Engine Operation and Troubleshooting

10-43 OIL 5 4 300 200 0 100 values in the engine manufacturer’s service or overhaul manual. Apply the torque with a slow, steady motion until the prescribed value is reached. Hold the tension on the wrench for a sufficient length of time to ensure that the nut or cap-screw tightens no more at the prescribed torque value. In many cases, additional turning of the cap-screw, or nut, as much as one-quarter turn can be done by maintaining the prescribed torque on the nut for a short period of time. After the stud nuts, or cap-screws, have been torqued to the prescribed value, safety them in the manner recommended in the engine manufacturer’s service manual.

Reinstall the push rods, push rod housings, rocker arms, barrel deflectors, intake pipes, ignition harness lead clamps and brackets, fuel injection line clamps and fuel injection nozzles (if removed), exhaust stack, cylinder head deflectors, and spark plugs. Remember that the push rods must be installed in their original locations and must not be turned end to end. Make sure that the push rod ball end seats properly in the tappet. If it rests on the edge or shoulder of the tappet during valve clearance adjustment and later drops into place, valve clearance is off. Furthermore, rotating the crankshaft with the push rod resting on the edge of the tappet may bend the push rod.

After installing the push rods and rocker arms, set the valve clearance. Before installing the rocker-box covers, lubricate the rocker arm bearings and valve stems. Check the rocker- box covers for flatness; re-surface them if necessary. After installing the gaskets and covers, tighten the rocker-box cover nuts to the specified torque. Always follow the recommended safety procedures. Cold Cylinder Check The cold cylinder check determines the operating characteristics of each cylinder of an air-cooled engine. The tendency for any cylinder, or cylinders, to be cold, or to be only slightly warm, indicates lack of combustion or incomplete combustion within the cylinder. This must be corrected if best operation and power conditions are to be obtained. The cold cylinder check is made with a cold cylinder indicator.

Engine difficulties that can be analyzed by use of the cold cylinder indicator are [Figure 10-55]: 1. Rough engine operation. 2. Excessive rpm drop during the ignition system check. 3. High manifold pressure for a given engine rpm during the ground check when the propeller is in the full low- pitch position. 4. Faulty mixture ratios caused by improper valve clearance. In preparation for the cold cylinder check, head the aircraft into the wind to minimize irregular cooling of the individual cylinders and to ensure even propeller loading during engine operation. Operate the engine on its roughest magneto at a speed between 1,200 and 1,600 rpm until the cylinder head temperature reading is stabilized. If engine roughness is encountered at more than one speed, or if there is an indication that a cylinder ceases operating at idle or higher speeds, run the engine at each of these speeds, and perform a cold cylinder check to pick out all the dead or intermittently operating cylinders. When low power output or engine vibration is encountered at speeds above 1,600 rpm when operating with the ignition switch on both, run the engine at the speed where the difficulty is encountered until the cylinder head temperatures have stabilized.

When cylinder head temperatures have reached the stabilized values, stop the engine by moving the mixture control to 10-44 Cylinder No. Right magneto Temperature readings Left magneto 1 2 3 4 5 6 180 170 100 145 70 60 170 175 170 60 155 45 the idle cutoff or full lean position. When the engine ceases firing, turn off both ignition and master switches. Record the cylinder head temperature reading registered on the flight deck gauge. As soon as the propeller has ceased rotating, apply the instrument to each cylinder head, and record the relative temperature of each cylinder. Start with number one and proceed in numerical order around the engine, as rapidly as possible. To obtain comparative temperature values, a firm contact must be made at the same relative location on each cylinder. Note any outstandingly low (cold) values. Compare the temperature readings to determine which cylinders are dead (cold cylinders) or are operating intermittently.

Difficulties that may cause a cylinder to be inoperative (dead) when isolated to one magneto, either the right or left positions, are: 1. Defective spark plugs. 2. Incorrect valve clearances. 3. Leaking intake pipes. 4. Lack of compression. 5. Defective spark plug lead. 6. Defective fuel-injection nozzle. Repeat the cold cylinder test for the other magneto positions on the ignition switch, if necessary. Cooling the engine between tests is unnecessary. The airflow created by the propeller, and the cooling effect of the incoming air-fuel mixture is sufficient to cool any cylinders that are functioning on one test and not functioning on the next.

