Reciprocating Engine Maintenance

10-13 Pilot Expander screw Stone Grinding Fluid hose Stone holder Spacing washers the spring to the specified height. This must be within the pressure limits established by the manufacturer. Refacing Valve Seats The valve seat inserts of aircraft engine cylinders usually are in need of refacing at every overhaul. They are refaced to provide a true, clean, and correct size seat for the valve. When valve guides or valve seats are replaced in a cylinder, the seats must be made concentric with the valve guide. Low power engines can use either bronze or steel seats. Bronze seats, although not widely used on current engines, are made of aluminum bronze or phosphor bronze alloys. Steel seats are commonly used for valve seats on higher powered engines and are made of heat-resistant steel with a layer of stellite steel alloy on the valve contact surface. Stellite seats can require a special stone to grind this very hard material.

Steel valve seats are refaced by grinding equipment. [Figure 10-16] Bronze seats are refaced preferably by the use of cutters or reamers, but they may be ground when this equipment is not available. The only disadvantage of using a stone on bronze is that the soft metal loads the stone to such an extent that much time is consumed in redressing the stone to keep it clean. The equipment used on steel seats can be either wet or dry valve seat grinding equipment. The wet grinder uses a mixture of soluble oil and water to wash away the chips and to keep the stone and seat cool; this produces a smoother, more accurate job than the dry grinder. The stones may be either silicon carbide or aluminum oxide.

Before refacing the seat, make sure that the valve guide is in good condition, clean, and does not have to be replaced. Mount the cylinder firmly in the hold down fixture. An expanding pilot is inserted in the valve guide from the inside of the cylinder, and an expander screw is inserted in the pilot from the top of the guide. [Figure 10-17] The pilot must be tight in the guide, because any movement can cause a poor grind. The fluid hose is inserted through one of the spark plug inserts. The three grades of stones available for use are classified as rough, finishing, and polishing stones. The rough stone is designed to true and clean the seat. The finishing stone must follow the rough to remove grinding marks and produce a smooth finish. The polishing stone does just as the name implies and is used only where a highly polished seat is desired.

The stones are installed on special stone holders. The face of the stone is trued by a diamond dresser. The stone should be refaced whenever it is grooved or loaded, and when the stone is first installed on the stone holder. The diamond dresser also may be used to cut down the diameter of the stone. Dressing of the stone should be kept to a minimum as a matter of conservation; therefore, it is desirable to have sufficient stone holders for all the stones to be used on the job. In the actual grinding job, considerable skill is required in handling the grinding gun. The gun must be centered accurately on the stone holder. If the gun is tilted off- center, chattering of the stone results, and a rough grind is produced. It is very important that the stone be rotated at a speed that permits grinding instead of rubbing. This speed is approximately 8,000 to 10,000 revolutions per minute (rpm).

Excessive pressure on the stone can slow it down. It is not a good technique to let the stone grind at slow speed by putting pressure on the stone when starting or stopping the gun. The maximum pressure used on the stone at any time should be no more than that exerted by the weight of the gun. 10-14 Seat out of alignment with guide Needs further rough grinding Excessive pitting True and clean ready for finish grind Another practice, conducive to good grinding, is to ease off on the stone every second or so to let the coolant wash away the chips on the seat. This rhythmic grinding action also helps keep the stone up to its correct speed. Since it is quite a job to replace a seat, remove as little material as possible during the grinding. Inspect the job frequently to prevent unnecessary grinding.

The rough stone is used until the seat is true to the valve guide and until all pits, scores, or burned areas are removed. [Figure 10-18] After refacing, the seat should be smooth and true. The finishing stone is used only until the seat has a smooth, polished appearance. Extreme caution should be used when grinding with the finishing stone to prevent chattering. The size and trueness of the seat can be checked by several methods. Runout of the seat is checked with a special dial indicator and should not exceed 0.002 inch. The size of the seat may be determined by using Prussian blue. Prussian blue is used to check for contact transfer from one surface to the other. To check the fit of the seat, spread a thin coat of Prussian blue evenly on the seat. Press the valve onto the seat. The blue transferred to the valve indicates the contact surface. The contact surface should be one-third to two-thirds the width of the valve face and in the middle of the face. In some cases, a-go and no-go gauge is used in place of the valve when making the Prussian blue check. If Prussian blue is not used, the same check may be made by lapping the valve lightly to the seat. Lapping is accomplished by using a small amount of lapping compound placed between the valve face and seat. The valve is then moved in a rotary motion back and forth until the lapping compound grinds slightly into the surface. After cleaning the lapping contact compound off, a contact area can be seen. Examples of test results are shown in Figure 10-19.

If the seat contacts the upper third of the valve face, grind off the top corner of the valve seat. [Figure 10-20] Such grinding is called narrowing grinding. This permits the seat to contact the center third of the valve face without touching the upper portion of the valve face. If the seat contacts the bottom third of the valve face, grind off the inner corner of the valve seat. [Figure 10-21] The seat is narrowed by a stone other than the standard angle. It is common practice to use a 15° angle and 45° angle cutting stone on a 30° angle valve seat, and a 30° angle and 75° angle stone on a 45° angle valve seat. [Figure 10-22] If the valve seat has been cut or ground too much, the valve contacts the seat too far up into the cylinder head, and the valve clearance, spring tension, and the fit of the valve to the seat is affected. To check the height of a valve, insert the valve into the guide, and hold it against the seat. Check the height of the valve stem above the rocker box or some other fixed position.

