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56                        U.S. CIVIL AERONAUTICS AUTHORITY
pronely the lightest in the engine. Constant and continued abuse can only lead to mechanical troubles and may even induce complete failure.
 Although most blower systems are equipped with a clutch to eliminate-inate as much of this strain as possible, such abrupt use is still a serious abuse and if continue will sooner or later result in trouble.
 The habit of smooth use of the throttle will eliminate all such hazards.
                                 WARMING UP THE ENGINE 
 This is a subject to which too few instructors and pilots pay sufficient-cient attention. All of the older engines requires that the oil tea-perature or water temperature be at a certain degree before proper operation could be depended upon, and many modern engines still require a warming-up period.
 Many air cooled engines are closely cowled and equipped with present-sure baffles which direct the flow of air to the proper places in sufficient quantities during flight. On the ground, however, insufficient air is forced around these baffles and through the cowling due to the design of propeller blades near the hubs, and any prolonged running causes serious overheating long before any indication of rising temperature is given by the oil temperature gauge.
 The recommendations o the engine manufacturer should be strictly followed if cracked heads, stuck rings, and the wrapping of other parts are to be avoided. Many modern engined are designed to be operated with very short warm-up periods, the initial sluggishness of the oil being taken care of in their design. A good rule to follow in these cases is to use a head temperature gauge religiously or to take off as soon as the engine can be eased open to full throttle without faltering or spitting. Where the latter procedure would be a serious abuse of the engine in some cases, it is an absolute necessity in others. Even in extremely cold weather the engine may overheat badly before any indication of such a condition shows on the oil must be preheated in cold weather if damage is to be avoided.
 Even though the student may be going to fly the simplest engine, these items should not be omitted from his instruction, for sooner or later he will fly this type of equipment and perhaps under circumstances that may prevent his learning of these facts before serious damage is done to expensive equipment.
 He should be impressed that the prolonged running of any engine at high speeds on the ground is a serious abuse. Competent engineers estimate that one hour's running at high speed on the ground imposes as much wear and tear on the engine as several hours of full throttle operation in the air. 
 The student should be taught that the engine is his best friend during his flying career and should be rested as such. Too students acquire the habit, mainly due to poor instruction and lack of understanding, of having only two positions for the throttle--closed and wide open. Wide open operation is a serious and needless abuse of the engine.