CLOSE PROX GA SMA TAXIING INTO POS AND A SECOND SMA XING THE THRESHOLD FOR LNDG.

Narrative

ON THE RAMP AT NAPA ARPT; WAS CLRED TO TAXI TO RWY 18L. WITHIN A FEW MINS WAS TOLD TO TAXI TO RWY 24 AS TWR WAS CHANGING RWYS. TAXIED TO RWY 24 RUNUP AREA AND CHKED OUT ACFT; THEN ADVISED TWR I WAS 'READY FOR TKOF RWY 24; REQUESTING L DOWNWIND DEP TOWARD SACRAMENTO'. TWR CLRED ME FOR TKOF. STARTED TO TAXI INTO POS FOR TKOF WHEN AN SMA Y CAME OVER THE RWY THRESHOLD AND LANDED 150 YARDS DOWN THE RWY FOR A TOUCH AND GO. NEVER HEARD ANY COM BTWN TWR AND SMA Y. TWR THEN TOLD ME TO HOLD POS; THEN WHEN THE SMA Y WAS AIRBORNE; TWR CLRED ME FOR TKOF. TWR AND I DID NOT DISCUSS INCIDENT (END OF STORY). WHILE TECHNICALLY THERE DOES NOT SEEM TO BE FAULT ON THE PART OF THE PLT; A MORE CAREFUL SURVEY OF APCH PATH BEFORE BEGINNING TO TAXI INTO POS FOR TKOF WOULD PRESUMABLY HAVE REVEALED THE APCHING SMA Y. THE SUBMITTER IS VERY CONSCIOUS OF THIS PRECAUTIONARY 'LOOK SEE' BUT MAY HAVE BEEN LULLED BECAUSE RWY IN USE HAD JUST BEEN CHANGED AND NO TWR-TO-SMA Y COMS WERE HEARD. THEREFORE; ASSUMED THAT IT WAS OK TO TAXI INTO POS FOR TKOF PER CLRNC FROM TWR.