In interpreting the results of a cold cylinder check, remember that the temperatures are relative. A cylinder temperature taken alone means little, but when compared with the temperatures of other cylinders on the same engine, it provides valuable diagnostic information. The readings shown in Figure 10-56 illustrate this point. On this check, the cylinder head temperature gauge reading at the time the engine was shut down was 160 °C on both tests. A review of these temperature readings reveals that, on the right magneto, cylinder number 3 runs cool and cylinders 5 and 6 run cold. This indicates that cylinder 3 is firing intermittently, and cylinders 5 and 6 are dead during engine operation on the plugs fired by the right magneto. Cylinders 4 and 6 are dead during operation on the plugs fired by the left magneto. Cylinder 6 is completely dead. An ignition system operational check would not disclose this dead cylinder, since the cylinder is inoperative on both right and left switch positions.

A dead cylinder can be detected during run-up, since an engine with a dead cylinder requires a higher than normal manifold pressure to produce any given rpm below the cut-in speed of the propeller governor. A dead cylinder could also be detected by comparing power input and power output with the aid of a torquemeter. Defects within the ignition system that can cause a cylinder to go completely dead are: 1. Both spark plugs inoperative. 2. Both ignition leads grounded, leaking, or open. 3. A combination of inoperative spark plugs and defective ignition leads. 4. Faulty fuel-injection nozzles, incorrect valve clearances, and other defects outside the ignition system.

In interpreting the readings obtained on a cold cylinder check, the amount the engine cools during the check must be considered. To determine the extent to which this factor should be considered in evaluating the readings, re-check some of the first cylinders tested, and compare the final readings with those made at the start of the check. Another factor to be considered is the normal variation in temperature between cylinders and between rows. This variation results from those design features that affect the airflow past the cylinders. Turbine Engine Maintenance Turbine powerplant maintenance procedures vary widely according to the design and construction of the particular engine being serviced. The detailed procedures recommended by the engine manufacturer should be followed when performing inspections or maintenance. Maintenance information presented in this section is not intended to specify the exact manner in which maintenance operations are to be performed but is included to convey a general idea of the procedures involved. For inspection purposes, the turbine engine is divided into two main sections: the cold and hot.

10-45

Compressor Section

Maintenance of the compressor, or cold section, is one of concern because damage to blades can cause engine failure. Much of the damage to the blades arises from foreign matter being drawn into the turbine engine air intakes. The atmosphere near the ground is filled with tiny particles of dirt, oil, soot, and other foreign matter. A large volume of air is introduced into the compressor, and centrifugal force throws the dirt particles outward so that they build up to form a coating on the casing, the vanes, and the compressor blades. Accumulation of dirt on the compressor blades reduces the aerodynamic efficiency of the blades with resultant deterioration in engine performance. The efficiency of the blades is impaired by dirt deposits in a manner similar to that of an aircraft wing under icing conditions. Unsatisfactory acceleration and high exhaust gas temperature can result from foreign deposits on compressor components.

An end result of foreign particles, if allowed to accumulate in sufficient quantity, would be inefficiency. The condition can be remedied by periodic inspection, cleaning, and repair of compressor components. Inspection & Cleaning Minor damage to axial-flow engine compressor blades may be repaired if the damage can be removed without exceeding the allowable limits established by the manufacturer. Typical compressor blade repair limits are shown in Figure 10-57. Well-rounded damage to leading and trailing edges that is evident on the opposite side of the blade is usually acceptable without re-work, provided the damage is in the outer half of the blade only, and the indentation does not exceed values specified in the engine manufacturer’s service and overhaul instruction manuals. When working on the inner half of the blade, damage must be treated with extreme caution. Repaired compressor blades are inspected by either magnetic particle or fluorescent penetrant inspection methods to ensure that all traces of the damage have been removed. All repairs must be well blended so that surfaces are smooth. [Figure 10-58] No cracks of any extent are tolerated in any area.

