TABLE 7-15. TABLE OF PINS (CONTINUED) Pin Number Description Pin Number Description NAS1332-1346 Pin, quick release NAS1535-1542 Pin, swage lock, 100° hd., tension, pul type, alum.
NAS1353-1366 Pin, quick release, NAS1546-1552 Pin, swage lock, positive locking, 100° hd., tension, double acting stump type, alum.
NAS1407 Pin, spring, NAS1583 Pin, 100° csk. hd., coiled hi-shear rivet, 1200 F NAS1436-1442 Pin, swage lock, NAS1584 Pin, fathead, 0000000000001 100° shear head, pull type, steel hi-shear rivet, 1200 F NAS1446-1452 Pin, swage lock, protruding head, pull type, steel NAS1456-1462 Pin, swage lock, 100° head, pull type, steel NAS1465-1472 Pin, swage lock, protruding head, OSCOIN pull type, steel NAS1475-1482 Pin, swage lock, 100° head, pull type, steel NAS1486-1492 Pin, swage lock, 100° head, stump type, steel NAS1496-1502 Pin, swage lock, stump type, steel protruding head NAS1525-1532 Pin, swage lock, protruding head, al. alloy
7-196.-7-206. [RESERVED.] Par 7-195 Page 7-83
Par 8-1 Page 8-1 CHAPTER 8. ENGINES, FUEL, EXHAUST, AND PROPELLERS SECTION 1. ENGINES
8-1. GENERAL. Consult the manufacturer’s manuals, Service Bulletins (SB), and instruction books regarding the repair and overhaul, inspection, installation, and maintenance of aircraft engines for that particular make, model, and type of engine. This section lists acceptable inspection and repair procedures that may be used in the absence of an engine manufacturer’s maintenance information.
8-2. SPECIAL INSPECTION. A visual inspection is needed to determine the condition of the engine and its components. An annual or 100-hour inspection should include the engine and nacelle group as follows.
a. Cold Cylinder Check. If an engine is running rough, the cause may be a bad ignition lead, a spark plug not firing, a partially clogged fuel injector, or a bad magneto. The dead cylinder will be colder than the surrounding cylinders and can be quickly determined by using the recommended cold cylinder checks. This should be done using a thermocouple probe, which is very sensitive to small differences in temperature, which is the case with a partially clogged injector. For a carbureted engine, the following check may be helpful:
(1) Using experienced personnel, run the engine on the bad magneto for approximately 30 seconds at 1200 revolutions per minute (rpm). Without switching the magneto switch back to both shut off the engine. Have another mechanic use a grease pencil (non-carbon) and quickly mark each exhaust stack approximately 1 inch from the flange that holds the exhaust stack to the cylinder. Next, check the exhaust stacks and look for the exhaust stack whose grease pencil mark has not turned to a grayish-white or ash color. This is the cold cylinder.
(2) The probable cause of the cold cylinder is either a defective spark plug or ignition lead. Switch the spark plugs to another cylinder and run the test again. If the problem stays with the original cylinder, the problem is either the ignition lead or the magneto.
b. Piston Engine Sudden Stoppage Inspection. Sudden stoppage is a very rapid and complete stoppage of the engine. It can be caused by engine seizure or by one or more of the propeller blades striking an object in such a way that rpm goes to zero in less than one complete revolution of the propeller. Sudden stoppage can cause internal damage to constant-speed propellers; reduction drive; gear train damage in the accessory section; crankshaft misalignment;
or damage to accessories such as magnetos, generators, vacuum pumps, and tach generators.
(1) Every engine that suffers a sudden stoppage must be inspected in accordance with the manufacturer’s maintenance instructions before being returned to service.
(2) If the engine manufacturer does not provide the required information, then the engine case must be opened, and every major component part must be inspected using visual and/or nondestructive inspection (NDI) procedures as applicable.
(3) The sudden-stoppage inspections include checking for cowling, spinner, and airframe cracks and hidden damage and alignment of the engine mount to the airframe, the
Par 8-2 Page 8-2 mounting hardware, isolation mounts, and bushings. The aircraft’s firewall must also be checked for distortion, cracks, and elongated bolt holes. The damaged propeller must be sent to a Federal Aviation Administration (FAA)-certificated repair station for complete NDI and repair.
