Multiple aircraft experienced significant engine malfunctions, including high EGT, rough running, and power loss, directly attributed to spark plug failures.
Common spark plug defects identified were internal resistor failure leading to excessive resistance or open circuits, insulator failure, loss of center electrodes, and complete plug departure from the engine.
These failures resulted in serious consequences such as preignition causing piston damage, magneto failures, and inadequate engine performance, with some defective plugs passing standard inspection tests.
The pilot noticed a high EGT reading. A magneto check revealed one of the eight spark plugs was not firing. Examination revealed resistance in five of the eight plugs exceeded the 2 MΩ limit. The internal resistors had failed and the current path was open. The resistors should have been about 1.5 kΩ.
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