Incident Description
The pilot reported that smoke began entering the cockpit, impairing visibility to the point that he could hardly see the propeller. He stated that at that moment, the indicator light for the cabin door seal pressurization pump became illuminated. Believing that the pump had been operating continuously due to an air leak in the system, the pilot turned the master switch off to eliminate electrical power to the pump. After landing the airplane, the pilot located a fire in the upholstery and extinguished it.
Investigation Findings
Post-incident examination revealed that the aluminum housing of a resistor (inverter) in the cabin door seal pressurization pump, which was part of an STC modification, had melted and deformed. Further investigation determined that the pump unit had not been installed properly due to lack of adequate written instructions.
Additionally, the design of the unit was found to have several deficiencies: it did not include a pilot-controlled on-off switch, there was inadequate space for cooling during operation, there were insufficient procedural directions for when the amber caution light comes on, and there was no means to monitor the pump's operating time limitation of 10 minutes. It was also determined that a wrong amperage fuse had been installed in the pump system.