CL604 FLT CREW FORGETS TO RESET ALTIMETER TO 29 PT 92 AT FL180 RESULTING IN 300 FT ALT DISCREPANCY.

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AFTER CRUISING AT FL180 FOR ABOUT 5 MINS; WE WERE ASSIGNED A LOWER ALT. JUST PRIOR TO AND DURING THE CLB AND LEVELOFF TO FL180; I WAS ATTEMPTING TO CONTACT THE FBO TO ARRANGE GND XPORTATION FOR OUR PAX WHILE THE PLT FLYING WAS CHKING THE ATIS AND SETTING UP THE FMS FOR THE APCH. AROUND THIS SAME TIMEFRAME; WE WERE GIVEN DIRECT TO MDW. AT NO TIME WHILE CRUISING AT FL180 DID ATC QUESTION US ABOUT OUR ALT. THERE WERE NO ACFT NEAR OUR ALT ACCORDING TO THE TCASII. LATER ON AT THE FBO; I SUDDENLY REALIZED THAT I HAD NOT SET MY ALTIMETER TO 29.92 FROM 30.22 WHEN WE WERE ASSIGNED FL180. THIS MEANT THAT WE WOULD HAVE BEEN ABOUT 300 FT OFF OUR ALT. I ASKED THE OTHER PLT IF HE HAD REMEMBERED TO CHANGE HIS ALTIMETER. HE SAID THAT HE ONLY REALIZED THE ERROR WHEN WE STARTED THE DSCNT FROM FL180. CONTRIBUTING FACTORS: 1) UNUSUAL CIRCUMSTANCE: IT IS EXTREMELY RARE THAT FL180 IS EVER ASSIGNED. TYPICALLY; WE ARE GIVEN A CLB OR DSCNT THROUGH FL180 AND WE ARE WELL AWARE THAT AN ALTIMETER CHANGE IS ALWAYS REQUIRED. WE ALSO HAVE THE BACKUP OF A FLASHING ALTIMETER WHEN WE PASS THROUGH FL180 AS A REMINDER. HOWEVER; WHEN WE LEVELED OFF AT FL180 FROM 16000 FT; THE ALTIMETER DIDN'T FLASH. THE UNDERLYING THOUGHT IS THAT WE; AS PLTS IN GENERAL; ARE PROGRAMMED TO CHANGE THE ALTIMETER WHEN PASSING THROUGH FL180. 2) WORKLOAD: THE PLT WAS GETTING ATIS AND SETTING UP FOR THE APCH AFTER RECEIVING A CLRNC DIRECT TO MDW. I WAS TALKING WITH THE FBO PERSONNEL ARRANGING GND XPORTATION FOR OUR PAX. THIS WAS ALL TAKING PLACE DURING THE CLB TO FL180. 3) FATIGUE: I HAD MISTAKENLY GOTTEN UP AT XA00 AFTER A FITFULLY SHORT NIGHT OF SLEEP BEFORE THE FLT. I WAS THANKFUL THAT THE OTHER PLT VOLUNTEERED TO FLY THIS LEG BECAUSE IT WOULD GIVE ME A CHANCE TO RELAX. 4) CRM FAILURE: THE VARIOUS TASKS NEEDING ATTN IN THE SHORT TIME REMAINING PRIOR TO OUR DSCNT WERE BEING SHARED BY BOTH PLTS IN ORDER TO GET THEM DONE. THE CREW SHOULD HAVE REMAINED FOCUSED ON THEIR NORMAL RESPECTIVE DUTIES. HUMAN PERFORMANCE CONSIDERATIONS: CREW FATIGUE RESULTING FROM THE EARLY MORNING DEP PROBABLY PLAYED A SIGNIFICANT ROLE IN THIS EVENT CONSIDERING BOTH PLTS ARE VERY EXPERIENCED IN JETS. ALTHOUGH THE CREW WAS SHARING THE WORKLOAD; GETTING THE ATIS AND SETTING UP THE FMS TOOK THE PLT'S FOCUS AWAY FROM FLYING THE AIRPLANE. THE COPLT WAS FOCUSED ON MAKING PAX XPORTATION ARRANGEMENTS INSTEAD OF ASSISTING THE PLT. IN SPITE OF THE FATIGUE; I BELIEVE THAT HAD WE FOLLOWED PROPER CRM; WE WOULD NOT HAVE COMMITTED THIS ERROR OR WOULD HAVE RECOGNIZED IT MUCH EARLIER IF WE HAD MADE THE ERROR.