(4) Engine accessories, such as magnetos, starters, fuel pumps, turbochargers, alternators, or generators, must be inspected in accordance with the manufacturer’s maintenance manual on sudden stoppage or overhaul procedures to determine the product’s airworthiness.
c. Reciprocating Engine (Direct Drive). Preliminary inspection before teardown.
(1) Remove the engine cowling and examine the engine for visible external damage and audible internal damage.
(2) Rotate the propeller shaft to determine any evidence of abnormal grinding or rubbing sounds.
(3) With the propeller removed, inspect the crankshaft flange or splines for signs of twisting, cracks, or other deformation. Remove the thrust-bearing nut and seal and thoroughly inspect the threaded area of the shaft for evidence of cracks.
(4) Rotate the shaft slowly in 90-degree increments while using a dial indicator or an equivalent instrument to check the concentricity of the shaft.
(5) Remove the oil sump drain plug and check for metal chips and foreign material.
(6) Remove the oil screens and inspect for metal particles and contamination.
(7) Visually inspect the engine case exterior for signs of oil leaks and cracks. Give particular attention to the propeller thrust-bearing area of the nose case section.
(8) Inspect cylinders and the cylinder hold-down area for cracks and oil leaks.
Thoroughly investigate any indication of cracks, oil leaks, or other damage.
d. Internal Inspection Requirements.
(1) On engines equipped with crankshaft vibration dampers, remove and inspect the cylinders and inspect the crankshaft dampers in accordance with the engine manufacturer’s inspection and overhaul manual. When engine design permits, remove the damper pins and examine the pins and damper liners for signs of nicks or brinelling.
(2) After removing the engine-driven accessories, remove the accessory drive case and examine the accessory and supercharger drive gear train, couplings, and drive case for evidence of damage.
(a) Check for cracks in the case in the area of accessory mount pads and gear shaft bosses.
(b) Check the gear train for signs of cracked, broken, or brinelled teeth.
Par 8-2 Page 8-3
(c) Check the accessory drive shaft couplings for twisted splines, misalignment, and run-out.
(d) Check connecting rods for cracks and straightness.
e. Reciprocating Engine (Gear-Drive). Inspect the engine, propeller (see Section 4 on propeller inspection), and components as described in the preceding paragraphs.
(1) Remove the propeller reduction gear housing and inspect for:
(a) Loose, sheared, or spalled cap screws or bolts.
(b) Cracks in the case.
(2) Disassemble the gear train and inspect the propeller shaft, reduction gears, and accessory drive gears for nicks, cracks, or spalling.
f. Engine-Mount Inspection.
(1) Examine the engine flex mounts, when applicable, for looseness of engine to mount, distortion, or signs of wear.
(2) Inspect the engine-mount structure for bent, cracked, or buckled tubes.
(3) Check the adjacent airframe structure firewall for cracks, distortion, or wrinkles.
(4) Remove engine-mount bolts and mount hold-down bolts and replace.
g. Exhaust-Driven Supercharger (Turbo) Inspection. Sudden stoppage of the powerplant can cause the heat in turbine parts to heat soak the turbine seals and bearings. This excessive heat causes carbon to develop in the seal area and varnish to form on the turbine bearings and journals.
(1) Inspect all air ducts and connections for air leaks, warpage, or cracks.
(2) Remove compressor housing and check the turbine wheel for rubbing or binding and coke or varnish buildup.
NOTE: Turbine turbosupercharger disk seal rubbing is not unusual and may be a normal condition. Consult the engine manufacturer’s inspection procedures and table of limits.
h. Accessory and Drive Inspection. Check the drive shaft of each accessory,
i.e., magnetos, generators, external superchargers, and pumps, for evidence of damage.