Whenever possible, stoning and local re-work of the blade should be performed parallel to the length of the blade. Re- work must be accomplished by hand, using stones, files, or emery cloth. Do not use a power tool to buff the entire area of the blade. The surface finish in the repaired area must be comparable to that of a new blade. On centrifugal flow engines, it is difficult to inspect the compressor inducers without first removing the air-inlet screen. After removing the screen, clean the compressor inducer and inspect it with a strong light. Check each vane for cracks by slowly turning the compressor. Look for cracks in the leading edges. A crack is usually cause for component rejection. The compressor inducers are normally the parts that are damaged by the impingement of foreign material during engine operation.

Compressor inducers are repaired by stoning out and blending the nicks and dents in the critical band (1 1⁄2 to 2 1⁄2 inches from the outside edge), if the depth of such nicks or dents does not exceed that specified in the engine manufacturer’s service or overhaul instruction manuals. Repair nicks by stoning out material beyond the depth of damage to remove the resulting cold-worked metal. A generous radius must be applied at the edges of the blend. After blending the nick, it should be smoothed over with a crocus cloth. Pitting nicks or corrosion found on the sides of the inducer vanes are similarly removed by blending.

Causes of Blade Damage Loose objects often enter an engine either accidentally or through carelessness. Foreign object damage (FOD), such as pencils, tools, and flashlights, are often drawn into the engine and can cause damage to the fan blades. [Figure 10-59] Do not carry any objects in pockets when working around operational turbine engines. A compressor rotor can be damaged beyond repair by tools that are left in the air intake, where they are drawn into the engine on subsequent starts. A simple solution to the problem is to check the tools against a tool checklist. Prior to starting a turbine engine, make a minute inspection of engine inlet ducts to assure that items, such as nuts, bolts, lock wire, or tools, were not left there after work had been performed.

axial-flow engine. The descriptions and possible causes of blade damage are given in Figure 10-61. Corrosion pitting is not considered serious on the compressor stator vanes of axial-flow engines if the pitting is within the allowed tolerance. Do not attempt to repair any vane by straightening, brazing, welding, or soldering. Crocus cloth, fine files, and stones are used to blend out damage by removing a minimum of material and leaving a surface finish comparable to that of a new part. The purpose of this blending is to minimize stresses that concentrate at dents, scratches, or cracks. The inspection and repair of air intake guide vanes, swirl vanes, and screens on centrifugal-flow engines necessitates the use of a strong light. Inspect screen assemblies for breaks, rips, or holes. Screens may be tin-dipped to tighten the wire mesh, provided the wires are not worn too thin. If the frame strip or lugs have separated from the screen frames, re-brazing may be necessary.

Inspect the guide and swirl vanes for looseness. Inspect the outer edges of the guide vanes, paying particular attention to 10-46 These dimensions controlled by depth limit Area E These dimensions controlled by depth limit Area C The limits referred to in this figure in areas C and E pertain to local, isolated, damaged areas only and must not be interpreted as authority for removal of material all across the tip and leading or trailing edges as might be done in a single machining cut. CAUTION Maximum allowable repair limits-inches 5/16 R 1/32 D 5/32 D .008 D 1/32 D 1/4 R 1/32 D 1/8 D .005 D 1/32 D 5/16 R 1/32 D 5/32 D NONE 1/32 D 1/4 R 1/32 D 1/8 D NONE 1/32 D Steel blades Titanium blades Stages Stages 1 through 4 5 through 9 1 through 4 5 through 9 Blade area A B C D E R—radius D—depth Area B Area A Area C Area EArea E Area D Fillet areaFillet area Concave and convex surfaceConcave and convex surface Approximate centerlineApproximate centerline RadiusRadius Concave and convex surface Concave and convex surface ¼" ¼" Damaged blade Damaged blade after blending Damaged blade Damaged blade after blending 10-47 the point of contact between the guides and swirl vanes for cracks and dents due to the impingement of foreign particles.

Inspect the edges of the swirl vanes. Inspect the downstream edge of the guide vanes very closely, because cracks are generally more prevalent in this area. Cracks that branch or fork out so that a piece of metal could break free and fall into the compressor are cause for vane rejection. Blending & Replacement Because of the thin-sheet construction of hollow vanes, blending on the concave and convex surfaces, including the leading edge, is limited. Small, shallow dents are acceptable if the damage is of a rounded or gradual contour type and not a sharp or V-type, and if no cracking or tearing of vane material is evident in the damaged area.

