SENECA PLTS HAVE A GEAR-UP LNDG AT ZZZ.

Narrative

ON OCT/SUN/02; I WAS ACTING AS SAFETY PLT/INSTRUCTOR IN A SENECA. THE PVT PLT MULTI-ENG AND ME (SITTING IN THE R SEAT) WERE DOING R TFC PATTERNS AT ZZZ (A NON-CTLED ARPT). AT THE SPECIFIED DAY; VMC PREVAILED AND WE WERE OPERATING UNDER THE PROVISIONS OF 14 CFR PART 91. AFTER THE COMPLETION OF 4 PATTERNS WITHOUT ANY PROBS; DOING THE CHKLIST AND ANNOUNCING OUR POS ON THE CTAF AT THE APPROPRIATE TIMES. WE CONTINUED FOR ANOTHER PATTERN; DURING R DOWNWIND; I NOTICED A PLANE HOLDING SHORT OF RWY. THE PLT CONTINUED AS NORMAL FOLLOWING THE PROCS WHILE I ANNOUNCED OUR POS. AS WE TURNED FINAL; THE OTHER PLANE TAXIED INTO POS WITHOUT ANNOUNCING ANYTHING ON THE CTAF. I TOLD THE PLT THAT WE MIGHT HAVE TO DO A GAR. I ANNOUNCED OUR POS ANOTHER TIME WHILE I SAW THE PLT BRINGING THE PROPS FULL FORWARD AND DOING A LAST CHK. SINCE I WAS PREOCCUPIED WITH THE TFC AND WITH THE POSSIBILITY OF HAVING TO DO A GAR; I DIDN'T CONFIRM THAT WE HAD THE 3 GREEN LIGHTS AND 1 IN THE MIRROR; SOMETHING I ALWAYS WANT THE PLT TO SAY (OR ME) AS PART OF A SHORT FINAL. MEANWHILE; THE OTHER PLANE WAS ON THE ROLL AND ROTATED WITHOUT ANNOUNCING ANYTHING. SINCE I AND THE PLT WERE IN THE ASSUMPTION THAT THE GEAR WAS DOWN; WE CONTINUED TO MAKE A FULL STOP LNDG. THE PLT STARTED WITH THE ROUND OUT AND EVERYTHING LOOKED LIKE IT WAS GOING TO BE A NICE LNDG; THERE WAS NO GEAR HORN WARNING TO INDICATE THAT THE GEAR WASN'T LOCKED. THE PLANE WAS TRYING TO SETTLE DOWN AND NEXT THING I HEARD WAS THE NOISE OF A SOFT IMPACT THAT I THOUGHT (INITIALLY) MIGHT BE THE GEAR (LOOKING BACK I THINK IT MIGHT HAVE BEEN THE STEP THAT WE HIT FIRST). WE BOTH KEPT THE NOSE OFF THE GND AS LONG AS POSSIBLE AND TRIED TO MAINTAIN DIRECTIONAL CTL WHILE THE UNDERSIDE OF THE PLANE WAS SLIDING OVER THE GND AND THE PROPS HIT THE RWY. WHILE WE WERE SLIDING AND TRYING TO KEEP SOME DIRECTIONAL CTL; I IMMEDIATELY TURNED OFF THE FUEL (MIXTURE IDLE CUT-OFF PLUS FUEL SELECTORS OFF) AND ASKED THE PLT TO TURN OFF THE MASTER SWITCH. I OPENED THE DOOR; AND AS SOON AS THE PLANE CAME TO A STOP; I TOLD EVERYONE TO LEAVE THE PLANE. THE CONCLUSION IS; NO MATTER HOW MANY HRS OF FLT EXPERIENCE YOU HAVE; AND EVEN THOUGH WE ARE BOTH QUALIFIED TO FLY THAT PLANE; ALWAYS SAY THE MAGIC WORDS: 3 GREENS; 1 IN THE MIRROR (NOSEWHEEL). AS LONG AS THAT ISN'T CONFIRMED; EITHER CONFIRM IT OR GO AROUND WITHOUT GETTING PREOCCUPIED WITH DISTRACTING FACTORS.