CESSNA 650 CAPT UNKNOWINGLY CROSSED RWY TO WHICH HE WAS TO HOLD SHORT OF DURING TAXI OUT.

Narrative

WE WERE IN FRONT OF HANGAR ACROSS FROM THE CTL TWR. I CALLED FOR CLRNC TO TAXI AND WE WERE CLRED TO RWY 16 TO HOLD SHORT OF RWY 21. I READ THE CLRNC BACK AND THE CAPT STARTED TO TAXI. I CONTINUED WITH CHKLIST AND SETTING NAV IN THE FMS WHICH IS INSTALLED ON THE PEDESTAL AND REQUIRES KEEPING YOUR HEAD INSIDE THE COCKPIT. THE NEXT RADIO CALL WAS GND CTL ASKING IF WE REALIZED THAT WE HAD JUST CROSSED RWY 21. I WAS SHOCKED AND LOOKED UP AND REALIZED THAT WE HAD. I RESPONDED THAT I REALIZED THAT WE HAD. THE CAPT TOLD ME HE HAD NOT HEARD THE HOLD SHORT OF RWY 21 PART; ONLY TAXI TO RWY 16 AND THAT HE HAD LOOKED UP THE APCH PATH BEFORE XING. EVEN THOUGH BOTH THE CTLR SAID TO HOLD SHORT OF RWY 21 AND I HAD READ BACK TO HOLD SHORT OF RWY 21; THE CAPT DID NOT HEAR THIS. THIS WOULD NOT HAVE OCCURRED IF I HAD NOT HAD MY HEAD INSIDE THE COCKPIT ENTERING INFO IN THE FMS. CONTRIBUTING TO THIS WAS THE FACT THAT IT WAS A SUNDAY MORNING WITH ONLY 1 PERSON RUNNING GND; CLRNC; TWR AND WHO KNOWS HOW MANY OTHER THINGS. OTHERWISE GND CTL MAY HAVE NOTICED THAT WE DIDN'T LOOK LIKE WE WERE SLOWING TO HOLD SHORT AND MIGHT HAVE REMINDED US BEFORE WE CROSSED RWY 21.