Trailing edge damage may be blended, if one-third of the weld seam remains after repair. [Figure 10-62] Concave surfaces of rubber-filled vanes may have allowable cracks extending inward from the outer airfoil, provided there is no suggestion of pieces breaking away. Using a light and mirror, inspect each guide vane trailing edge and vane body for cracks or damage caused by foreign objects. Any inspection and repair of turbine compressor section components require that the technician always use the specific manufacturer’s current information for evaluation and limits of repairs. Combustion Section Inspection One of the controlling factors in the service life of the turbine engine is the inspection and cleaning of the hot section.

Emphasis must be placed on the importance of careful inspection and repair of this section. The following are general procedures for performing a hot section (turbine and combustion section) inspection. It is not intended to imply that these procedures are to be followed when performing repairs or inspections on turbine engines. However, the various practices are typical of those used on many turbine engines. Where a clearance or tolerance is shown, it is for illustrative purposes only. Always follow the instructions contained in the applicable manufacturer's maintenance and overhaul manuals. The entire external combustion case should be inspected for evidence of hot spots, exhaust leaks, and distortions before the case is opened. After the combustion case has been opened, the combustion chambers can be inspected for localized overheating, cracks, or excessive wear. [Figure 10-63] Inspect the first stage turbine blades and nozzle guide vanes for cracks, warping, or FOD. Also inspect the combustion chamber outlet ducts and turbine nozzle for cracks and for evidence of FOD.

One of the most frequent discrepancies that are detected while inspecting the hot section of a turbine engine is cracking. These cracks may occur in many forms, and the only way to determine that they are within acceptable limits or if they are allowed at all, is to refer to the applicable engine manufacturer’s service and overhaul manuals. Cleaning the hot section is not usually necessary for a repair in the field, but in areas of high salt water or other chemicals a turbine rinse should be accomplished. Engine parts can be degreased by using the emulsion-type cleaners or chlorinated solvents. The emulsion-type cleaners are safe for all metals, since they are neutral and noncorrosive.

Cleaning parts by the chlorinated solvent method leaves the parts absolutely dry. If they are not to be subjected to further cleaning operations, they should be sprayed with a corrosion- preventive solution to protect them against rust or corrosion. The hot section, which generally includes the combustion section and turbine sections, normally require inspections at regular intervals. The extent of disassembly of the engine to accomplish this inspection varies from different engine types. Most engines require that the combustion case be open for the inspection of the hot section. However, in performing this disassembly, numerous associated parts are readily accessible for inspection. The importance of properly supporting the engine and the parts being removed cannot be overstressed.

The alignment of components being removed and installed is also of the utmost importance. After all the inspections and repairs are made, the manufacturer’s detailed assembly instructions should be followed. These instructions are important in efficient engine maintenance, and the ultimate life and performance of the engine. Extreme care must be 10-48 Dent Calling Scratches Score Cracks Burr Burn Damage repair (blend) Corrosion (putting) Term Appearance Usual Causes • Smooth repair of ragged edge or surface into the contour of surrounding area. • Bent blade. • Damage to surfaces evidenced by discoloration or, in severe cases, by flow of material.

• A ragged or turned out edge. • Breakdown of the surface; pitted appearance. • A partial fracture (separation). • Small, smoothly rounded hollow. • A transfer of metal from one surface to another. • Displacement of material from a surface; a cutting or tearing effect. • Elongation of blade. • (See corrosion). • Contour of a blade or surface. • Deep scratches. • Narrow shallow marks. • Blend • Bow • Burning • Burr • Corrosion (pits) • Cracks • Dent • Gall • Gouging • Growth • Pit • Profile • Score • Scratch • Foreign objects. • Excessive heat. • Grinding or cutting operation. • Corrosive agents—moisture, etc.

• Excessive stress due to shock, overloading, or faulty processing; defective materials; overheating. • Striking of a part with a dull object. • Severe rubbing. • Presence of a comparatively large foreign body between moving parts. • Continued and/or excessive heat and centrifugal force. • Presence of chips between surfaces. • Sand or fine foreign particles; careless handling. 10-49 Before blending After blending taken during assembly to prevent dirt, dust, cotter pins, lock wire, nuts, washers, or other foreign material from entering the engine. Marking Materials for Combustion Section Parts Certain materials may be used for temporary marking during assembly and disassembly. Always refer to manufacturer’s information for marking parts. Layout dye (lightly applied), a felt tip marker, or chalk may be used to mark parts that are directly exposed to the engine’s gas path, such as turbine blades and discs, turbine vanes, and combustion chamber liners. A wax marking pencil may be used for parts that are not directly exposed to the gas path. Do not use a wax marking pencil on a liner surface or a turbine rotor. The use of carbon alloy or metallic pencils is not recommended because of the possibility of causing intergranular corrosion attack, that could result in a reduction in material strength and cracking.