Before refacing a valve seat, consult the overhaul manual for the particular model engine. Each manufacturer specifies the desired angle for grinding and narrowing the valve seat. Valve Reconditioning One of the most common jobs during engine overhaul is grinding the valves. The equipment used should preferably be a wet valve grinder. With this type of machine, a mixture of soluble oil and water is used to keep the valve cool and carry away the grinding chips. Like many machine jobs, valve grinding is mostly a matter of setting up the machine. The following points should be checked or accomplished before starting a grind. True the stone by means of a diamond nib. The machine is turned on, and the diamond is drawn across the stone, cutting just deep enough to true and clean the stone. Determine the face angle of the valve being ground and set the movable head of the machine to correspond to this valve angle. Usually, valves are ground to the standard angles of 30° or 45°. However, in some instances, an interference fit of 0.5° or 1.5° less than the standard angle may be ground on the valve face.

10-15 Valve and seat ground to 45°Grind seat to 45° Seat 45° stone Valve Top edge reduced to 30°Reduce seat 1/3 with 30° stone 30° stone Seat Valve Lower edge reduced to 75°Reduce seat 1/3 with 75° stone 75° stone Seat Valve 45° 75° 30° 45° 15° Inner corner Inner corner Upper corner Seat SeatInner corner Inner corner Line contact 45° 44° The interference fit is used to obtain a more positive seal by means of a narrow contact surface. [Figure 10-23] Theoretically, there is a line contact between the valve and seat. With this line contact, the load that the valve exerts against the seat is concentrated in a very small area, thereby increasing the unit load at any one spot. The interference fit is especially beneficial during the first few hours of operation after an overhaul. The positive seal reduces the possibility of a burned valve or seat that a leaking valve might produce. After the first few hours of running, these angles tend to pound down and become identical.

Notice that the interference angle is ground into the valve, not the seat. It is easier to change the angle of the valve grinder work head than to change the angle of a valve seat grinder stone. Do not use an interference fit unless the manufacturer approves it. Install the valve into the chuck and adjust the chuck so that the valve face is approximately 2 inches from the chuck. [Figure 10-24] If the valve is chucked any further out, there is danger of excessive wobble and also a possibility of grinding into the stem. There are various types of valve grinding machines. In one type, the stone is moved across the valve face; in another, the valve is moved across the stone. Whichever type is used, the following procedures are typical of those performed when refacing a valve.

10-16 Travel adjusted by stops Valve adjusted correctly “In” stop prevents grinding the stem Out In Workhead table 2" Feather edge Normal margin edge. Check the travel of the valve face across the stone. The valve should completely pass the stone on both sides, yet not travel far enough to grind the stem. There are stops on the machine that can be set to control this travel. With the valve set correctly in place, turn on the machine and the grinding fluid so that it splashes on the valve face. Back the grinding wheel off all the way. Place the valve directly in front of the stone. [Figure 10-25] Slowly bring the wheel forward until a light cut is made on the valve. The intensity of the grind is measured by sound more than anything else.

Slowly draw the valve back and forth across the stone without increasing the cut. Move the work head table back and forth using the full face of the stone, but always keep the valve face on the stone. When the sound of the grind diminishes, indicating that some valve material has been removed, move the workhead table to the extreme left to stop rotation of the valve. Inspect the valve to determine if further grinding is necessary. If another cut must be made, bring the valve in front of the stone, then advance the stone out to the valve. Do not increase the cut without having the valve directly in front of the stone.

An important precaution in valve grinding, as in any kind of grinding, is to make light cuts only. Heavy cuts cause chattering, that may make the valve surface so rough that much time is lost in obtaining the desired finish. After grinding, check the valve margin to be sure that the valve edge has not been ground too thin. A thin edge is called a feather edge and can lead to pre-ignition; the valve edge would burn away in a short period of time, and the cylinder would have to be overhauled again. Figure 10-26 shows a valve with a normal margin and one with a feather edge. The valve tip may be resurfaced on the valve grinder. The tip is ground to remove cupping or wear, and also to adjust valve clearances on some engines.

10-17 Stone Shield Smooth, even gray band Sharply defined edges Piston Repairs Piston repairs are not required as often as cylinder repairs since most of the wear is between the piston ring and cylinder wall, valve stem and guide, and valve face and seat. A lesser amount of wear is encountered between the piston skirt and cylinder, ring and ring groove, or piston pin and bosses. The most common repair is the removal of scores. Usually, these may be removed only on the piston skirt if they are very light. On engines where the entire rotating and reciprocating assembly is balanced, the pistons must weigh within one-fourth ounce of each other. When a new piston is installed, it must be within the same weight tolerance as the one removed. It is not enough to have the pistons matched alone; they must be matched to the crankshaft, connecting rods, piston pins, etc. To make weight adjustments on new pistons, the manufacturer provides a heavy section at the base of the skirt. To decrease weight, file metal evenly off the inside of this heavy section. The piston weight can be decreased easily, but welding, metalizing, or plating cannot be done to increase the piston weight.

If ring grooves are worn or stepped, the pistons are normally replaced. Small nicks on the edge of the piston pin boss may be sanded down. Deep scores inside the boss, or anywhere around the boss, are definite reasons for rejection. It has become more economical to replace pistons rather than reconditioning and reusing old ones, especially during overhaul. Cylinder Grinding & Honing If a cylinder has excessive taper, out-of-roundness, step, or its maximum size is beyond limits, it can be reground to the next allowable oversize. If the cylinder walls are lightly rusted, scored, or pitted, the damage may be removed by honing or lapping.

