AC 43.13-1B - Engines, Fuel, Exhaust, and Propellers - Engines

FAA Advisory Circular 43.13-1B

AC 43.13-1B CHG 1

9/27/01

FIGURE 8-6. Typical carbon-fouled spark plug.

b. Fused Electrodes. There are many dif-

ferent types of malfunctions which result in

fused spark plug electrodes; however, most are

associated with pre-ignition either as the cause

or the effect. For this reason, any time a spark

plug is found with the following defects, fur-

ther investigation of the cylinder and piston

should be conducted.

(1) Occasionally, the ceramic nose core

will crack, break away, and remain trapped be-

hind the ground electrode. This piece of insu-

lation material will then buildup heat to the

point it will ignite the fuel/air mixture prema-

turely. The high temperatures and pressures

encountered during this condition can cause

damage to the cylinder and piston and ulti-

mately lead to fusing and shorting out of the

plug. (See figure 8-6c.)

FIGURE 8-6a. Typical lead-fouled spark plug.

(2) Corrosive gases formed by combus-

tion and the high voltage spark have eroded the

electrodes. Spark plugs in this condition re-

quire more voltage to fire—often more than

the ignition system can produce. (See fig-

ure 8-6d.)

c. Bridged Electrodes. Occasionally, free

combustion chamber particles will settle on the

electrodes of a spark plug and gradually bridge

the electrode gap, resulting in a shorted plug.

Small particles may be dislodged by slowly

cycling the engine as described for the oil-

fouled condition; however, the only remedy for

more advanced cases is removal and replace-

ment of the spark plug. This condition is

shown in figure 8-6e.

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