A PA44 ON LNDG FAILED TO EXTEND THE LNDG GEAR INCURRING DAMAGE TO BOTH PROPS AND 1 ENG.

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OUR FLT SCHOOL'S ONLY MULTI ENG AIRPLANE; A PIPER SEMINOLE; JUST CAME OUT OF 1 WK MAINT. DUE TO MY SCHEDULE AND UNAVAILABILITY OF THE SEMINOLE; WE ARE NOW PRESSED FOR TIME. THE ONLY TIME THE SEMINOLE IS AVAILABLE FOR US IS AFTER XA00 PM. SINCE MY STUDENT STILL NEEDS 5.5 HRS NIGHT TIME TO MEET THE COMMERCIAL REQUIREMENTS WE OPT TO DEPART AT XB30 PM. WE ARE BEING RADAR VECTORED FOR ZZZ NDB RWY 6 APCH. ON BASE LEG WE ARE AT OUR INITIAL ALT OF 1600 FT MSL. A COUPLE MINS BEFORE ATC GIVES US THE LAST HDG TO FLY AND INTERCEPT THE INBOUND COURSE I FAIL THE L ENG. IT TAKES MY STUDENT ABOUT 10 SECONDS TO GO THROUGH THE ENG FAILURE PROCS. THE L ENG NOW HAS 11 INCHES MANIFOLD PRESSURE SET FOR ZERO THRUST. OVER THE LOM (THE FAF) THE 1 ENG NON PRECISION APCH PROC IS PERFORMED: 1) FULL PWR AT 100 KTS. 2) INTERCEPTED 15 INCHES 100 KTS. 3) LNDG CHKLIST. 4) MDA G_MP (NOTICE THE 'U' IN GUMP IS MISSING. IF THE GEAR WERE LOWERED THIS WOULD CREATE DRAG CAUSING THE AIRPLANE TO DSND QUICKER AND POSSIBLY LAND SHORT OF THE RWY). 5) WHEN FIELD CAN BE MADE; GEAR DOWN. NEARING THE RWY; I TOLD MY STUDENT TO REMOVE HIS FOGGLES (SIMULATING HE JUST BROKE OUT). UPON LNDG; SCRAAAAPE. A NOISE I HAD NEVER HEARD BEFORE. SINCE THE NOSE WAS HIGH AND THE TAIL LOW; I DIDN'T THINK THE PROPS HIT. I SHOVED BOTH THROTTLES FORWARD; REMAINED IN GND EFFECT; LOWERED THE GEAR; 3 GREEN; THROTTLES TO IDLE; LANDED; AND TAXIED TO THE FBO TO INSPECT THE DAMAGE. STAYING UP AL NIGHT THINKING; IT DAWNED ON ME THAT THE GEAR WARNING HORN DID NOT SOUND. I KNOW THAT IT WORKS. AT 5000 FT WHEN I DEMONSTRATE A FULL FEATHER SHUTDOWN AND AIR START ON THE L ENG; TO VERIFY THAT IT IS THE L ENG THAT FAILED; THE L THROTTLE IS BROUGHT BACK TO IDLE AND THE GEAR WARNING HORN WILL SOUND. HOWEVER; THE GEAR WARNING HORN WILL NOT SOUND WHEN THE R THROTTLE IS BROUGHT BACK TO IDLE. ACCORDING TO THE SEMINOLE'S POH; 'A MICROSWITCH INCORPORATED IN THE THROTTLE QUADRANT ACTIVATES THE GEAR WARNING HORN UNDER THE FOLLOWING CONDITIONS: A) THE GEAR IS NOT LOCKED DOWN AND THE MANIFOLD PRESSURE HAS FALLEN BELOW 14 INCHES ON EITHER ONE OR BOTH ENGS....' AFTER READING THIS; IT APPEARS THE GEAR WARNING HORN SHOULD SOUND REGARDLESS WHICH THROTTLE IS BROUGHT BACK TO IDLE. THE POH SAYS 'A MICRO SWITCH' NOT 'MICRO SWITCHES' PLURAL. THIS CAN BE VERIFIED BY THE LNDG GEAR WIRING DIAGRAM ON THE NEXT PAGE. IT SHOWS ONLY 1 MICRO SWITCH FOR THE THROTTLE. OBVIOUSLY; THE L ONE AS PREVIOUSLY DISCUSSED. SITTING IN THE AIRPLANE ON THE GND YOU CAN HEAR THE SWITCH CLICK BY MOVING THE L THROTTLE. THERE IS NO CLICK ON THE R THROTTLE. BACK TO OUR LNDG. WE HAD 11 INCHES MANIFOLD PRESSURE ON THE L ENG. THIS IS BELOW THE 14 INCHES THE POH CALLS FOR. THE GEAR WARNING HORN AND LIGHT SHOULD HAVE ALERTED US THE GEAR WAS NOT DOWN. IF THE WARNING HORN AND LIGHT DID SOUND WE WOULD HAVE THE 1600 FT MSL AND APPROX 3 1/2 MINS OF A CONTINUOUS BRIGHT RED 'GEAR UNSAFE WARNING' LIGHT GLOWING IN OUR FACES AND THE GEAR WARNING HORN ITSELF TO REMIND US TO PUT THE GEAR DOWN. ACCORDING TO THE SEMINOLE'S POH WHEN 'THE MANIFOLD PRESSURE HAS FALLEN BELOW 14 INCHES ON EITHER...ENGS' THE GEAR HORN SHOULD SOUND. SO WHEN MY STUDENT PULLED THE R THROTTLE BACK TO LAND; THE GEAR WARNING HORN AND LIGHT SHOULD HAVE SOUNDED. BUT I DON'T THINK THIS IS POSSIBLE AS I BELIEVE THERE IS ONLY ONE SWITCH ON THE L THROTTLE AS PREVIOUSLY DISCUSSED. I THINK THERE SHOULD BE CLARIFICATION OF THE POH. IS THERE 1 OR ARE THERE 2 SWITCHES? IF THERE IS ONLY 1; I THINK THERE SHOULD BE AN AIRWORTHINESS DIRECTIVE FOR PIPER SEMINOLES TO HAVE A R THROTTLE SWITCH INSTALLED. IF THE GEAR HORN SOUNDED; WOULD WE HAVE PUT THE GEAR DOWN AND MADE A SAFE LNDG? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LNDG GEAR WARNING HORN AND THE RED LNDG GEAR UNSAFE LIGHT DID NOT OPERATE ON FINAL APCH. THE RPTR SAID THE AIRPLANE WAS CONFIGURED WITH THE LNDG GEAR UP AND LOCKED; FLAPS AT 10 DEGS AND L THROTTLE RETARDED TO LESS THAN 14 INCHES MANIFOLD PRESSURE. THE RPTR SAID IN THISCONFIGN THE LNDG GEAR WARNING SHOULD HAVE BEEN ACTIVATED. THE RPTR STATED IT APPEARED THE R THROTTLE SWITCH WAS MISSING BUT ON CHKING THE POH IT MENTIONS 'SWITCH' AND NOT 'SWITCHES.' THE RPTR SAID THE POH WIRE DIAGRAM SHOWS ONLY 1 SWITCH AND COMPANY MAINT CONFIRMS 1 SWITCH. THE RPTR SAID DAMAGE WAS INCURRED TO THE PROPS AND TO 1 ENG. THE RPTR STATED THIS AIRPLANE HAS NO METHOD OF SILENCING THE LNDG GEAR WARNING HORN.