Regrinding a cylinder is a specialized job that the powerplant mechanic is not usually expected to be able to do. However, The valve is held by a clamp on the side of the stone. [Figure 10-27] With the machine and grinding fluid turned on, the valve is pushed lightly against the stone and swung back and forth. Do not swing the valve stem off either edge of the stone. Because of the tendency for the valve to overheat during this grinding, be sure plenty of grinding fluid covers the tip. Grinding of the valve tip may remove, or partially remove, the bevel on the edge of the valve. To restore this bevel, mount a V-way approximately 45° to the grinding stone.

Hold the valve onto the V-way and twist the valve tip onto the stone. With a light touch, grind all the way around the tip. This bevel prevents scratching the valve guide when the valve is installed. Valve Lapping & Leak Testing After the grinding procedure is finished, it is sometimes necessary that the valve be lapped to the seat. This is done by applying a small amount of lapping compound to the valve face, inserting the valve into the guide, and rotating the valve with a lapping tool until a smooth, gray finish appears at the contact area. The appearance of a correctly lapped valve is shown in Figure 10-28.

After the lapping process is finished, be sure that all lapping compound is removed from the valve face, seat, and adjacent areas. The final step is to check the mating surface for leaks to see if it is sealing properly. This is done by installing the valve in the cylinder, holding the valve by the stem with the fingers, and pouring kerosene or solvent into the valve port. While holding finger pressure on the valve stem, check to see if the kerosene is leaking past the valve into the combustion chamber. If it is not, the valve re-seating operation is finished. If kerosene is leaking past the valve, continue the lapping operation until the leakage is stopped. The incorrect indications are of value in diagnosing improper valve and valve seat grinding. Incorrect indications, their cause, and remedy are shown in Figure 10-29.

10-18 Indication Fuzzy edge Cause Rough grind Remedy Regrind valve or continue lapping Too narrow contact Unintentional interference fit Grind both valve and set Too lap bands Improper narrowing of seat Renarrow seat the mechanic must be able to recognize when a cylinder needs regrinding, and they must know what constitutes a good or bad job. Generally, standard aircraft cylinder oversizes are 0.010 inch, 0.015 inches, 0.020 inch, or 0.030 inch. Aircraft cylinders have relatively thin walls and may have a nitrided surface, that must not be ground away. Nitriding is a surface hardening process that hardens the steel surface to a depth of several thousandths of an inch. Any one manufacturer usually does not allow all of the above oversizes. Some manufacturers do not allow regrinding to an oversize at all. The manufacturer’s overhaul manual, or parts catalog, usually lists the oversizes allowed for a particular make and model engine.

To determine the regrind size, the standard bore size must be known. This usually can be determined from the manufacturer’s specifications or manuals. The regrind size is figured from the standard bore. For example, a certain cylinder has a standard bore of 3.875 inches. To have a cylinder ground to 0.015 inches oversize, it is necessary to grind to a bore diameter of 3.890 inch (3.875 + 0.015). A tolerance of ±0.0005 inches is usually accepted for cylinder grinding. Another factor to consider when determining the size to which a cylinder must be reground is the maximum wear that has occurred. If there are spots in the cylinder wall that are worn larger than the first oversize, then obviously it is necessary to grind to the next oversize to clean up the entire cylinder.

The type of finish desired in the cylinder is an important consideration when ordering a regrind. Some engine manufacturers specify a fairly rough finish on the cylinder walls, that allows the rings to seat even if they are not lapped to the cylinder. Other manufacturers desire a smooth finish to which a lapped ring seats without much change in ring or cylinder dimensions. The latter type of finish is more expensive to produce. The standard used when measuring the finish of a cylinder wall is known as micro-inch root mean square (micro-inch RMS). In a finish where the depth of the grinding scratches are one-millionth (0.000001) of an inch deep, it is specified as 1 micro-inch RMS. Most aircraft cylinders are ground to a finish of 15 to 20 micro-inch RMS. Several low-powered engines have cylinders that are ground to a relatively rough 20- to 30-micro-inch RMS finish. On the other end of the scale, some manufacturers require a superfinish of approximately 4- to 6-micro-inch RMS.

Cylinder grinding is accomplished by a firmly mounted stone that revolves around the cylinder bore, as well as up and down the length of the cylinder barrel. [Figure 10-30] The cylinder, the stone, or both may move to get this relative movement. The size of the grind is determined by the distance the stone is set away from the centerline of the cylinder. Some cylinder bore grinding machines produce a perfectly straight bore, while others are designed to grind a choked bore. A choked bore grind refers to the manufacturing process in which the cylinder walls arc ground to produce a smaller internal diameter at the top than at the bottom. The purpose of this type grind or taper is to maintain a straight cylinder wall during operation. As a cylinder heats up during operation, the head and top of the cylinder are subjected to more heat than the bottom. This causes greater expansion at the top than at the bottom, thereby maintaining the desired straight wall.

After grinding a cylinder, it may be necessary to hone the cylinder bore to produce the desired finish. In this case, specify the cylinder regrind size to allow for some metal removal during honing. The usual allowance for honing is 0.001 inch. If a final cylinder bore size of 3.890 inches is desired, specify the regrind size of 3.889 inches, and then hone to 3.890 inches. There are several different makes and models of cylinder hones. The burnishing hone is used only to produce the desired finish on the cylinder wall. The more elaborate 10-19 Stone Grinder Cylinder Stone micromatic hone can also be used to straighten out the cylinder walls. A burnishing hone should not be used in an attempt to straighten cylinder walls. [Figure 10-31] Since the stones are only spring loaded, they follow the contour of the cylinder wall and may aggravate a tapered condition.

