Aircraft Systems

which is established by the aircraft or engine manufacturer. slightly higher power output. If one of the magnetos fails, the When evaluating the performance of the turbocharging other is unaffected. The engine continues to operate normally, system, be aware that if the manifold pressure begins although a slight decrease in engine power can be expected. decreasing before the specified critical altitude, the engine The same is true if one of the two spark plugs in a cylinder fails. and turbocharging system should be inspected by a qualified aviation maintenance technician (AMT) to verify that the The operation of the magneto is controlled in the flight deck system is operating properly. by the ignition switch. The switch has five positions: 1. OFF Ignition System 2. R (right) In a spark ignition engine, the ignition system provides a spark that ignites the fuel-air mixture in the cylinders and is 3. L (left) made up of magnetos, spark plugs, high-tension leads, and 4. BOTH an ignition switch. [Figure 7-16] 5. START A magneto uses a permanent magnet to generate an electrical With RIGHT or LEFT selected, only the associated magneto current completely independent of the aircraft’s electrical is activated. The system operates on both magnetos when system. The magneto generates sufficiently high voltage BOTH is selected. to jump a spark across the spark plug gap in each cylinder. The system begins to fire when the starter is engaged and the A malfunctioning ignition system can be identified during crankshaft begins to turn. It continues to operate whenever the pretakeoff check by observing the decrease in rpm that the crankshaft is rotating. occurs when the ignition switch is first moved from BOTH to RIGHT and then from BOTH to LEFT. A small decrease Most standard certificated aircraft incorporate a dual ignition in engine rpm is normal during this check. The permissible system with two individual magnetos, separate sets of wires, decrease is listed in the AFM or POH. If the engine stops and spark plugs to increase reliability of the ignition system. running when switched to one magneto or if the rpm drop Each magneto operates independently to fire one of the two exceeds the allowable limit, do not fly the aircraft until spark plugs in each cylinder. The firing of two spark plugs the problem is corrected. The cause could be fouled plugs, improves combustion of the fuel-air mixture and results in a Upper magneto wires Upper spark plugs Lower spark plugs Lower magneto wires 2 1 4 3 Left magneto Right magneto Figure 7-16. Ignition system components. 7-15