BRYCE CANYON, UT, USA
N731NX
Cessna P210N
The pilot said that smoke began coming into the cockpit and 'I could hardly see the propeller.' He stated that 'at that point, we discovered the indicator light for the cabin door seal pressurization pump was on.' The pilot believes that it had been operating continuously due to an air leak in the system. In order to eliminate electrical power to the pressurization pump, the pilot turned the master switch off. After landing the airplane, the pilot located a fire in the upholstery, which he extinguished. Postincident examination revealed that the aluminum housing of a resistor (inverter) in the cabin door seal pressurization pump (an STC modification) had melted and deformed. Further investigation revealed that the pump unit had not been installed properly due to lack of adequate written instructions. Additionally, the design of the unit did not include a pilot controlled on-off switch, adequate space for cooling during operation, sufficient procedural directions for when the amber caution light comes on, and there was no way to monitor the pumps operating time limitation (10 minutes). It was also determined that the wrong amperage fuse had been installed in the pump system.
On October 15, 1997, approximately 1036 mountain daylight time, a Cessna P210N, N731NX, received minor damage due to an airborne fire while in cruise flight near Bryce Canyon, Utah. The commercial pilot and his passenger were not injured. The airplane was operated by the Mesa County Sheriff's Department in Grand Junction, Colorado, under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the cross-country public use flight which originated from Grand Junction, Colorado, approximately 60 minutes before the incident. A VFR flight plan had been filed. According to the pilot, he was cruising at 16,500 feet msl and had just passed Hanksville, Utah, when smoke began coming into the cockpit. The pilot reported that the smoke increased to where "I could hardly see the propeller." The pilot said that "at that point, we discovered the indicator light for the cabin door seal pressurization pump was on." In order to eliminate electrical power which was powering the pressurization pump, the pilot turned the master switch off. The smoke continued to come into the cockpit and the two occupants reported that they used paper towels to filter the air they were breathing. The pilot said that when he began descending to a lower altitude, he noticed "the airspeed was indicating above the red line." He said that he "changed the power and the pitch to slow the aircraft up with no change in the indicated airspeed." After the pilot landed the airplane, he noticed that the upholstery was burning. The pilot proceeded to remove the circuit breaker panel and pull the inside paneling out. He reported observing visible flames which he extinguished with the airplane's seat-mounted fire extinguisher. Postincident examination revealed that the aluminum housing of a resistor (inverter) in the cabin door seal pressurization pump (an STC modification) had melted and deformed (aluminum melts at 900 F. to 1200 F.). According to the NTSB's Systems Group Chairman's Factual Report, there are two resistors, which are connected in series and located adjacent to one another, which reduce the 28-volt aircraft power supply to the 14-volt air pump motor. Each of the resistors has an attached mounting tab. The tabs facilitate the mounting of the resistors onto a "metal plate" for dissipation of heat by conduction, as per the installation instructions. Dissipation of the heat is required because wire-wound, high-power resistors of this type generate a significant amount of heat due to the current that passes through them. The attached mounting tabs on the two resistors did not exhibit any score marks, indicating that no mounting screws were installed at the time of the fire. The installation instructions for the Bob Fields electric inflatable door seal pump state: "be sure to mount the resistors pak to a metal plate to make a heat sink, this plate and resistors can be mounted at the parking brake support angle under the instrument panel." No other verbiage is found in the instructions related to the resistor mounting. Test data from the vendor indicates that resistor housing temperatures may rise in excess of 600 degrees F. if the resistor is not mounted to conductive material for heat dissipation. The manufacturer's design calls for the system to be installed behind the pilot's "kick panel." According to the NTSB's Systems Group Chairman's Report, the kick panel area is a confined space between the external skin of the airplane just forward of the left door and an upholstery panel in the cockpit. This confined space lacks an engineered means for ventilation to meet the requirements of a continuously operating electrical pump and its associated high-power resistors. Additionally, the design does not provide a means to manually turn the pump off. The pilot's only alternative is to turn off the airplane's entire electrical power via the master switch. The design does incorporate an amber light which illuminates when the pump is operating. According to the manufacturer's instructions, a placard is supposed to be placed near the light which states: "CAUTION/DOOR SEAL PUMP ON." However, no information is provided by the manufacturer to instruct the pilot as to what actions must be taken when the light illuminates. The manufacturer of the air pump motor, which was used in the electrical door seal pressurization system, stated that continuous use of the pressure supply module "should not exceed 10 minutes without stopping for 30 minutes." The module was originally intended to be used in an automotive cigarette lighter socket to inflate tires. Typical time required to inflate an automobile tire is about 5 minutes. However, in the application of pressurizing airplane door seals, a possible leak in the door seals or the lines that connect the pump to the door seals would enable the module to operate in excess of 10 minutes. The pilot stated that he examined the door seals and did not detect an air leak. He further stated that he believes that the door seal pump was running continuously due to some form of compromise in the air line running from the pressurization module to the door seals. Examination of the in-line fuse indicated that the fuse was not blown. Further examination revealed that a fuse rated for 10 amps had been installed in the system, which exceeded the 7.5 amp rated fuse specified by the manufacturer.
The failure of the resistor (inverter) in the door seal pressurization air pump due to continuous operation. Factors were a failure in the door seal pressurization system, its inadequate design by the manufacturer, its inadequate written instructions for installation and operation, and the installation of the improper electrical fuse by unknown persons.
Source: NTSB Aviation Accident Database
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