FROM OTHER SYS WHEN A 'DIRECT TO' COMMAND IS INSERTED BUT NOT ACTIVATED. SPECIFICALLY STRESSING THE LACK OF 'DISTANCE TO' DISPLAY TO THE MODIFIED BUT UNEXECUTED WAYPOINT.