8-3. CRANKSHAFT INSPECTION AND REPAIR REQUIREMENTS. Carefully inspect for misalignment and replace if bent beyond the manufacturer’s permissible service limit. Worn journals may be repaired by regrinding in accordance with manufacturers’ instructions. It is recommended that grinding operations be performed by appropriately rated repair stations or the
Par 8-3 Page 8-4 original engine manufacturer. Common errors that occur in crankshaft grinding are the removal of nitrided journal surface, improper journal radii, unsatisfactory surfaces, and grinding tool marks on the journals. If the fillets are altered, do not reduce their radii. Polish the reworked surfaces to assure removal of all tool marks. Most opposed engines have nitrided crankshafts, and engine manufacturers specify that these crankshafts must be re-nitrided after grinding.
NOTE: Rapid deceleration or momentary slowing of a propeller may occur due to contact with tall grass, water, or snow. If this occurs, the engine and propeller should be inspected in accordance with the manufacturer’s instruction or SBs.
8-4. REPLACEMENT PARTS IN CERTIFICATED ENGINES. Engine replacement parts must be approved under Title 14 of the Code of Federal Regulations (14 CFR) part 21.
Serviceable parts obtained from the engine manufacturer, authorized service facility, and those which are approved FAA/Parts Manufacturer Approval (PMA) or Technical Standard Order (TSO) and meet the requirements of part 21 are acceptable for use as replacement parts. Used engine parts can be installed if that part either conforms to new part tolerances or meets the manufacturer’s service limits. Ensure that used parts are airworthy and properly identified as a PMA or TSO part.
8-5. OIL SYSTEM LINES INSPECTION. The inspection of the plumbing for an oil system is similar to the inspection of any other plumbing system. The tubing, hose, tube fittings, hose fittings, hose clamps, and all other components of the system are inspected for cracks, holes, dents, bulges, and other signs of damage that might restrict the oil flow or cause a leak. All lines are inspected to ensure that they are properly supported and are not rubbing against a structure.
Fittings should be checked for signs of improper installation, overtorquing, excessive tension, or other conditions which may lead to failure.
8-6. OIL FILTER INSPECTION. The oil filter provides an excellent method for discovering internal engine damage. During the inspection of the engine oil filter, the residue on the screens, disks, or disposable filter cartridge and the residue in the filter housing are carefully examined for metal particles. A new engine or a newly overhauled engine will often have a small amount of fine metal particles in the screen or filter, but this is not considered abnormal. After the engine has been operated for a time and the oil has been changed one or more times, there should not be an appreciable amount of metal particles in the oil screen. If an unusual residue of metal particles is found in the oil screen, the engine must be taken out of service and disassembled to determine the source of the particles. As an additional precaution, an oil analysis/trend analysis may prevent an engine failure in flight. At oil changes, oil samples are often taken and sent to laboratories to be analyzed for wear by determining the amount of metal in the sample. Over time, a trend is developed, and the engine can be removed from service before failure.
8-7. CYLINDER HOLD-DOWN NUTS AND CAP SCREWS. Great care is required in tightening cylinder hold-down nuts and cap screws. They must be tightened to the recommended torque limits to prevent improper stressing and to ensure even loading on the cylinder flange.
The installation of baffles, brackets, clips, and other extraneous parts under nuts and cap screws is not a good practice and is discouraged. If these baffles, brackets, etc., are not properly fabricated or made of suitable material, they may cause loosening of the nuts or cap screws even
Par 8-7 Page 8-5 though the nuts or cap screws were properly tightened and locked at installation. Improper prestressing or loosening of any one of these nuts or cap screws will introduce the danger of progressive stud failure with the possible loss of the engine cylinder in flight.
8-8. REUSE OF SAFETYING DEVICES. Do not use cotter pins and safety wire a second time. Flat, steel-type wrist pin retainers and lock washers, likewise, must be replaced at overhaul unless the manufacturer’s recommendations permit their reuse.
8-9. SELF-LOCKING NUTS FOR AIRCRAFT ENGINES AND ACCESSORIES.
