DESPITE TRIPLE GUMPS CHK; SMA ACFT LANDED WITH GEAR RETRACTED. MECHANICAL INDICATORS SHOWING GEAR DOWN.

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MY INSTRUCTOR AND I STARTED OUT BY DOING MANEUVERS. WE THEN PROCEEDED TOWARDS LANCASTER ARPT. ABOUT 10 MI S OF THE ARPT WE ASKED FOR ARPT ADVISORY. UNICOM ANNOUNCED THAT THE WINDS WERE FROM THE S AND THEY WERE USING RWY 13. I ANNOUNCED THAT WE WOULD CROSS MIDFIELD AT 2000 MSL. I RPTED WHILE XING MIDFIELD AND TOLD LANCASTER TFC THAT I WOULD BE TURNING AROUND TO ENTER A L DOWNWIND ON A 45 DEG ANGLE FOR RWY 13. I RPTED WHEN WE WERE ESTABLISHED ON DOWNWIND. ABEAM THE NUMBERS ON THE RWY. I DID A GUMPS CHK WHILE SAYING IT OUT LOUD. GAS - ON FULLEST MAIN; UNDERCARRIAGE - DOWN BELOW 142. I LOOKED UP THE AIRSPD INDICATOR SAID 140 SO I SAID GEAR IS COMING DOWN. THE GEAR CAME DOWN; THE GREEN INDICATOR LIGHT WAS ON AND THE MECHANICAL POINTER WAS DOWN. (I ALSO FELT AND HEARD IT COMING DOWN.) I THEN PUT 10 DEG OF FLAPS AND RETRIMMED THE AIRPLANE. MIXTURE - WAS FULL RICH; PROPS - WOULD BE ON FINAL; AND SEATBELTS - WERE CHKED. WE CONTINUED DOWNWIND WAITING FOR A PLANE TO TURN ONTO FINAL AND WE TURNED BASE AFTER THE PLANE WAS BEYOND OUR L WING. WE TURNED BASE AND DID ANOTHER GUMPS CHK ALONG WITH VERBALLY SAYING IT OUT LOUD. UNDERCARRIAGE STILL INDICATED THAT IT WAS DOWN. ON FINAL DID ANOTHER GUMPS CHK AND THE UNDERCARRIAGE STILL INDICATED THAT IT WAS DOWN. PUT PROPS FULL FORWARD AND LAST NOTCH OF FLAPS. AT NO TIME DID I THINK THE UNDERCARRIAGE WAS NOT DOWN. BY THE TIME WE REALIZED THAT THE INDICATORS WERE WRONG IT WAS TOO LATE. LNDG GEAR HORN DID NOT COME ON. SPEAKING TO ANOTHER PLT; DURING AN INSTRUCTIONAL FLT; THE GEAR WAS LOWERED AND RETRACTED ON ITS OWN.