An aircraft’s ignition system is the result of careful design and thorough testing. The ignition system usually provides reliable service when it is properly maintained and inspected. However, normal wear and deterioration can affect ignition system performance, particularly in magneto-ignition systems. Possible defects associated with magneto-ignition systems include deterioration of insulating materials, breaker point wear, corrosion, bearing and oil seal wear, and electrical connection problems.
Ignition timing requires precise adjustment to ensure that the following four conditions occur at the correct instant:
- The piston in the No. 1 cylinder must be positioned at the prescribed number of degrees before top dead center (BTDC) on the compression stroke.
- The rotating magnet of the magneto must be in the E-gap position.
- The breaker points must be just opening on the No. 1 cam lobe.
- The distributor finger must be aligned with the electrode serving the No. 1 cylinder.
If any of these conditions is not correctly synchronized with the others, the ignition system is improperly timed. Improper ignition timing causes the spark to occur at the wrong point in the engine cycle, resulting in reduced engine performance.
When ignition occurs before the optimum crankshaft position is reached, the timing is said to be advanced, or early. If ignition occurs too early, combustion pressure develops while the piston is still rising, opposing piston movement. Excessively advanced ignition timing can result in power loss, overheating, and an increased risk of abnormal combustion, including detonation.
When ignition occurs after the optimum crankshaft position, the timing is said to be retarded, or late. If ignition occurs too late, insufficient time is available for efficient combustion of the air-fuel charge, resulting in incomplete combustion. As a result, the engine loses power and requires a greater throttle opening to carry a given propeller load.
Moisture accumulation on various parts of the ignition system is a common cause of ignition system irregularities. Moisture can enter ignition system units through cracks or loose covers, or it can result from condensation.
“Breathing” can occur as pressure changes cause air to move into and out of ignition system components, potentially drawing moisture-laden air into the system. Ordinarily, engine heat is sufficient to evaporate this moisture, but occasionally the moist air condenses as the engine cools. The resulting moisture accumulation can reduce the electrical resistance of insulating materials.
Even a small amount of moisture contamination may reduce magneto output by allowing part of the high-voltage current intended for the spark plug to leak to ground. If moisture accumulation is significant, much or all of the magneto output may be dissipated to ground through flashover or carbon tracking.
Moisture accumulation during flight is uncommon because the system's high operating temperature helps prevent condensation. Problems caused by moisture accumulation are more likely to become evident during engine starting and ground operation.
Spark plugs are often incorrectly identified as the source of a malfunction when the actual problem exists elsewhere in the engine. Carburetor malfunctions, poor fuel distribution, excessive valve overlap, a leaking primer system, or incorrect idle speed and mixture settings can produce symptoms similar to those of an ignition system fault.
Replacing the spark plugs may temporarily improve some of these conditions, but the problem can soon recur if the underlying cause has not been corrected. A thorough understanding of the various engine systems, combined with careful inspection and proper maintenance practices, can help prevent such diagnostic errors.