AN ALLEGED LNDG OVER MEN AND EQUIP WORKING ON A RWY FOR SNOW REMOVAL AT NIGHT WITH TWR CLOSED.

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ON DEC/XA/01 AT APPROX XA40; I WAS SHOOTING THE ILS RWY 18 INTO JAC. NIGHT CONDITIONS PREVAILED WITH LIGHT SNOW. THE CTL TWR WAS NOT IN OP. IT WOULD OPEN AT XB00. WE BROKE OUT ON THE ILS APPROX 500 FT AGL WITH 4-5 MI OF VISIBILITY. WE LANDED WITHOUT INCIDENT. UPON OPENING THE DOOR OF THE ACFT; THE LCL SHERIFF WAS THERE. HE TOLD ME THAT WE HAD LANDED RIGHT OVER TOP A MAN WORKING AT THE THRESHOLD ON A SNOW BLOWING MACHINE. THE CREW NEVER SAW ANY EQUIP ON THE RWY. THIS INFO IS STRICTLY SECONDHAND FROM THE SHERIFF. HE SAID THAT THE WORKERS HAD NOT HEARD MY CALLS ON THE CTAF FREQ. I TOLD HIM I GAVE POS CALLS ON THE LOC AT 18 MI; 8 MI; AND 4 MI OUT. THIS WAS ON THE JAC TWR/ADVISORY FREQ. I HAVE NO EXPLANATION WHY THEY DID NOT HEAR US. ZLC HAD TOLD ME OF MEN AND EQUIP ON TXWYS AND RWYS NOTAM PRIOR TO ME SWITCHING OVER TO CTAF. IF WHAT THE SHERIFF TOLD ME WAS CORRECT; A HAZARDOUS CONDITION EXISTED FOR THE ACFT. BECAUSE THE ARPT WAS OPEN OUR ACFT HAD A RIGHT TO LAND. I BELIEVE THERE SHOULD BE A WAY FOR AN ENRTE CTR TO CONTACT ARPT WORKERS OF AN APCHING ACFT. OBVIOUSLY; THIS SIT BEGS A BACK-UP FORM OF NOTIFICATION TO JUST THE CTAF.