Deglazing the cylinder walls is accomplished with the use of a deglazing hone. A cross-hatch pattern must be placed on the cylinder wall to allow for piston ring break-in. This is accomplished by a deglazing hone turned by a drill being moved in and out of the cylinder rapidly. [Figure 10-32] After the cylinders have been reground or deglazed, or both, check the size and wall finish, and check for evidence of overheating or grinding cracks before installing on an engine. Reassembly Before starting reassembly, all serviceable and new engine components need to be cleaned, organized, and laid out in the order they are to be assembled. A popular method of engine assembly is for the engine to be assembled at one work station with the same technicians completing the total assembly of the engine. It is also important to refer to the parts catalog to ensure that the correct hardware is used during the assembly of the engine. The engine overhaul manual should be referred to for information on the use of safety wire, self-locking nuts, and torque values. During assembly, the components should be pre-lubricated as the overhaul manual sets forth. It is important to follow the manufacturer’s overhaul assembly procedures completely and perform all checks and procedures that are called for in the manual.

Installation & Testing Testing Reciprocating Engines The procedures and equipment used in determining that an engine is ready for airworthy service and is in excellent mechanical condition, normally requires the use of a test stand, or test cell, although the aircraft can be used. 10-20 [Figure 10-33] The method of engine testing or run-in that takes place during overhaul prior to delivery of the engine is critical to the airworthiness of the engine. It must be emphasized that engine run-in is as vital as any other phase of engine overhaul, for it is the means by which the quality of a new or newly overhauled engine is checked, and it is the final step in the preparation of an engine for service.

Thus, the reliability and potential service life of an engine is in question until it has satisfactorily passed the cell test. The test serves a dual purpose. First, it accomplishes piston ring run-in and bearing burnishing. Second, it provides valuable information that it used to evaluate engine performance and determine engine condition. To provide proper oil flow to the upper portion of the cylinder barrel walls with a minimum loss of oil, it is important that piston rings be properly seated in the cylinder in which they are installed. The process is called piston ring run-in (break- in) and is accomplished chiefly by controlled operation of the engine in the high-speed range. Improper piston ring conditioning, or run-in, may result in unsatisfactory engine operation with high oil consumption. A process called bearing burnishing creates a highly polished surface on new bearings and bushings installed during overhaul. The burnishing is usually accomplished during the first periods of the engine run-in at comparatively slow engine speeds.

The failure of any part during engine testing or run-in requires that the engine be returned, repaired, and completely retested. After an engine has successfully completed test requirements, it is then specially treated to prevent corrosion, if it is shipped or stored before being installed in an aircraft. During the final run-in period during testing, the engines are operated on the proper grade of fuel prescribed for the particular kind of engine. The oil system is serviced with a mixture of corrosion-preventive compound and engine oil. The temperature of this mixture is maintained at 105 °C to 121 °C. Near the end of final run-in, corrosion-preventive mixture (CPM) is used as the engine lubricant. The engine induction passages and combustion chambers are also treated with CPM by an aspiration method. CPM is drawn or breathed into the engine.

Test Cell Requirements The test cell requires an area to mount and hold the engine for testing. The cell needs to have the controls, instruments, and any special equipment to evaluate the total performance of the engine. A test club should be used for testing instead of a flight propeller. [Figure 10-34] A test club provides more cooling air flow and the correct amount of load. The operational tests and test procedures vary with individual engines, but the basic requirements are generally closely related. Engine Instruments The test cell control room contains the controls used to operate the engine and the instruments used to measure various temperatures and pressures, fuel flow, and other factors. These devices are necessary in providing an accurate check and an evaluation of the operating engine. The control room is separate from, but adjacent to, the space (test cell) that houses the engine being tested. The safe, economical, and reliable testing of modern aircraft engines depends largely upon the use of instruments. In engine run-in procedures, the same basic engine instruments are used as when the engine is installed in the aircraft, plus some additional connections to these instruments, and some indicating and measuring devices that cannot be practically installed in the aircraft.

Instruments used in the testing procedures are inspected and calibrated periodically, as are instruments installed in the aircraft; thus, accurate information concerning engine operation is ensured. Engine instruments can operate using different methods, some mechanically, some electrically, and some by sensing 10-21 Yellow Green Red 150 100 50 -50 0 150 100 50 -50 0 C° 50 0 the direct pressure of air or liquid. Some of the basic instruments are: 1. Carburetor air temperature gauge. 2. Fuel pressure gauge. 3. Fuel-flow meter. 4. Manifold pressure gauge. 5. Oil temperature gauge. 6. Oil pressure gauge.

7. Tachometer. 8. Exhaust gas temperature gauge. 9. Cylinder head temperature gauge. 10. Torquemeter. Instrument markings, ranges of operation, minimum and maximum limits, and the interpretation of these markings are general to all the instruments. Generally, the instrument marking system consists of three colors: red, yellow, and green. A red line, or mark, indicates a point beyond which a dangerous operating condition exists. A red arc indicates a dangerous operating range due generally to an engine propeller vibration range. This arc can be passed through, but the engine cannot be operated in this area. Of the two, the red mark is used more commonly and is located radially on the cover glass or dial face. The yellow arc covers a given range of operation and is an indication of caution. Generally, the yellow arc is located on the outer circumference of the instrument cover glass or dial face. The green arc shows a normal and safe range of operation. When the markings appear on the cover glass, a white line is used as an index mark, often called a slippage mark. The white radial mark indicates any movement between the cover glass and the case, a condition that would cause mislocation of the other range and limit markings.

