After checking the magnetos are functioning correctly, the next item on the check list is the CSU governor. The RPM is increased further into the prop governor range for cycling the prop; 2000 RPM is a normal figure to use. The prop lever is retarded into full coarse pitch where the extra blade drag will cause the RPM to rapidly drop. When this occurs, the prop control must be quickly and smoothly returned to the fine position. The RPM should then return to its original value to indicate the blades are moving freely throughout the entire pitch range and returning to the fine/flat pitch stop, ready for take-off. If the engine has just been started from cold, cycle the props three times; if the engine is warm, twice should be sufficient. Cold oil in the system, either from standing overnight or from flying at high altitude, will be too thick to operate the system smoothly and the prop may hunt, it will not maintain the required RPM. Hence the need to cycle the prop three times on the first start-up of the day to ensure the thin oil is flowing freely through the CSU. Therefore check oil temperatures are in the green before run-up. During the prop cycling, the prop should respond to the coarse pitch selection within 2-3 seconds of coarse pitch application. If it does not, it requires the attention of a power plant technician.
The next checklist item if the prop is of the feathering type, especially during the first run-up of the day, is to check the feathering system. After checking the magnetos and cycling the prop, the engine is throttled back to 1700 RPM or thereabouts depending on the type The prop pitch lever is then retarded to the feathering gate where initially there should be no change in the RPM; an RPM change indicates a faulty CSU. Retarding the prop pitch lever through the gate into the feathering position, results in an RPM drop due to propeller drag; the prop blades are at a 90 degree angle to the relative airflow when the aircraft is stationary. The RPM will drop to about 600-800 RPM and the sound of the engine changes from a steady hum to a throbbing sound as the blades turn into the feathered position. With this change in engine sound, the prop pitch lever is then returned to its fine/flat pitch position. The RPM should not be allowed to drop below 1000 RPM because this will place undue stress on the engine. In cold weather, the oil in the CSU may not initially run freely preventing a smooth PRM drop when performing the feathering check Repeat the procedure until the RPM drops evenly.
The term feathering is taken from boat rowing where the oar blade is turned parallel to the water’s surface as it is returned ready for the next rowing stroke.
During normal cruise flight, a power setting of 24 inches Hg manifold pressure and 2400 RPM would be a commonly used figure for modern light aircraft with a normally aspirated (non-turbocharged) engine. This power setting is known as ‘running the engine square’ or, ‘24 square’ for short. [Knock off the last two digits of the RPM number; in this example it is 2400 minus the last two zeros to equal 24]. If the manifold pressure is increased to a higher figure than the 2400 RPM the engine would be running ‘over square’.
Conversely, at a manifold pressure below 24" Hg the engine would be running ‘under square’. During the normal operation of a non-turbocharged engine, it is usual practice to run the engine either square or under square. Except for turbocharged engines, running the engine over square is not normally recommended for low time pilots new to constant – speed props.
Running some engines too far over square results in overboosting which can cause serious damage to the engine in the form of pre-ignition, detonation and high cylinder head temperatures. To avoid this problem a set sequence is recommended to increase or decrease power settings When increasing power, lead with the RPM/pitch control lever followed by the MP/throttle control. The procedure is reversed for decreasing power; reduce MP/throttle first followed by the RPM/pitch change. However, there is an exception to the rule, which will be discussed shortly.