Inspection & Repair of Combustion Chambers Inspect the combustion chambers and covers for cracks by using visible dye or fluorescent penetrant inspection method. Any cracks, nicks, or dents are usually cause for rejecting the component. Inspect the covers, noting particularly the area around the fuel drain bosses for any pits or corrosion. When repairing the combustion chamber liner, the procedures given in the appropriate engine manufacturer’s overhaul instruction manual should be followed. If there is doubt that the liner is serviceable, it should be replaced. Combustion chambers should be replaced or repaired if two cracks are progressing from a free edge so that their meeting is imminent and could allow a piece of metal that could cause turbine damage to break loose. Separate cracks in the baffle are acceptable. Cracks in the cone are rare but, at any location on this component, is cause for rejection of the liner. Cracks in the swirl vanes are cause for rejection of the liner. Loose swirl vanes may be repaired by silver brazing.

Cracks in the front liner emanating from the air holes are acceptable, provided they do not exceed allowable limits. If such cracks fork or link with others, the liner must be repaired. If two cracks originating from the same air hole are diametrically opposite, the liner is acceptable. Radial cracks extending from the interconnector and spark igniter boss are acceptable, if they do not exceed allowable limits and if such cracks do not fork or link with others. Circumferential 10-50 cracks around the boss pads should be repaired prior to re-use of the liner. Baffle cracks connecting more than two holes should be repaired.

After long periods of engine operation, the external surfaces of the combustion chamber liner location pads often show signs of fretting. This is acceptable, provided no resultant cracks or perforation of the metal is apparent. Any cover or chamber inadvertently dropped on a hard surface or mishandled should be thoroughly inspected for minute cracks that may elongate over a period of time and then open, creating a hazard. Parts may be found where localized areas have been heated to an extent to buckle small portions of the chamber. Such parts are considered acceptable if the burning of the part has not progressed into an adjacent welded area, or to such an extent as to weaken the structure of the liner weldment.

Buckling of the combustion chamber liner can be corrected by straightening the liner. Moderate buckling and associated cracks are acceptable in the row of cooling holes. More severe buckling that produces a pronounced shortening or tilting of the liner is cause for rejection. Upon completion of the repairs by welding, the liner should be restored as closely as possible to its original shape. Fuel Nozzle & Support Assemblies Clean all carbon deposits from the nozzles by washing with a cleaning fluid approved by the engine manufacturer and remove the softened deposits with a soft bristle brush. It is desirable to have filtered air passing through the nozzle during the cleaning operation to carry away deposits as they are loosened. Make sure all parts are clean. Dry the assemblies with clean, filtered air. Because the spray characteristics of the nozzle may become impaired, no attempt should be made to clean the nozzles by scraping with a hard implement or by rubbing with a wire brush. Inspect each component part of the fuel nozzle assembly for nicks and burrs. Many fuel nozzles can be checked by flowing fluid through the nozzle under pressure and closely checking the flow pattern coming for the nozzle.

Turbine Disc Inspection The inspection for cracks is very important because cracks are not normally allowed. Crack detection, when dealing with the turbine disc and blades, is mostly visual, although structural inspection techniques can be used, such as penetrant methods and others to aid in the inspection. Cracks on the disc necessitate the rejection of the disc and replacement of the turbine rotor. Slight pitting caused by the impingement of foreign matter may be blended by stoning and polishing. Turbine Blade Inspection Turbine blades are usually inspected and cleaned in the same manner as compressor blades. However, because of the extreme heat under which the turbine blades operate, they are more susceptible to damage. Using a strong light and a magnifying glass, inspect the turbine blades for stress rupture cracks and deformation of the leading edge.