Carburetor Air Temperature (CAT) Indicator Measured at the carburetor entrance, carburetor air temperature (CAT) is regarded by many as an indication of induction system ice formation. Although it serves this purpose, it also provides many other important items of information. The powerplant is a heat machine, and the temperature of its components, or the fluids flowing through it, affects the combustion process either directly or indirectly. The temperature level of the induction air affects not only the charge density, but also the vaporization of the fuel. CAT is also useful for checking induction system condition.

Backfiring is indicated as a momentary rise on the gauge, provided it is of sufficient severity for the heat to be sensed at the carburetor air-measuring point. A sustained induction system fire shows a continuous increase of CAT. The CAT should be noted before starting and just after shutdown. The temperature before starting is the best indication of the temperature of the fuel in the carburetor body and tells whether vaporization is sufficient for the initial firing, or whether the mixture must be augmented by priming. If an engine has been shut down for only a short time, the residual heat in the carburetor may make it possible to rely on the vaporizing heat in the fuel and powerplant. Priming would then be unnecessary.

After shutdown, a high CAT is a warning that the fuel trapped in the carburetor will expand, producing high internal pressure. When a high temperature is present at this time, the fuel line and manifold valves should be open so that the pressure can be relieved by allowing fuel passage back to the tank. The CAT gauge indicates the temperature of the air before it enters the carburetor. The temperature reading is sensed by a bulb or electric sensor. In the test cell, the sensor is located in the air intake passage to the engine and, in an aircraft, it is located in the ram-air intake duct. The CAT gauge is calibrated in the centigrade scale. [Figure 10-35] This gauge, like many other multi-engine aircraft instruments, is a dual gauge; two gauges, each with a separate pointer and scale, are used in the same case.

Notice the range markings used. The yellow arc indicates a range from –10 °C to +15 °C, since the danger of icing occurs between these temperatures. The green range indicates the normal operating range from +15 °C to +40 °C. The red line indicates the maximum operating temperature of 40 °C; any operation at a temperature over this value places the engine in danger of detonation. Fuel Pressure Indicator The fuel pressure gauge is calibrated in pounds per square inch (psi) of pressure. It is used during the test run-in to measure engine fuel pressure at the carburetor inlet, the fuel 10-22 feed valve discharge nozzle, and the main fuel supply line.

Fuel gauges are located in the operator’s control room and are connected by flexible lines to the different points at which pressure readings are desired during the testing procedures. In some aircraft installations, the fuel pressure is sensed at the carburetor or fuel injection unit inlet of each engine, and the pressure is indicated on individual gauges on the instrument panel. [Figure 10-36] The dial is calibrated in graduations and is extended and numbered. The numbers range from 0 to 10 in this example. The red line on the dial at the 2 pounds psi graduation shows the minimum fuel pressure allowed during flight. The green arc shows the desired range of operation, which is 2 to 9 psi. The red line at the 9 psi graduation indicates the maximum allowable fuel pressure.

Fuel pressures vary with the type of fuel system installation and the size of the engine. When fuel injection systems are used, the fuel pressure range is much higher; the minimum allowable pressure is approximately 10 psi, and the maximum is generally 25 psi. Oil Pressure Indicator The main oil pressure reading is taken at the pressure side of the oil pump. Generally, there is only one oil pressure gauge for each aircraft engine. The oil pressure gauge dial does not show the pressure range or limits for all installations. [Figure 10-36] The actual markings for specific aircraft may be found in the aircraft specifications or Type Certificate Data Sheets. The lower red line at 25 psi indicates the minimum oil pressure permissible in flight. The green arc between 60 to 85 psi illustrates the desired operating oil pressure range. The red line at 100 psi indicates maximum permissible oil pressure.

The oil pressure gauge indicates the pressure, in psi, that the oil of the lubricating system is being supplied to the moving parts of the engine. The engine should be shut down immediately if the gauge fails to register pressure when the engine is operating. Excessive oscillation of the gauge pointer indicates that there is air in the lines leading to the gauge, or that some unit of the oil system is functioning improperly. Oil Temperature Indicator During engine run-in in the test cell, engine oil temperature readings are taken at the oil inlet and outlet. From these readings, it can be determined if the engine heat transferred to the oil is low, normal, or excessive. This information is of extreme importance during the breaking-in process of large reciprocating engines. The oil temperature gauge line in the aircraft is connected at the oil inlet to the engine.

Three range markings are used on the oil temperature gauge. The green arc in Figure 10-36, on the dial, shows the minimum oil temperature permissible for ground operational checks or during flight. The green mark between 25 °F and below 245 °F shows the desired oil temperature for continuous engine operation. The red mark at 245 °F indicates the maximum permissible oil temperature. Fuel-Flow Meter The fuel-flow meter measures the amount of fuel delivered to the engine. During engine testing procedures, the fuel-flow to the engine can be measured by three different methods: a direct flow meter, a pressure-based flow meter, or a turbine sensor-based flow meter. The direct reading flow meter uses a series of calibrated tubes located in the control room. The tubes are of various sizes to indicate different volumes of fuel-flow. Each tube contains a float that can be seen by the operator, and as the fuel-flow through the tube varies, the float is either raised or lowered, indicating the amount of fuel-flow. From these indications, the operator can determine whether an engine is operating at the correct air-fuel mixture for a given power setting. Reciprocating engines on light aircraft usually use a fuel pressure gauge that is also used for the flow meter. This is because the fuel-flow is proportional to the fuel pressure in this system. Fuel-flow is normally measured in gallons per hour.

