INSTRUCTOR WITH COMMERCIAL FLT STUDENT HAD NOSE GEAR COLLAPSE ON LNDG. PERFORMED ALL PROPER CHKS; LANDS.

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I WENT UP IN AN SMA WITH A STUDENT WHO WAS WORKING ON HER COMMERCIAL PLT'S CERTIFICATE AND NEEDED TIME IN A COMPLEX ACFT. AFTER SEVERAL TOUCH-AND-GOES; I TOOK THE CTLS FOR THE LAST LNDG. AT NO POINT DURING THE FLT DID WE MAKE A NOSEWHEEL LNDG NOR DID WE MAKE ANY EXCESSIVELY HARD LNDGS. I PUT THE GEAR DOWN ON DOWNWIND; AND CHKED THAT I HAD 3 GREEN DOWN AND LOCKED LIGHTS. I ALSO PERFORMED A GUMPS CHK ON FINAL; AGAIN SEEING THAT I HAD 3 GREEN. I TOUCHED DOWN MAIN WHEELS FIRST; AND BEGAN TO LET THE NOSE DOWN. I SAW THAT THE NOSE WAS DROPPING MUCH TOO FAR; SO I ADDED FULL PWR; RETRACTED 1 NOTCH OF FLAPS; AND EXECUTED A GAR. IT WAS NOT UNTIL THIS TIME THAT THE GEAR UNSAFE LIGHT CAME ON AND THE NOSE GEAR LIGHT WENT OUT. I NOTIFIED THE TWR OF THE PROB AND THEY ASKED US TO BUZZ THE TWR SO THEY COULD SEE IF THE NOSE GEAR WAS DOWN OR NOT. WE DID SO; AND THE TWR SHINED A LIGHT ON THE NOSE GEAR AND SAID IT APPEARED TO BE DOWN; BUT THEY COULDN'T TELL IF IT WAS LOCKED. WE CLBED TO 2500 MSL AND CIRCLED THE FIELD; AND PERFORMED THE EMER GEAR EXTENSION CHKLIST; AND RECYCLED THE GEAR. THE GEAR UNSAFE LIGHT NEVER WENT OUT; AND I NEVER GOT A GREEN LIGHT ON THE NOSE GEAR. (YES; I DID CHANGE THE BULBS.) WE BUZZED THE TWR TWICE MORE; AND ONE OF MY FELLOW CFI'S FROM OUR FLT SCHOOL CAME UP ON UNICOM (122.95) AND SAID IT APPEARED THAT THE NOSE GEAR WAS FULLY EXTENDED. (I WAS MONITORING TWR; AND TALKING ON 122.95; THEY WERE TALKING WITH SOME MECHS AND THE OWNER OF THE PLANE.) THE OWNER SAID TO GO AHEAD AND LAND THE PLANE; AND SINCE I HAD DONE EVERYTHING I AND THE PEOPLE ON THE GND COULD THINK OF; DID SO. THE NOSE GEAR DID HOLD UP ON THIS LNDG; AND WE TAXIED BACK TO THE RAMP. UPON SHUTDOWN; I SAW THAT THE PROP HAD HIT THE GND; EVIDENTLY ON THE GAR. IT WAS DETERMINED THAT THE RECEPTACLE FOR THE DOWN AND LOCKED PIN FAILED; AND THAT WHEN I TOUCHED DOWN THE NOSE GEAR COLLAPSED. THAT WAS WHEN THE PROP HIT; BUT I DID NOT KNOW THIS UNTIL WE SHUT THE ENG DOWN AFTER WE FINALLY GOT THE PLANE DOWN SAFELY. I DON'T KNOW WHY THE NOSE GEAR STAYED LOCKED ON THE LAST TOUCHDOWN; THE NOSE GEAR LIGHT NEVER DID COME ON AND THE UNSAFE LIGHT REMAINED ON. I AM JUST GLAD THAT THE NOSE GEAR HELD THE LAST TIME AND THAT NO ONE WAS HURT.