Self-locking nuts may be used on aircraft engines provided the following criteria are met:
a. When their use is specified by the engine manufacturer in the assembly drawing, parts list, and bills of material.
b. When the nuts will not fall inside the engine should they loosen and come off.
c. When there is at least one full thread protruding beyond the nut.
d. Where the temperature will not exceed the maximum limits established for the self-locking material used in the nut. On many engines, the cylinder baffles, rocker box covers, drive covers and pads, and accessory and supercharger housings are fastened with fiber insert lock nuts, which are limited to a maximum temperature of 250 °F. Above this temperature, the fiber insert will usually char and, consequently, lose its locking characteristic. For locations such as the exhaust pipe attachment to the cylinder, a locknut, which has good locking features at elevated temperatures, will give invaluable service. In a few instances, fiber insert lock nuts have been approved for use on cylinder hold-down studs. This practice is not generally recommended since especially tight stud fits to the crankcase must be provided and extremely good cooling must prevail so that low temperatures exist where the nut is installed.
e. Information concerning approved self-locking nuts and their use on specific engines are usually found in engine manufacturer’s manuals or bulletins. If the desired information is not available, it is suggested that the engine manufacturer be contacted.
f. See Chapter 7, Aircraft Hardware, Control Cables, and Turnbuckles, for additional information on self-locking nuts.
8-10. METALLIZING. Metallizing internal parts of aircraft engines is not acceptable unless it is proven to the FAA that the metallized part will not adversely affect the airworthiness of the engine. Metallizing the finned surfaces of steel cylinder barrels with aluminum is acceptable since many engines are originally manufactured in this manner.
8-11. PLATING. Before restoring the plating on any engine part in accordance with the manufacturer’s instructions, the part should be visually inspected and have an NDI performed before any cylinder reconditioning. In general, chromium plating would not be applied to highly stressed engine parts. Certain applications of this nature have been found to be satisfactory;
however, engineering evaluation of the details for the processes used should be obtained.
Par 8-11 Page 8-6
a. Dense chromium plating of the crankpin and main journals of some small engine crankshafts has been found satisfactory except where the crankshaft is already marginal in strength. Plating to restore worn, low-stress engine parts, such as accessory drive shafts and splines, propeller shaft ends, and seating surfaces of roller and ball-type bearing races, is acceptable but requires compliance with FAA-approved data.
b. Porous chromium-plated walls of cylinder barrels have been found to be satisfactory for practically all types of engines. Dense or smooth chromium plating without roughened surfaces, on the other hand, has not been found to be satisfactory.
(1) Cylinder barrel pre-grinding and chromium plating techniques used by the military are considered acceptable for all engines, and military-approved facilities engaged in doing this work in accordance with military specifications are eligible for approval by the FAA.
(2) Chromium-plated cylinder barrels have been required for some time to be identified in such a manner that the markings are visible with the cylinder installed. Military-processed cylinders are banded with orange enamel above the mounting flange. It has been the practice to etch on either the flange edge or on the barrel skirt the processor’s initials and the cylinder oversize. Most plating facilities use the orange band as well as the permanent identification marks.
(3) A list of engine and maximum permissible cylinder barrel oversize are referenced in Table 8-1.
TABLE 8-1. CURRENT ENGINE AND MAXIMUM PERMISSIBLE CYLINDER
BARREL OVERSIZE Engine Manufacturer Engine Series Max. Oversize (in) Aircooled Motors (Franklin) No oversize for sleeved cylinders.
Solid cylinders
0.017 Continental Motors R-670, W-670, R9A 0.010 to 0.020 GTSIO-520, 550 0.005 All others 0.015 Jacobs All 0.015 Kinner All 0.015 Pigman, LeBlond, Rearwin, Ken Royce All 0.025 Lycoming All 0.010 to 0.020 Menasco All 0.010
Par 8-11 Page 8-7 Engine Manufacturer Engine Series Max. Oversize (in) Pratt & Whitney R-2800B, C, CA, CB 0.025 *R-959 and R-1830 0.030 All others 0.020 Ranger 6-410 early cyls. 6-390 0.010 6-410 late cyls. 6-440 (L-440) series 0.120 Warner All 0.015 Wright All 0.020 *(The above oversize limits correspond to the manufacturer’s requirements, except for P&W R-985 and R-1830 series engines.)
NOTE: (Check for latest manufacturer specifications.)