[Figures 10-64 and 10-65] Stress rupture cracks usually appear as minute hairline cracks on or across the leading or trailing edge at a right angle to the edge length. Visible cracks may range in length from one-sixteenth inch upward. Deformation, caused by over-temperature, may appear as waviness and/or areas of varying airfoil thickness along the leading edge. The leading edge must be straight and of uniform thickness along its entire length, except for areas repaired by blending. Do not confuse stress rupture cracks or deformation of the leading edge with foreign material impingement damage or with 10-51 ½" ¼" Nicks at tip edge not critical blend raised edge only Examine carefully for indication of cracks at edges of fir-tree serrations. Reject for any indication of cracks.

Tip curling permissible in these areas, if under certain circumstances. Tip curling permissible in these areas, if under certain circumstances. Nicks over .008-inch deep are cause for rejection except that if nicks are slightly deeper than .006-inch but do not exceed .012-inch in depth and are well away from the lead or trail edge the blade is acceptable for continued use. Nicks on the convex surface away from the leading and the trailing edges need not be completely blended out. Same for the concave surface. Nicks on the trailing edge must be completely removed. If too much material must be removed, reject blade.

Entire trailing edge is critical area. Nicks at outer tip end not as critical as nicks nearer the root end, since the nearer the root end, the greater the moment-weight tending to produce a crack. Nicks that “come through” to underside of blade are cause for rejection. Nicks on the leading edge must be completely blended out. If too much material must be removed, reject blade. blending repairs to the blade. When any stress rupture cracks or deformation of the leading edges of the first-stage turbine blades are found, an over-temperature condition must be suspected. Check the individual blades for stretch and the turbine disc for hardness and stretch. Blades removed for a detailed inspection or for a check of turbine disc stretch must be re-installed in the same slots from which they were removed. Number the blades prior to removal.

The turbine blade outer shroud should be inspected for air seal wear. If shroud wear is found, measure the thickness of the shroud at the worn area. Use a micrometer or another suitable and accurate measuring device that ensures a good reading in the bottom of the comparatively narrow wear groove. If the remaining radial thickness of the shroud is less than that specified, the stretched blade must be replaced. Typical blade inspection requirements are indicated in Figure 10-66. Blade tip curling within a one-half inch square area on the leading edge of the blade tip is usually acceptable if the curling is not sharp. Curling is acceptable on the trailing edge if it does not extend beyond the allowable area. Any sharp bends that may result in cracking or a piece breaking out of the turbine blade is cause for rejection, even though the curl may be within the allowable limits. Each turbine blade should be inspected for cracks.

Turbine Blade Replacement Procedure Turbine blades are generally replaceable, subject to moment- weight limitations. These limitations are contained in the 10-52 DK 1DV 54DU 53DU 52DU 51DU50 DU49 DU48 DT47 DT46 DT45 DS44 DR43 DR42 DP41 DP40 DN39 DN38 DM37 DM36 DM 35 DL 34 DL 33 DL 32 DL 31 DL 30 DK 29 DK 28 DV 27 DU 26 DU 25 DU 24 DU23 DU22 DU21 DT20 DT19 DT18 DS17 DR16 DR15 DP14 DP13 DN12 DN11 DM10 DM 9 DM 8 DL 7 DL 6 DL 5 DL 4 DL 3 DK 2 Spherical indent on face of discs Clockwise Viewed from rearViewed from rear distribution. for inspection. The blade limits specified in the engine manufacturer’s overhaul and service instruction manual should he adhered to. Figure 10-69 shows where cracks usually occur on a turbine nozzle assembly. Slight nicks and dents are permissible if the depth of damage is within limits.

Inspect the nozzle vanes for nicks or cracks. Small nicks are not cause for vane rejection, provided such nicks blend out smoothly. Inspect the nozzle vane supports for defects caused by the impingement of foreign particles. Use a stone to blend any doubtful nicks to a smooth radius. Like turbine blades, it is possible to replace a maximum number of turbine nozzle vanes in some engines. If more than the maximum vanes are damaged, a new turbine nozzle vane assembly must be installed. With the tailpipe (exhaust nozzle) removed, the rear turbine stage can be inspected for any cracks or evidence of blade stretch. Additional nozzle stages can also be inspected with a strong light by looking through the rear-stage turbine.

