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