In most turbine aircraft installations, the fuel-flow indicating system consists of a transmitter and an indicator for each engine. The fuel-flow transmitter is conveniently mounted in the engine’s accessory section and measures the fuel-flow between the engine-driven fuel pump and the fuel control device. The transmitter is an electrical device that contains a turbine that turns faster as the flow increases, which increases the electrical signal to the indicator. The fuel-flow transmitter 10-23 Green Red MAN. IN. HG. ABS. 10 15 20 25 30 35 40 45 50 55 60 6570 75 15 20 25 30 35 40 45 50 55 60 65 75 is connected electrically to the indicator located on the aircraft flight deck, or on the test cell operator’s panel. The reading on the indicator on turbine aircraft is calibrated to record the amount of fuel-flow in pounds of fuel per hour.

Manifold Pressure Indicator The preferred type of instrument for measuring the manifold pressure on reciprocating engines is a gauge that records the pressure as an absolute pressure reading. Absolute pressure takes into account the atmospheric pressure plus the pressure in the intake manifold. To read the manifold pressure of the engines, a specially designed manifold pressure gauge that indicates absolute manifold pressure in inches of mercury ("Hg) is used. The red line indicates the maximum manifold pressure permissible during takeoff. The manifold pressure gauge range markings and indications vary with different kinds of engines and installations.

gauge and shows how the range markings are positioned. The green arc starts at 35 "Hg and continues to the 44 "Hg. The red line on the gauge, at 49 "Hg shows the manifold pressure recommended for takeoff. This pressure should not be exceeded. Tachometer Indicator The tachometer for reciprocating engines shows the engine crankshaft rpm. The system used for testing the engine is the same as the system in the aircraft installation. The tachometer, often referred to as TACH, is calibrated in hundreds with graduations at every 100-rpm interval. The dial shown in The green arc indicates the rpm range within operation that is permissible. The red line indicates the maximum rpm permissible during takeoff; any rpm beyond this value is an overspeed condition.

Turbine engines use percent rpm indicators due to the high rpm that the engines generally operate. Each rotating assembly in an engine has its own percent rpm indicator. The tachometer indicates speed of the compressor section. The 100 percent position on the indicator is the highest rpm at which the engine can operate. Red lines and green arcs operate the same as with reciprocating engines. Rotorcraft use two synchronous tachometers. Cylinder Head Temperature Indicator During the engine test procedures, the cylinder head temperatures of various cylinders on the reciprocating engine are normally tested. Thermocouples are connected to several cylinders and, by a selector switch, any cylinder head temperature can be indicated on the indicators.

When installed in the aircraft, there is sometimes only one thermocouple lead and indicator for each engine installed in an aircraft. Cylinder head temperatures are indicated by a gauge connected to a thermocouple attached to the cylinder, that tests show to be the hottest on an engine in a particular installation. The thermocouple may be placed in a special gasket located under a rear spark plug, or in a special well in the top or rear of the cylinder head. The temperature recorded at either of these points is merely a reference or control temperature; but as long as it is kept within the prescribed limits, the temperatures inside 10-24 Blue Green Red 300 200 0 100 300 200 0 100 CYL. TEMP.

HEAD C° 300 200 0 100 300 200 0 100 0 5 10 152025 30 35 40 45 50 TORQUE FTLB x100 the cylinder dome, exhaust valve, and piston is within a satisfactory range. Since the thermocouple is attached to only one cylinder, it can do no more than give evidence of general engine temperature. While normally it can be assumed that the remaining cylinder temperatures are lower, conditions such as detonation are not indicated unless they occur in the cylinder that has the thermocouple attached. The cylinder head temperature gauge range marking is similar to that of the manifold pressure and tachometer indicator. The cylinder head temperature gauge is a dual gauge that incorporates two separate temperature scales.

[Figure 10-39] The scales are calibrated in increments of 10°, with numerals at the 0°, 100°, 200°, and 300° graduations. The space between any two graduation marks represents 10 °C. The blue arc on the gauge indicates the range within which operation is permitted in auto-lean. The bottom of this arc, 100 °C, indicates the minimum desired temperature to ensure efficient engine operation during flight. The top of the blue arc, 230 °C, indicates the temperature at which the mixture control must be moved to the “auto-rich” position. The green arc describes the range within which operation must be in auto-rich. The top of this arc, 248 °C, indicates maximum continuous power; all operation above this temperature is limited in time (usually 5 to 15 minutes). The red line indicates maximum permissible temperature, 260 °C.

Torquemeter Most torque systems use an oil pressure output from a torque valve to indicate actual engine power output at various power settings. The torquemeter indicates the amount of torque being produced at the propeller shaft. A helical gear moves back and forth as the torque on the propeller shaft varies. This gear, acting on a piston, positions a valve that meters the oil pressure proportionally to the torque being produced. A change in pressure from the valve that is connected to a transducer is then converted to an electrical signal and is transmitted to the flight deck. The torquemeter can read out in foot-pounds of torque, percent of horsepower, or horsepower. The earlier systems read out in psi, and the flight engineer converted this to the correct power setting. [Figure 10-40] Some systems use strain gauges to attach to the ring gear to provide an electrical signal directly to the readout.

Warning Systems Many of the miscellaneous gauges and devices indicate only that a system is functioning or has failed to function. On some aircraft, a warning light illuminates when the fuel pressure or oil pressure is low. Reciprocating Engine Operation The operation of the powerplant is controlled from the flight deck. Some installations have numerous control handles and levers connected to the engine by rods, cables, bellcranks, pulleys, etc. In most cases, the control handles are conveniently mounted on quadrants in the flight deck. Placards, or markings, are placed on the quadrant to indicate the functions and positions of the levers. In some installations, friction clutches are installed to hold the controls in place.

