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9/8/98 AC 43.13-1B classification of spark plugs according to their ability to transfer heat from the firing end of the spark plug to the cylinder head. (1) Spark plugs have been classified as “hot,” “normal,” and “cold.” However, these terms may be misleading because the heat range varies through many degrees of tem- perature from extremely hot to extremely cold. Thus the words “hot,” “cold,” and “normal” do not necessarily tell the whole story. (2) Since the insulator is designed to be the hottest part of the spark plug, its tempera- ture can be related to the pre-ignition and fouling regions as shown in figure 8-2. Pre- ignition is likely to occur if surface areas in the combustion chamber exceed critical limits or if the spark plug core nose temperature exceeds 1,630 °F (888 °C). However, fouling or short- circuiting of the plug due to carbon deposits is likely to occur if the insulator tip temperature drops below approximately 800 °F (427 °C). Since spark plugs must operate between fairly well-defined temperature limits, they must be supplied in various heat ranges to meet the re- quirements of different engines under a variety of operating conditions. FIGURE 8-2. Chart of spark plug temperature ranges. (3) From the engineering standpoint, each individual plug must be designed to offer the widest possible operating range. This means that a given type of spark plug should operate as hot as possible at low speeds and light loads and as cool as possible under cruising and takeoff power. Plug performance, therefore, depends on the operating tempera- ture of the insulator nose, with the most desir- able temperature range falling between 1,000 °F and 1,250 °F (538 °C and 677 °C). (4) Fundamentally, an engine which runs hot requires a relatively cold spark plug, whereas an engine which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which runs hot, the spark plug tip will be overheated and cause pre- ignition. If a cold spark plug is installed in an engine which runs cool, the tip of the spark plug will collect unburned carbon, causing fouling of the plug. The principal factors gov- erning the heat range of aircraft spark plugs are: (a) the distance between the copper sleeve around the insulator and the insulator tip; (b) the thermal conductivity of the insulating material; (c) the thermal conductivity of the electrode; (d) the rate of heat transfer between the electrode and the insulator; (e) the shape of the insulator tip; (f) the distance between the insulator tip and the shell; and (g) the type of outside gasket used. (5) “Hot” plugs have a long insulator nose; thereby, creating a long heat transfer path, whereas “cold” plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head. (See figure 8-3.) Par 8-15 Page 8-9PDF Image | AFS-640
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