(4) Cylinder barrels, which have been plated by an agency whose process is approved by the FAA and which have not been worked beyond maximum permissible limits, will be considered acceptable for installation on certificated engines. It will be the responsibility of the owner or the repairing agency to provide this proof. In some cases, it may be necessary to remove cylinders to determine the amount of oversize since this information may be etched on the mating surface of the cylinder base flange.
8-12. CORROSION. Accomplish corrosion preventive measures for temporary and long-term storage in accordance with the instructions issued by the pertinent engine manufacturer. Avoid the use of solutions which contain strong caustic compounds and all solutions, polishes, cleaners, abrasives, etc., which might possibly promote corrosive action (see Chapter 6, Corrosion, Inspection, and Protection).
8-13. ENGINE RUN-IN. After an aircraft engine has been overhauled, it is recommended that the pertinent aircraft engine manufacturer’s run-in instructions be followed. Observe the manufacturer’s recommendations concerning engine temperatures and other criteria. Repair processes employed during overhaul often necessitate amending the manufacturer’s run-in procedures. Follow the approved amended run-in procedures in such instances.
NOTE: Do not run up engines on the ground for long periods of time with the cowling off. The engine will overheat because cylinder cooling has been disrupted.
8-14. COMPRESSION TESTING OF AIRCRAFT ENGINE CYLINDERS. A test to determine the internal condition of the combustion chamber cylinder assembly by ascertaining if any appreciable internal leakage is occurring is compression testing of aircraft engine cylinders.
If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine and procedures in subparagraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected. To determine the cylinder’s problem area, have someone hold the propeller at the weak cylinder’s top dead center (TDC) and, with compressed air still being applied, listen. If air is heard coming out of the exhaust pipe, the cylinder’s exhaust
Par 8-14 Page 8-8 valve is not seating properly. If air is heard leaking out of the air cleaner/carburetor heat box, the intake valve is leaking. With the oil dipstick removed, and air is rushing out, the piston rings are defective. Remove and repair/overhaul the defective cylinder.
a. Differential Compression Test. The most common type of compression tester currently in use is the differential pressure-type tester. It provides a cross-reference to validate the readings obtained and tends to assure that the cylinder is defective before it is removed.
Before beginning a compression test, consider the following points:
(1) When the spark plugs are removed from the engine, identify them to coincide with the cylinder and location from which they were removed. Close examination of the plugs will reveal the actual operating conditions and aid in diagnosing problems within each individual cylinder.
(2) The operating and maintenance records of the engine should be reviewed. Records of previous compression tests are of assistance in determining progressive wear conditions and help to establish the necessary maintenance corrective actions.
b. Differential Pressure Compression Test. The differential pressure tester is designed to check the compression of aircraft engines by measuring the leakage through the cylinders caused by worn or damaged components. The operation of the compression tester is based on the principle that for any given airflow through a fixed orifice, a constant pressure drop across that orifice will result. The restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:
(1) For an engine cylinder having less than a 5.00-inch bore, 0.040-inch orifice diameter, 0.250-inch long, and a 60-degree approach angle.
(2) For an engine cylinder with 5.00-inch bore and over 0.060-inch orifice diameter,
0.250-inch long, and a 60-degree approach angle.
(3) A typical schematic diagram of the differential pressure tester is shown in Figure 8-1.
Par 8-14 Page 8-9 FIGURE 8-1. SCHEMATIC OF DIFFERENTIAL PRESSURE COMPRESSION TESTER (
4) As the regulated air pressure is applied to one side of the restrictor orifice with the air valve closed, there will be no leakage on the other side of the orifice, and both pressure gauges will read the same. However, when the air valve is opened and leakage through the cylinder increases, the cylinder pressure gauge will record a proportionally lower reading.
(5) While performing the check, the following procedures are listed to outline the principles involved and are intended to supplement the manufacturer’s instructions for the particular tester being used.
(a) Perform the compression test as soon as possible after the engine is shut down to ensure that the piston rings, cylinder walls, and other engine parts are well lubricated.
(b) Remove the most accessible spark plug from each cylinder.
(c) With the air valve closed, apply an external source of clean air (approximately 100 to 120 pounds per square inch (psi)) to the tester.
