A PA31 GEAR UP LNDG IS PREVENTED BY A TIMELY CALL ON THE RADIO FROM AN OUTSIDE OBSERVER AS THE ACFT DSNDED THROUGH 100 FT.

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DURING STRAIGHT-IN LNDG APCH; TWR ADVISED OF FLOATPLANE TFC ON BASE LEG FOR THE FLOAT POND WHICH IS PARALLEL TO THE RWY. I BEGAN LOOKING FOR THIS TFC; WHICH WOULD BE XING MY FINAL APCH PATH. AFTER EXTENDING APCH FLAPS; I BECAME CONCERNED THAT I DID NOT HAVE THE XING TFC IN SIGHT. THE TWR WAS FAIRLY BUSY; AND I DID NOT ASK FOR AN UPDATE ON THE XING TFC'S POS. EVENTUALLY; I SAW THE FLOATPLANE ON FINAL APCH TO THE FLOATPOND AND PREFERRED TO LAND. AT ABOUT 100 FT AGL; ANOTHER ACFT RADIOED; 'CHK GEAR' AND I THEN NOTICED THAT I HAD FAILED TO EXTEND THE LNDG GEAR. A GAR WAS COMMENCED; AND I RETURNED FOR A NORMAL LNDG. 1) THE 'BEFORE LNDG' CHKLIST WAS IN FRONT OF ME ON THE GLARE SHIELD; BUT I DID NOT COMPLETE THE CHKLIST DUE TO THE DISTR OF XING TFC. 2) I AM IN THE HABIT OF MAKING A 'PRE-FLARE' CHK OF THE LNDG GEAR POS LIGHTS. BECAUSE I WAS WARNED BY THE RADIO CALL FROM THE OTHER ACFT; I DON'T KNOW IF I WOULD HAVE CAUGHT THE ERROR BEFORE LNDG WITH THIS CHK. 3) IT IS COMMON KNOWLEDGE THAT THE LNDG GEAR WARNING SYS ON THE PA31 IS INEFFECTIVE IN PREVENTING GEAR UP LNDG ACCIDENTS. THE WARNING HORN IS ACTIVATED ONLY BY CLOSING THE THROTTLES TO NEAR IDLE. THIS DOESN'T NORMALLY HAPPEN UNTIL WELL INTO THE LNDG FLARE OR SOMETIMES AFTER THE WHEELS TOUCH (PWR ON LNDG). 4) HAVE FLOWN OTHER PA31'S THAT WERE EQUIPPED WITH AN AFTER-MARKET VOICE ANNUNCIATOR THAT GIVES A VOICE WARNING IF THE FLAPS ARE EXTENDED BEYOND THE APCH SETTING WITHOUT THE LNDG GEAR EXTENDED. 5) I REVIEWED SEVERAL NTSB ACCIDENT RPTS INVOLVING GEAR UP LNDGS IN PA31'S. THERE WERE SEVERAL CASES WHERE THE PLT WAS DISTR; FAILED TO COMPLETE THE BEFORE LNDG CHK; AND THE LNDG GEAR WARNING HORN FAILED TO SOUND. 6) THE PA31 IS NORMALLY FLOWN SINGLE-PLT IN COMMUTER SVC; USING AN AUTOPLT IN LIEU OF AN SIC. CONCLUSION: IN SPITE OF SEVERAL YR'S EXPERIENCE; GOOD TRAINING; AND CONSIDERABLE TIME IN TYPE; I NEARLY HAD AN ACCIDENT DUE TO DISTR. IN THE SINGLE-PLT ENVIRONMENT; THERE IS NO BACKUP 'LINE OF DEFENSE' (SIC) TO CATCH ERRORS. THE PLT IS ON HIS OR HER OWN. THIS; COMBINED WITH THE PA31'S INADEQUATE LNDG GEAR WARNING SYS; HAS RESULTED IN A NUMBER OF GEAR UP LNDGS. THE INCIDENT DESCRIBED HERE COULD HAVE BEEN AVOIDED IF THERE HAD BEEN AN SIC (GENERALLY CONSIDERED TO BE UNFEASIBLE IN THE INDUSTRY); OR IF THE AIRPLANE HAD BEEN EQUIPPED WITH THE AFTER-MARKET VOICE WARNING SYS DESCRIBED EARLIER.