What was found
AFTER T/O CREW ADJUSTED TEMP CONTROL TO WARM UP CABIN. SHORTLY AFTER CABIN START’S TO FILL WITH SMOKE. TEMP WAS REDUCED, ATC NOTIFIED OF PROBLEM, QRH DRILL FOLLOWED. SMOKE CLEARED, A/C LANDED BACK IN 4 MM WITHOUT INCIDENT. MX INSPECTION REVEALED, INSULATION ON THE UNDERSIDE OF HEAT DUCT BENEATH FLOOR HAD ABSORBED HYD FLUID FROM A PREVIOUS HYD LEAK. TEMP INCREASE WITHIN DUCTING C CAUSED OIL SOAKED INSULATION TO EMIT A HAZE OR SMOKE CONDITION. HEAT DUCT WAS REPLACED AND FUNCTION TEST SERVICEABLE.