Par 8-14 Page 8-10
(d) Install an adapter in the spark plug bushing and connect the compression tester to the cylinder.
(e) Adjust the pressure regulator to obtain a reading of 20 psi on the regulator pressure gauge. At this time, the cylinder pressure gauge should also register 20 psi.
(f) Turn the crankshaft, by hand, in the direction of rotation until the piston (in the cylinder being checked) is coming up on its compression stroke. Slowly open the air valve and pressurize the cylinder to 80 psi.
CAUTION: Care must be exercised in opening the air valve since sufficient air pressure will have built up in the cylinder to cause it to rapidly rotate the propeller if the piston is not at TDC.
(g) Continue rotating the engine against this pressure until the piston reaches TDC.
Reaching TDC is indicated by a flat spot or sudden decrease in force required to turn the crankshaft. If the crankshaft is rotated too far, back up at least one-half revolution and start over again to eliminate the effect of backlash in the valve operating mechanism and to keep piston rings seated on the lower ring lands.
(h) Open the air valve completely. Check the regulated pressure and readjust, if necessary, to read 80 psi.
(i) Observe the pressure indication of the cylinder pressure gauge. The difference between this pressure and the pressure shown by the regulator pressure gauge is the amount of leakage through the cylinder. A loss in excess of 25 percent of the input air pressure is cause to suspect the cylinder of being defective; however, recheck the readings after operating the engine for at least 3 minutes to allow for sealing of the rings with oil.
(j) If leakage is still occurring after a recheck, it may be possible to correct a low reading. This is accomplished by placing a fiber drift on the rocker arm directly over the valve stem and tapping the drift several times with a hammer to dislodge any foreign material between the valve face and seat.
NOTE: When correcting a low reading in this manner, rotate the propeller so the piston will not be at TDC. This is necessary to prevent the valve from striking the top of the piston in some engines. Rotate the engine before rechecking compression to reseat the valves in the normal manner.
8-15. SPARK PLUGS. The spark plug provides the high-voltage electrical spark to ignite the fuel/air mixture in the cylinder. The types of spark plugs used in different engines will vary with regard to heat range, reach, thread size, and other characteristics required by the particular installation.
a. Heat Range. The heat range of a spark plug is the principal factor governing aircraft performance under various service conditions. The term “heat range” refers to the classification of spark plugs according to their ability to transfer heat from the firing end of the spark plug to the cylinder head.
Par 8-15 Page 8-11
(1) Spark plugs have been classified as “hot,” “normal,” and “cold.” However, these terms may be misleading because the heat range varies through many degrees of temperature from extremely hot to extremely cold. Thus, the words “hot,” “cold,” and “normal” do not necessarily tell the whole story.
(2) Since the insulator is designed to be the hottest part of the spark plug, its temperature can be related to the pre-ignition and fouling regions as shown in Figure 8-2.
Pre-ignition is likely to occur if surface areas in the combustion chamber exceed critical limits or if the spark plug core nose temperature exceeds 1,630 ºF (888 ºC). However, fouling or short circuiting of the plug due to carbon deposits is likely to occur if the insulator tip temperature drops below approximately 800 ºF (427 ºC). Since spark plugs must operate between fairly well-defined temperature limits, they must be supplied in various heat ranges to meet the requirements of different engines under a variety of operating conditions.
FIGURE 8-2. CHART OF SPARK PLUG TEMPERATURE RANGES (
3) From the engineering standpoint, each individual plug must be designed to offer the widest possible operating range. This means that a given type of spark plug should operate as hot as possible at low speeds and light loads and as cool as possible under cruising and takeoff power. Plug performance, therefore, depends on the operating temperature of the insulator nose with the most desirable temperature range falling between 1,000 ºF and 1,250 ºF (538 ºC and 677 ºC).
(4) Fundamentally, an engine which runs hot requires a relatively cold spark plug, whereas an engine which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which runs hot, the spark plug tip will be overheated and cause pre-ignition. If a cold spark plug is installed in an engine which runs cool, the tip of the spark plug will collect unburned carbon, causing fouling of the plug. The principal factors governing the heat range of aircraft spark plugs are: