C172 INSTRUCTOR AND STUDENT FAILED TO SEE ANOTHER C172 ON FINAL PRIOR TO TAXIING INTO POS ON RWY AT AN UNCTLED ARPT RESULTING IN THE AIRBORNE C172 MAKING A GAR.

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ACFT #1 ON DUAL TRAINING FLT WITH PRE-SOLO STUDENT. WE STOPPED ON TXWY SHORT OF HOLD LINE TO REVIEW TFC CLRING; RADIO AND XWIND TKOF PROCS. AFTER VISUALLY CLRING DOWNWIND; BASE; AND (I THOUGHT) FINAL APCH AREAS; I HAD STUDENT MAKE 'DEP' RADIO CALL ON CTAF; 122.7; AND TAXI ONTO RWY 28 FOR TKOF. STUDENT PLT STOPPED MOMENTARILY ON RWY AFTER GETTING ALIGNED TO CHK ACFT CONFIGN. AT THAT TIME; I NOTICED ACFT #2 OVERHEAD ABOUT 100 FT R AND 500 FT ABOVE THE RWY; APPARENTLY ON A GAR. NEITHER I NOR THE STUDENT EITHER SAW THIS ACFT OR HEARD A RADIO CALL ANNOUNCING ITS POS OR INTENTIONS PRIOR TO THAT TIME. WE OBSERVED THE SECOND ACFT RE-ENTER THE PATTERN AND LAND AFTER WE HAD DEPARTED THE RWY FOR OUR HOME BASE. WE DID NOT HEAR ANY RADIOED POS RPTS THROUGHOUT THE PATTERN; LNDG AND EXIT FROM RWY. A CHK OF OUR COM RADIO IN USE DETERMINED THAT IT WAS PROPERLY TUNED; TURNED ON; AND VOLUME UP AND IT CONTINUED TO FUNCTION NORMALLY ON OUR RETURN TRIP TO THE CTLED ARPT THAT WAS OUR ORIGINAL DEST. THE PRIMARY CAUSE OF THIS OCCURRENCE APPEARS TO BE THAT NEITHER I NOR THE STUDENT REALLY PROPERLY CLRED FINAL APCH PRIOR TO TAXI ONTO THE RWY; ALTHOUGH I THOUGHT WE HAD. CONTRIBUTING FACTORS: HIGH WING CONFIGN OF C172 MAY HAVE BLOCKED IN PART FINAL APCH; CONCENTRATION ON INSTRUCTION AND CONVERSATION AT THAT POINT; PERHAPS AN UNCONSCIOUS RELIANCE ON RADIO POS RPTING BY OTHER TFC -- AS IS THE NORM AT THIS ARPT; AND THE DELAY ON THE RWY. IT IS ALSO POSSIBLE ACFT #2 WAS ON SAME TYPE OF 'MODIFIED' FINAL. I DO NOT KNOW IF IT HAD RADIO FAILURE OR TRIED TO MAKE POS RPTS. POSSIBLE AIDS IN PREVENTION; OTHER THAN MORE VIGILANCE ON MY STUDENT'S PART; MIGHT BE TO SLIGHTLY ANGLE TXWYS BY PERHAPS 10 DEGS SO ENTIRE FINAL APCH AREA AND AREA ON OTHER SIDE WOULD BE VISIBLE FROM HIGH WING ACFT. NORMALLY; I HAVE STUDENTS STOP AND ANGLE TO RWY SO THIS IS POSSIBLE; BUT DIDN'T DO SO THIS TIME. ANOTHER THOUGHT WOULD BE TO MOVE 'STOP LINE' TO THE CORNER OF THE TXWY TO FORCE THIS TO HAPPEN.