broken or shorted wires between the magneto and the plugs,  or improperly timed firing of the plugs. It should be noted  that “no drop” in rpm is not normal, and in that instance, the  aircraft should not be flown.  Following engine shutdown, turn the ignition switch to the  OFF position. Even with the battery and master switches  OFF, the engine can fire and turn over if the ignition switch  is left ON and the propeller is moved because the magneto  requires no outside source of electrical power. Be aware of  the potential for serious injury in this situation.  Even with the ignition switch in the OFF position, if  the ground wire between the magneto and the ignition  switch becomes disconnected or broken, the engine could  accidentally start if the propeller is moved with residual fuel  in the cylinder. If this occurs, the only way to stop the engine  is to move the mixture lever to the idle cutoff position, then  have the system checked by a qualified AMT.  Reciprocating engines use either a wet-sump or a dry-sump  oil system. In a wet-sump system, the oil is located in a sump  that is an integral part of the engine. In a dry-sump system,  the oil is contained in a separate tank and circulated through  the engine by pumps.  [Figure 7-17]  The main component of a wet-sump system is the oil pump,  which draws oil from the sump and routes it to the engine. After  the oil passes through the engine, it returns to the sump. In  some engines, additional lubrication is supplied by the rotating  crankshaft, which splashes oil onto portions of the engine.  An oil pump also supplies oil pressure in a dry-sump  system, but the source of the oil is located external to the  engine in a separate oil tank. After oil is routed through  the engine, it is pumped from the various locations in the  engine back to the oil tank by scavenge pumps. Dry-sump  systems allow for a greater volume of oil to be supplied to  the engine, which makes them more suitable for very large  reciprocating engines.  The engine oil system performs several important functions:  The oil pressure gauge provides a direct indication of the oil  system operation. It ensures the pressure in pounds per square  inch (psi) of the oil supplied to the engine. Green indicates  the normal operating range, while red indicates the minimum  and maximum pressures. There should be an indication of  oil pressure during engine start. Refer to the AFM/POH for  manufacturer limitations.  Figure 7-17.  Wet-sump oil system.  Oil Systems  •  Lubrication of the engine’s moving parts  •  Cooling of the engine by reducing friction  •  Removing heat from the cylinders  •  Providing a seal between the cylinder walls and pistons  •  Carrying away contaminants  Engine and  Accessory Bearings  200  245  I50  I00  75  °F  P  S  I  II5  60  I00  20  0  OIL  P  R  E  S  T  E  M  P  Oil cooler and filter  Oil pressure relief valve  Oil filler cap and dipstick  High pressure oil screen  Oil pump  Low pressure oil screen  Oil sump  Pressure oil from  oil pump  Sump oil and return oil from relief valve  7-16 