Engine Instruments The term engine instruments usually includes all instruments required to measure and indicate the functioning of the powerplant. The engine instruments are generally installed on the instrument panel so that all of them can easily be observed at one time. Manifold pressure, rpm, engine temperature, oil temperature, CAT, and the air-fuel ratio can be controlled by manipulating the flight deck controls. Coordinating the movement of the controls with the instrument readings protects against exceeding operating limits. Engine operation is usually limited by specified operating ranges of the following: 1. Crankshaft speed (rpm).

2. Manifold pressure. 10-25 Draining engine oil Partially filled intake pipe 3. Cylinder head temperature. 4. CAT. 5. Oil temperature. 6. Oil pressure. 7. Fuel pressure. 8. Fuel-flow meter. 9. Air-fuel mixture setting. The procedures, pressures, temperatures, and rpm used throughout this section are solely for the purpose of illustration and do not have general application. The operating procedures and limits used on individual makes and models of aircraft engines vary considerably from the values shown here. For exact information regarding a specific engine model, consult the applicable instructions.

Engine Starting Before starting the engine, observe the manifold pressure gauge that should read approximate atmospheric (barometric) pressure when the engine is not running. At sea level, this is approximately 30 "Hg, and at fields above sea level, the atmospheric pressure is less, depending on the height above sea level. Also, observe all engine gauges for the correct reading for engine off settings. Correct starting technique is an important part of engine operation. Improper procedures often are used, because some of the basic principles involved in engine operation are misunderstood. Read more about typical procedures for starting reciprocating engines in the Aviation Maintenance Technician Handbook - General.

Pre-Oiling Engines that have undergone overhaul or major maintenance can have air trapped in some of the oil passages that must be removed before the first start. This is done by pre-oiling the engine by cranking, with the spark plugs removed, the engine with the starter or by hand (turning) until oil pressure is indicated. A second method is to pump oil under pressure through the oil system using an external pump until oil comes out of the oil outlet of the engine. Hydraulic Lock Whenever a radial engine remains shut down for any length of time beyond a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes ready to be drawn into the cylinders when the engine starts. [Figure 10-41] As the piston approaches top center of the compression stroke (both valves closed), this liquid being incompressible, stops piston movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine with a hydraulic lock of this nature may cause the affected cylinder to blow out or, more likely, may result in a bent or broken connecting rod.

To eliminate a lock, remove either the front or rear spark plug of the lower cylinders and pull the propeller through in the direction of rotation. The piston expels any liquid that may be present. If the hydraulic lock occurs as a result of over- priming prior to initial engine start, eliminate the lock in the same manner (i.e., remove one of the spark plugs from the cylinder and rotate the crankshaft through two turns). Never attempt to clear the hydraulic lock by pulling the propeller through in the direction opposite to normal rotation. This tends to inject the liquid from the cylinder into the intake pipe with the possibility of a complete or partial lock occurring on the subsequent start.

Engine Warm-Up Proper engine warm-up is important, particularly when the condition of the engine is unknown. Improperly adjusted idle mixture, intermittently firing spark plugs, and improperly adjusted engine valves all have an overlapping effect on engine stability. Therefore, the warm-up should be made at the engine speed where maximum engine stability is obtained. Experience has shown that the optimum warm-up speed is from 1,000 to 1,600 rpm. The actual speed selected should be 10-26 the speed at which engine operation is the smoothest, since the smoothest operation is an indication that all phases of engine operation are the most stable.

Some engines incorporate temperature-compensated oil pressure relief valves. This type of relief valve results in high engine oil pressures immediately after the engine starts, if oil temperatures are very low. Consequently, start the warm- up of these engines at approximately 1,000 rpm and then move to the higher, more stable engine speed as soon as oil temperature reaches a warmer level. During warm-up, watch the instruments associated with engine operation. This aids in making sure that all phases of engine operation are normal. For example, engine oil pressure should be indicated within 30 seconds after the start.

Furthermore, if the oil pressure is not up to or above normal within 1 minute after the engine starts, the engine should be shut down. Cylinder head or coolant temperatures should be observed continually to see that they do not exceed the maximum allowable limit. A lean mixture should not be used to hasten the warm-up. Actually, at the warm-up rpm, there is very little difference in the mixture supplied to the engine, whether the mixture is in a rich or lean position, since metering in this power range is governed by throttle position. Carburetor heat can be used as required under conditions leading to ice formation. For engines equipped with a float-type carburetor, it is desirable to raise the CAT during warm-up to prevent ice formation and to ensure smooth operation.

The magneto safety check can be performed during warm- up. Its purpose is to ensure that all ignition connections are secure, and that the ignition system permits operation at the higher power settings used during later phases of the ground check. The time required for proper warm-up gives ample opportunity to perform this simple check, which may disclose a condition that would make it inadvisable to continue operation until after corrections have been made. The magneto safety check is conducted with the propeller in the high rpm (low pitch) position, at approximately 1,000 rpm. Move the ignition switch from “both” to “right” and return to “both;” from “both” to “left” and return to “both;” from “both” to “off” momentarily and return to “both.” While switching from “both” to a single magneto position, a slight but noticeable drop in rpm should occur. This indicates that the opposite magneto has been properly grounded out.

Complete cutting out of the engine when switching from “both” to “off” indicates that both magnetos are grounded properly. While in the single magneto position, failure to obtain any rpm drop, or failure of the engine to cut out while switching to off, indicates that one or both ground connections are faulty. This indicates a safety problem; the magnetos are not secured at shut down and may fire if the propeller is turned. Ground Check The ground check is performed to evaluate the functioning of the engine by comparing power input, as measured by manifold pressure, with power output, as measured by rpm or torque.

