The oil temperature gauge measures the temperature of oil.  A green area shows the normal operating range, and the red  line indicates the maximum allowable temperature. Unlike  oil pressure, changes in oil temperature occur more slowly.  This is particularly noticeable after starting a cold engine,  when it may take several minutes or longer for the gauge to  show any increase in oil temperature.  Check oil temperature periodically during flight especially  when operating in high or low ambient air temperature.  High oil temperature indications may signal a plugged oil  line, a low oil quantity, a blocked oil cooler, or a defective  temperature gauge. Low oil temperature indications may  signal improper oil viscosity during cold weather operations.  The oil filler cap and dipstick (for measuring the oil quantity)  are usually accessible through a panel in the engine cowling. If  the quantity does not meet the manufacturer’s recommended  operating levels, oil should be added. The AFM/POH or  placards near the access panel provide information about  the correct oil type and weight, as well as the minimum and  maximum oil quantity.  [Figure 7-18]  The burning fuel within the cylinders produces intense  heat, most of which is expelled through the exhaust system.  Much of the remaining heat, however, must be removed, or  at least dissipated, to prevent the engine from overheating.  Otherwise, the extremely high engine temperatures can lead  to loss of power, excessive oil consumption, detonation, and  serious engine damage.  While the oil system is vital to the internal cooling of the  engine, an additional method of cooling is necessary for the  engine’s external surface. Most small aircraft are air cooled,  although some are liquid cooled.  Air cooling is accomplished by air flowing into the engine  compartment through openings in front of the engine  cowling. Baffles route this air over fins attached to the engine  cylinders, and other parts of the engine, where the air absorbs  the engine heat. Expulsion of the hot air takes place through  one or more openings in the lower, aft portion of the engine  cowling.  [Figure 7-19]  The outside air enters the engine compartment through an  inlet behind the propeller hub. Baffles direct it to the hottest  parts of the engine, primarily the cylinders, which have fins  that increase the area exposed to the airflow.  The air cooling system is less effective during ground  operations, takeoffs, go-arounds, and other periods of high- power, low-airspeed operation. Conversely, high-speed  descents provide excess air and can shock cool the engine,  subjecting it to abrupt temperature fluctuations.  Operating the engine at higher than its designed temperature  can cause loss of power, excessive oil consumption, and  detonation. It will also lead to serious permanent damage,  such as scoring the cylinder walls, damaging the pistons and  rings, and burning and warping the valves. Monitoring the  flight deck engine temperature instruments aids in avoiding  high operating temperature.  Under normal operating conditions in aircraft not equipped  with cowl flaps, the engine temperature can be controlled  Figure 7-18.  Always check the engine oil level during the preflight  inspection.  Figure 7-19.  Outside air aids in cooling the engine.  7-17  Engine Cooling Systems  Fixed cowl opening  Baffle  Baffle Cylinders  Air inlet 