RWY INCURSION AFTER LWOC BY A KC135 MAKING A VISUAL APCH TO WHAT HE THOUGHT WAS EGLIN AFB BUT WAS ACTUALLY AN AUX FIELD 10 NM N OF VPS; EGI; FL.

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A SINGLE SHIP KC-135 MIL ACFT ON A XCOUNTRY BUSINESS MISSION FROM EILSON AFB; FAIRBANKS; AK; TO EGLIN AFB; VALPARAISO; FL. EVENT HAPPENED DURING THE FINAL APCH AND DSCNT PHASE (AND LNDG PHASE). BEGINNING AT APPROX 300 NM N OF EGLIN AFB; MIL FLT BEGAN ALTERING COURSE TO AVOID NUMEROUS EMBEDDED TSTMS. MIL FLT WAS UNDER INTERMITTENT IMC/VMC UNTIL APPROX 15 MI N OF DEST (EGLIN AFB); AT WHICH POINT THE WX CLRED SIGNIFICANTLY TO VMC. PRIOR TO BREAKOUT FROM IMC; MIL FLT WAS ON AN EXTENDED DOGLEG TO FINAL FOR RWY 19; AND CLRED FOR ILS TO RWY 19 AT EGLIN. UPON SEEING THE RWY AT APPROX 1-2 O'CLOCK POS AND FLYING CLOSER TO ITS EXTENDED CTRLINE; MIL FLT ADVISED CTLR THAT HE WAS TURING FINAL TO INTERCEPT FINAL; NAV #1 AND #2 WERE BOTH TUNED TO THE ILS FREQ FOR RWY 19; BUT IDENT WAS SCRATCHY; WE THINK; DUE TO AN INTERCOM PROB THAT WAS SUBSEQUENTLY WRITTEN UP AFTER THE MISSION. (BY THE WAY; WE ABORTED THE SAME JET 2 DAYS LATER FOR THE SAME PROB. INTERCOM IN ICS MODE HAD ALL CREW MEMBERS CUTTING IN AND OUT AND READABLE ONLY APPROX 80% OF THE TIME.) L SEAT PLT WAS FLYING THE APCH AND NOTICED WHAT HE THOUGHT TO BE INOP ILS LOC COURSES. ON FINAL; MIL FLT WAS THEN TOLD TO RPT A 5 MI FINAL; AND TO FOLLOW A CESSNA ACFT ON BASE. UNKNOWN TO MIL FLT X AT THE TIME; MIL FLT Y WAS ON FINAL FOR RWY 19 AS WELL. MIL FLT X TOLD TWR HE DIDN'T SEE THE CESSNA AND (AFTER A PERIOD OF TIME; ASKED TWR IF CLRED TO LAND). TWR CLRED MIL FLT X TO LAND (BUT IN REALITY; WE THINK IT WAS MIL FLT Y THEY SAW ON FINAL; AN IDENTICAL KC-135 ACFT FROM OUR SAME SQUADRON. MIL FLT X THEN LANDED AND WAS TOLD TO TAKE A L TO EXIT THE RWY (BUT SINCE MIL FLT Y LANDED AT THE SAME TIME; WE THINK TWR SAW MIL FLT Y AS FLT X). BOTH ACFT; MIL X OR MIL Y; NOW AT SEPARATE AIRFIELDS; TURNED L TO EXIT THE RWY. AFTER SEVERAL MINS OF COM CONFUSION WITH TWR; IT WAS FINALLY SORTED OUT THAT MIL FLT X WAS NOT AT EGLIN; BUT INSTEAD AT EGLIN AUX. MIL FLT X THEN COORD FOR A VFR DEP FROM EGLIN AUX TO EGLIN; AND LANDED AT EGLIN UNEVENTFULLY. RELEVANT FACTORS (IN MY OPINION): 1) CREW FATIGUE - WE THINK THIS COULD HAVE BEEN A CONTRIBUTING FACTOR. (BECAUSE OUR SHOW TIME WAS XA00; AND BETTER REST I THINK WOULD HAVE HELPED. 2) PERCEIVED INOP LOC (IN REALITY; THE LOC WAS FINE BUT THOUGHT TO BE INOP); MORE INQUIRY (TO TWR; FOR EXAMPLE) TO GET AN UPDATE ON ITS STATUS WOULD HAVE HELPED); IE; MORE INQUIRY ON THE PART OF THE CREW AT THE PRECISE TIME THE PERCEIVED AMBIGUITY WAS DISCOVERED. 3) SIMULTANEOUS; SIMILAR ACFT; ON THE SAME APCH IN THE SAME RELATIVE POS ON FINAL LED TO FURTHER CONFUSION AND ULTIMATELY MIL FLT X BEING CLRED TO LAND.