AIR TOUR PLT INVOLVED IN SINGLE PLT OP EXPERIENCES RWY INCURSION AT VGT.

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I WAS SCHEDULED FOR A FLT FROM THE N LAS VEGAS ARPT TO GRAND CANYON NATIONAL ARPT IN A PA 31-350. AFTER BOARDING THE PASSENGERS AND START UP I GOT THE TAXI CLRNC AND TAXIED TO RWY 07. WHEN I WAS READY I TAXIED UP TO THE HOLD SHORT LINE OF RWY 07 AND CONTACTED N LAS VEGAS TWR THAT I WAS READY FOR DEP. THE TWR CTLR GAVE ME THE CLRNC; 'ACFT X; CROSS RWY 07 TAXI TO RWY 12.' I MISINTERPRETED THIS CLRNC AS CROSS RWY 07 AND TAXI ONTO RWY 12 AND HOLD; SO I TAXIED INTO POS AND HELD ON RWY 12 AND SAW A TWIN OTTER (Y) ROLLING OUT THAT HAD JUST LANDED ON RWY 30. THE TWIN OTTER LEFT THE RWY AT INTERSECTION C WHICH IS ABOUT 2500 FT FROM THE NUMBERS OF RWY 12. THIS IS A RWY INCURSION INCIDENT. MAJOR REASON: MISUNDERSTANDING OF THE CLRNC BY THE PLT IN COMMAND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR NOTED THAT HE WAS CONDUCTING A SINGLE PLT OP; AND THAT WHILE HE WAS MONITORING COMPANY FREQ WHILE COMMUNICATING WITH LOCAL; HE WAS NOT DISTRACTED. HE ALSO NOTES A HIGH LEVEL OF ACTIVITY; AS THE VARIOUS AIR TOUR OPERATORS ALL DEPART AT ABOUT THE SAME TIME; BUT STATES THAT COMMUNICATION WAS ORDERLY. HE HAS FLOWN OUT OF THIS ARPT BOTH AS AN INSTRUCTOR AND AIR TOUR PLT FOR A SIGNIFICANT PERIOD OF TIME AND BELIEVES THAT COMPLACENCY ON HIS PART WAS THE PRIMARY CAUSE OF THE RWY INCURSION.