As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW OBSERVED SMOKE IN CABIN WITH HEATER ON CLIMB-OUT. THE PIC DECIDED TO ABORT THE FLIGHT AND RETURN TO STARTING POINT. UPON INVESTIGATION, IT WAS DISCOVERED THAT THIS WAS THE FIRST TIME THAT HEATER HAS BEEN TURNED ON SINCE THE SPRING MONTHS. LINT AND DUST WITHIN THE HEATER CAUSED THE BURNING SMELL. THE ASSISTANT CHIEF PILOT SAID THAT THIS IS COMMON WITH ALL KING AIR 90'S WITH AN ELECTRIC HEATER.