The engine may be capable of producing a prescribed power, even rated takeoff, and not be functioning properly. Only by comparing the manifold pressure required during the check against a known standard is an unsuitable condition disclosed. The magneto check can also fail to show shortcomings, since the allowable rpm dropoff is only a measure of an improperly functioning ignition system and is not necessarily affected by other factors. Conversely, it is possible for the magneto check to prove satisfactory when an unsatisfactory condition is present elsewhere in the engine. The ground check is made after the engine is thoroughly warm. It consists of checking the operation of the powerplant and accessory equipment by ear, by visual inspection, and by proper interpretation of instrument readings, control movements, and switch reactions. During the ground check, the aircraft should be headed into the wind, if possible, to take advantage of the cooling airflow. A ground check procedure is outlined below: 1. Control position check.

2. Cowl flaps (if equipped)—open. 3. Mixture—rich. 4. Propeller—high rpm. 5. Carburetor heat—cold. 6. Check propeller according to propeller manufacturer’s instruction. 7. Open throttle to the run-up rpm setting as per manufacturer’s instructions (specified RPM and manifold pressure). 8. Ignition system operational check. In performing the ignition system operational check (magneto check), the power-absorbing characteristics of the propeller in the low fixed-pitch position are utilized. In switching to individual magnetos, cutting out the opposite plugs results in a slower rate of combustion, which gives the same effect as retarding the spark advance. The drop in engine speed is a measure of the power loss at this slower combustion rate.

10-27 When the magneto check is performed, a drop in torquemeter pressure indication is a good supplement to the variation in rpm. In cases where the tachometer scale is graduated coarsely, the torquemeter variation may give more positive evidence of the power change when switching to the individual magneto condition. A loss in torquemeter pressure not to exceed 10 percent can be expected when operating on a single magneto. By comparing the rpm drop with a known standard, the following are determined: 1. Proper timing of each magneto. 2. General engine performance as evidenced by smooth operation.

3. Additional check of the proper connection of the ignition leads. Any unusual roughness on either magneto is an indication of faulty ignition caused by plug fouling or by malfunctioning of the ignition system. The operator should be very sensitive to engine roughness during this check. Lack of dropoff in rpm may be an indication of faulty grounding of one side of the ignition system. Complete cutting out when switching to one magneto is definite evidence that its side of the ignition system is not functioning. Excessive difference in rpm drop off between the left and right switch positions can indicate a difference in time between the left and right magnetos.

Sufficient time should be given to the check on each single switch position to permit complete stabilization of engine speed and manifold pressure. There is a tendency to perform this check too rapidly with resultant wrong indications. Operation as long as 1 minute on a single ignition system is not excessive. Another point that must be emphasized is the danger of sticking tachometer. The tachometer should he tapped lightly to make sure the indicator needle moves freely. In some cases using older mechanical tachometers, sticking has caused errors in indication to the extent of 100 rpm. Under such conditions, the ignition system could have had as much as a 200 rpm drop with only a 100 rpm drop indicated on the instrument. In most cases, tapping the instrument eliminates the sticking and results in accurate readings.

In recording the results of time ignition system check, record the amount of the total rpm drop that occurs rapidly and the amount that occurs slowly. This breakdown in rpm drop provides a means of pinpointing certain troubles in the ignition system. This can reduce unnecessary work by confining maintenance to the specific part of the ignition system that is responsible for the trouble. Fast rpm drop is usually the result of either faulty spark plugs or faulty ignition harness. This is true because faulty plugs or leads, take effect at once. The cylinder goes dead or starts firing intermittently the instant the switch is moved from “both” to the “right” or “left” position.

Slow rpm drop usually is caused by incorrect ignition timing or faulty valve adjustment. With late ignition timing, the charge is fired too late (in relation to piston travel) for the combustion pressures to build up to the maximum at the proper time. The result is a power loss greater than normal for single ignition because of the lower peak pressures obtained in the cylinder. However, this power loss does not occur as rapidly as that which accompanies a dead spark plug. This explains the slow rpm drop as compared to the instantaneous drop with a dead plug or defective lead. Incorrect valve clearances, through their effect on valve overlap, can cause the mixture to be too rich or too lean. The too rich or too lean mixture may affect one plug more than another, because of the plug location and show up as a slow rpm drop on the ignition check. Switch from “both” to “right” and return to “both.” Switch from “both” to “left” and return to “both.” Observe the rpm drop while operating on the right and left positions.

The maximum drop should not exceed that specified by the engine manufacturer. Fuel Pressure & Oil Pressure Check Fuel pressure and oil pressure must be within the established tolerance (green arc) for the engine. Propeller Pitch Check The propeller is checked to ensure proper operation of the pitch control and the pitch-change mechanism. The operation of a controllable pitch propeller is checked by the indications of the tachometer and manifold pressure gauge when the propeller governor control is moved from one position to another. Because each type of propeller requires a different procedure, the applicable manufacturer’s instructions should be followed.

Power Check Specific rpm and manifold pressure relationship should be checked during each ground check. This can be done at the time the engine is run-up to make the magneto check. The purpose of this check is to measure the performance of the engine against an established standard. Calibration tests have determined that the engine is capable of delivering a given power at a given rpm and manifold pressure. The original calibration, or measurement of power, is made by means of a dynamometer in a test cell. During the ground check, power is measured with the propeller. With constant conditions of air density, the propeller, at any fixed-pitch position, always requires the same rpm to absorb the same horsepower from the engine. This characteristic is used in determining the condition of the engine.