THE INSTRUCTOR PLT AND STUDENT PLT OF A C172RG FORGET TO PLACE THE GEAR DOWN DURING A PRACTICE ENG FAILURE; HIGH APCH; AFTER HAVING TO AVOID OPPOSITE DIRECTION TFC ON THE SAME RWY AT SPX; TX.

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SPX ARPT. I WAS DOING A REVIEW FLT IN PREPARATION FOR A COMMERCIAL CHKRIDE WITH A STUDENT. I REDUCED THE THROTTLE ON THE C172RG TO SIMULATE ENG FAILURE. WE WERE WITHIN PWR-OFF GLIDING DISTANCE FROM RWY 31 AND WINDS WERE CALM WITH NO ONE ELSE RPTING IN THE PATTERN. THE STUDENT PROMPTLY DID ALL THE CHKLIST ITEMS WHILE CTLING THE ACFT AND HEADING TOWARD THE RWY. THE STUDENT MADE A COMMENT ABOUT WAITING UNTIL 'LNDG ASSURED' TO PUT THE GEAR DOWN. EVERYTHING WAS LOOKING GREAT. WE WERE LINED UP WITH FINAL AND GLIDING IN. WE HAD BEEN MAKING RADIO CALLS AND THOUGHT THAT WE WERE THE ONLY ONES IN THE PATTERN. SUDDENLY WE SAW A SMALL BIPLANE WAS HDG DIRECTLY AT US. HE HAD TAKEN OFF ON RWY 13 WITH NO RADIO CALLS. AT THIS POINT; WE WERE ABOUT 1 MI OUT STILL AND SLIGHTLY HIGH. BOTH THE STUDENT AND I WERE STARTLED; AND I TOOK CTL OF THE ACFT. WE WENT AROUND THE BIPLANE AND CONTINUED WITH THE LNDG. WE WERE SLIGHTLY HIGH. AFTER ASKING THE STUDENT IF THEY COULD SLIP IT IN; I WAS ASKED TO DEMONSTRATE. I CONTINUED THE LNDG WITH A SLIP DOWN. I FAILED TO DOUBLECHK THE GEAR. I THOUGHT THE STUDENT HAD DONE IT. THE STUDENT THOUGHT I HAD DONE IT. WE WERE BOTH PREOCCUPIED WITH THE NEAR MISS AND THE SLIP. NEITHER OF US HEARD THE GEAR WARNING HORN BECAUSE ITS VOLUME WAS TOO LOW TO BE AUDIBLE. I WAS SO SURE THAT THE GEAR WAS DOWN; WHEN I HEARD A GRINDING NOISE; I THOUGHT IT WAS OUR WHEEL BEARINGS OR BRAKES. WHEN I REALIZED WHAT HAPPENED; I SHUT OFF ALL FUEL AND ELECTRICAL. A CHAIN OF EVENTS LED TO THIS INCIDENT; BEGINNING WITH BEING STARTLED BY THE UNEXPECTED TFC. THIS WAS FOLLOWED BY A BREAKDOWN IN COM BTWN THE STUDENT AND INSTRUCTOR ABOUT THE GEAR. THE FAULTY GEAR HORN SPEAKER ALSO PLAYED A ROLE; BUT IS NO EXCUSE. THANKFULLY; NO ONE WAS HURT AND DAMAGE TO THE ACFT WAS MINIMAL.