Narrative:

I was at FL200 en route to ZZZ to pick up passenger and moved the vent blower switch from automatic to high; as there was not much air coming out of the overhead cockpit vents. Doing this; however; did not increase airflow. I started to noticed a smoky smell; like an electrical fire; then I started feeling it when I could breathe in. At this point; I informed center that I was experiencing what I believed to be smoke in the cockpit and was going to turn back towards the city and requested a lower altitude. Once I made the turn back; I could see the smoke; so I turned the vent blower to automatic and the environmental switch to off. I also put on the oxygen mask and smoke goggles at this point and completed both the environmental smoke and electrical smoke checklists; with the exception of leaving the avionics on for a little bit. At this point; I declared an emergency and diverted to ZZZ1. I turned the avionics off momentarily to see if they were the problem at this point. Once they were off; there was no change in the smoke; so I turned them back on; but only 1 communication; 1 navigation; and the transponder. I made a normal landing at ZZZ1 and shut down without further incident. Having the smoke goggles in the BE90; even though not required; was a tremendous help during this event.

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Original NASA ASRS Text

Title: A BE90 AT FL200 DEVELOPED SMOKE IN THE COCKPIT AFTER THE PILOT SELECTED THE VENT BLOWER TO HIGH FROM AUTO. AN EMER WAS DECLARED FOLLOWED BY A LNDG AT A NEARBY ARPT.

Narrative: I WAS AT FL200 ENRTE TO ZZZ TO PICK UP PAX AND MOVED THE VENT BLOWER SWITCH FROM AUTO TO HIGH; AS THERE WAS NOT MUCH AIR COMING OUT OF THE OVERHEAD COCKPIT VENTS. DOING THIS; HOWEVER; DID NOT INCREASE AIRFLOW. I STARTED TO NOTICED A SMOKY SMELL; LIKE AN ELECTRICAL FIRE; THEN I STARTED FEELING IT WHEN I COULD BREATHE IN. AT THIS POINT; I INFORMED CTR THAT I WAS EXPERIENCING WHAT I BELIEVED TO BE SMOKE IN THE COCKPIT AND WAS GOING TO TURN BACK TOWARDS THE CITY AND REQUESTED A LOWER ALT. ONCE I MADE THE TURN BACK; I COULD SEE THE SMOKE; SO I TURNED THE VENT BLOWER TO AUTO AND THE ENVIRONMENTAL SWITCH TO OFF. I ALSO PUT ON THE OXYGEN MASK AND SMOKE GOGGLES AT THIS POINT AND COMPLETED BOTH THE ENVIRONMENTAL SMOKE AND ELECTRICAL SMOKE CHKLISTS; WITH THE EXCEPTION OF LEAVING THE AVIONICS ON FOR A LITTLE BIT. AT THIS POINT; I DECLARED AN EMER AND DIVERTED TO ZZZ1. I TURNED THE AVIONICS OFF MOMENTARILY TO SEE IF THEY WERE THE PROB AT THIS POINT. ONCE THEY WERE OFF; THERE WAS NO CHANGE IN THE SMOKE; SO I TURNED THEM BACK ON; BUT ONLY 1 COM; 1 NAV; AND THE XPONDER. I MADE A NORMAL LNDG AT ZZZ1 AND SHUT DOWN WITHOUT FURTHER INCIDENT. HAVING THE SMOKE GOGGLES IN THE BE90; EVEN THOUGH NOT REQUIRED; WAS A TREMENDOUS HELP DURING THIS EVENT.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.