FLC OF A LEARJET 60; (LR60); MADE A VISUAL APCH TO THE WRONG ARPT CAUSING A CONFLICT WITH A HELI. AFTER RECOGNIZING THEIR MISTAKE; THE CAPT LANDED AT THE INTENDED ARPT WITHOUT FURTHER PROBS.

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WE WERE ON AN IFR FLT FROM OSH TO F54. WE WERE ON RADAR VECTORS FROM REGIONAL APCH (DFW) COMING IN FROM THE NE. APCH ASKED IF WE HAD THE ARPT IN SIGHT AND MY COPLT SAID THAT HE HAD IT AND I HAD IT ON E9S WITH THE EXTENDED RWY CTRLINE FOR RWY 16 ON MY FMS; EFIS. APCH CLRED US FOR THE VISUAL AFTER WE TOLD HIM WE HAD THE ARPT AT 10 O'CLOCK POS AND 7 MI. HE SAID THAT'S IT (APCH SAID) THE ARPT WE WERE ON THE 45 DEG TO FINAL LOOKED TO BE LIKE IT WAS TOO CLOSE TO BE 5 MI THAT THE FMS TOLD ME IT WAS; SO WE DECIDED TO GAR. WE ROLLED TO A 270 DEG HDG AND CLBED TO PATTERN ALT (APPROX 2100 FT MSL) AND LOCATED THE CORRECT ARPT ABOUT 9 O'CLOCK POS AND 4 MI. PLEASE NOTE THAT BOTH ARPTS ARE SINGLE RWY. BOTH LINED UP APPROX RWYS 16 AND 34; AND VERY CLOSE TO EACH OTHER. THE IRONIC THING IS GPM IS 4000 FT BY 75 FT AND F54 IS 5000 FT BY 100 FT AND GPM HAS A TWR; F54 IS UNCTLED. ANYWAY; OUR TCASII SHOWED NUMEROUS ACFT IN THE PATTERN AT F54; NONE THAT WE NOTICED AT GPM. BY THEN WE WERE ON THE UNICOM FREQ FOR F54 AND HEARD SEVERAL AIRPLANES ON XWIND; DOWNWIND AND FINAL FOR RWY 16 AT F54. WE CROSSED OVER THE FIELD HEADING WBOUND; ANNOUNCED OUR INTENTIONS TO CROSS BACK OVER THE FIELD (EBOUND) AND ENTERED L DOWNWIND FOR RWY 16. THERE WAS A CESSNA ON DOWNWIND INSIDE AND AHEAD OF US THAT GRACIOUSLY EXTENDED HIS DOWNWIND TO LET US LAND ON RWY 16 AHEAD OF HIM. WE LANDED WITHOUT FURTHER INCIDENT. WE DROPPED OFF OUR PAX AND WERE ABOUT TO DEPART FOR DAL; WHEN THE FBO MGR CAME OUT AND TOLD US THAT GPM TWR HAD CALLED AND WANTED TO TALK TO US. WE HAD BOTH ENGS RUNNING AND DECIDED TO TALK TO HIM ON TWR FREQ AND TRY TO EXPLAIN WHAT HAPPENED. HE SAID 'NO PROB; BUT THAT WE GOT KIND OF CLOSE TO A HELI W OF THE FIELD.' HE DIDN'T SAY HOW CLOSE. LESSON LEARNED; NEVER ACCEPT A VISUAL UNLESS CERTAIN OF CORRECT ARPT AND IF UNFAMILIAR; PLEASE APCH; LET US KNOW OF SIMILAR LOOKING ARPTS IN SUCH CLOSE PROX TO EACH OTHER.