Clearances Checking the clearances is one of the procedures in the maintenance of the turbine section of a turbine engine. The manufacturer’s service and overhaul manual gives the procedures and tolerances for checking the turbine. Turbine clearances being measured at various locations are shown in Figures 10-70 and 10-71. To obtain accurate readings, special tools provided by each manufacturer must be used engine manufacturer’s applicable technical instructions. If visual inspection of the turbine assembly discloses several broken, cracked, or eroded blades, replacing the entire turbine assembly may be more economical than replacing the damaged blades. [Figure 10-67] In the initial buildup of the turbine, a complete set of 54 blades made in coded pairs (two blades having the same code letters) is laid out on a bench in the order of diminishing moment-weight. The code letters, indicating the moment- weight balance in ounces, are marked on the rear face of the fir-tree section of the blade (viewing the blade as installed at final assembly of the engine). The pair of blades having the heaviest moment-weight is numbered 1 and 28; the next heaviest pair of blades is numbered 2 and 29; the third heaviest pair is numbered 3 and 30. This is continued until all the blades have been numbered. Mark a number 1 on the face of the hub on the turbine disc. The number 1 blade is then installed adjacent to the number 1 on the disc. [Figure 10-68] The remaining blades are then installed consecutively in a clockwise direction, viewed from the rear face of the turbine disc. If there are several pairs of blades having the same code letters, they are installed consecutively before going to the next code letters. If a blade requires replacement, the diametrically opposite blade must also be replaced. Computer programs generally determine the location for turbine blades for turbine wheels on modern engines.

Turbine Nozzle Inlet Guide Vane Inspection After removing the required components, the first stage turbine blades and turbine nozzle vanes are accessible 10-53 Turbine nozzle assembly Spot weld cracks on inner duct Cracked area along spot weld line on inner duct Turbine nozzle assembly at junction of combustion chamber outlet duct and turbine nozzle outer case 10-54 clearances. as described in the service instructions for specific engines.

Exhaust Section

The exhaust section of the turbine engine is susceptible to heat cracking. This section must be thoroughly inspected along with the inspection the combustion section and turbine section of the engine. Inspect the exhaust cone and exhaust nozzle for cracks, warping, buckling, or hot spots. Hot spots on the tail cone are a good indication of a malfunctioning fuel nozzle or combustion chamber. The inspection and repair procedures for the hot section of any one gas turbine engine share similarities to those of other gas turbine engines. One usual difference is the nomenclature applied to the various parts of the hot section by the different manufacturers. Other differences include the manner of disassembly, the tooling necessary, and the repair methods and limits.

Engine Ratings The flat rating of a turbine engine is the thrust performance that is guaranteed by the manufacturer for a new engine under specific operating conditions, such as takeoff, maximum continuous climb, and cruise power settings. The turbine inlet temperature is proportional to the energy available to turn the turbine. This means that the hotter the gases are that are entering the turbine section of the engine, the more power is available to turn the turbine wheel. The exhaust temperature is proportional to the turbine inlet temperature. Regardless of how or where the exhaust temperature is taken on the engine for the flight deck reading, this temperature is proportional to the temperature of the exhaust gases entering the first stage of inlet guide vanes. A higher EGT corresponds to a larger amount of energy to the turbine so it can turn the compressor faster. This works fine until the temperature reaches a point when the turbine inlet guide vanes start to be damaged. EGT must be held constant or lowered as the result of a prolonged hot section life and, at the same time, provide the thrust to meet the certification requirements.

Before high bypass turbofan engines, some older types of engines used water injection to increase thrust for takeoff (wet). This is the maximum allowable thrust for takeoff. The rating is obtained by actuating the water-injection system and setting the computed wet thrust with the throttle, in terms of a predetermined turbine discharge pressure or engine pressure ratio for the prevailing ambient conditions. The rating is restricted to takeoff, is time-limited, and has an altitude limitation. Water injection is not used very much on turbine engines any more. Turbine Engine Instruments Engine Pressure Ratio Indicator Engine pressure ratio (EPR) is an indication of the thrust being developed by a turbofan engine and is used to set power for takeoff on many types of aircraft. It is instrumented by total pressure pickups in the engine inlet (Pt2) and in the turbine exhaust (Pt7). The reading is displayed in the flight deck by the EPR gauge, which is used in making engine power settings. [Figure 10-72]