limits of the engine. For example, a particular tank engine utilized a piston design that developed a compression ratio well in excess of the crankshaft absorption rate of the aircraft engine counterpart-result, crankshaft failure.
682. CYLINDER HOLDDOWN NUTS AND CAP- SCREWS. Great care is required in tightening cylinder holddown nuts and capscrews. They must be tightened to recommended torque limits to prevent improper stressing and to insure even loading on the cylinder flange. The installation of baffies, brackets, clips, and other extraneous parts under nuts and capscrews is not considered good practice and should be discouraged. If these baffles, brackets, etc., are not properly fabricated or made of suitable material, they may cause loosening of the nuts or capscrews even though the nuts or capscrews were properly tightened and locked at installation. Either improper prestressing or loosening of any one of these nuts or capscrews will introduce the danger of progressive stud failure with the possible loss of the engine cylinder in flight. Do not install parts made from aluminum alloy or other soft metals under cylinder holddown nuts or capscrews.
683. REUSE OF SAFETYING DEVICES. Do not use cotter pins and safety wire a second time. Flat steel-type wristpin retainers and thin lockwashers likewise should be replaced at overhaul unless the manufacturer's recommendations permit their reuse.
684. SELF-LOCKING NUTS FOR AIRCRAFT ENGINES AND ACCESSORIES. Self-locking nuts may be used on aircraft engines provided the following criteria are met:
a. Where their use is specified by the engine manufacturer in his 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. When the edges of cotter pin or lockwire holes are well rounded to preclude damage to the locknut, Par 681 AC 49.13-1A 1972
•. Prior to reuse, the effectiveness of the selflocking feature is found to be satisfactory,
f. 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 locknuts 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 locknuts have been approved for use on cylinder holddown 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 at this location on the specific engine for which such use is approved.
g. All proposed applications of new types of locknuts or new applications of currently used self-locking nuts must be investigated since many engines require specifically designed nuts. Such specifically designed nuts are usually required for one or more of the following reasons to provide:
(1) Heat resistance,
(2) Adequate clearance for installation and removal,
(3) For the required degree of tightening or locking ability which sometimes requires a stronger, specifically heat-treated material, a heavier cross-section, or a special locking means,
(4) Ample bearing area under the nut to reduce unit loading on softer metals, and
(5) To prevent loosening of studs when nuts are removed.
h. Information concerning approved self-locking nuta and their use on specific engines is 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.
Chap 14
AC 49.13-1А Page 271 1972
685. WELDING OF HIGHLY STRESSED ENGINE a. Dense chromium plating of the crankpin and PARTS. In general, welding of highly-stressed main journals of some small engine crankengine parts is not recommended for parts that shafts has been found satisfactory, except were not originally welded. However, under the where the crankshaft is already marginal in conditions given below, welding may be accom- strength. Plating to restore worn low-stress plished if it can be reasonably expected that the engine parts such as accessory driven shafts welded repair will not adversely affect the air- and splines, propeller shaft ends, and seating worthiness of the engine when: surfaces of roller- and ball-type bearing races
a. The weld is externally situated and can be is acceptable.
inspected easily, b. Porous chromium plated walls of cylinder * b. The part has been cracked or broken as barrels have been found to be satisfactory for practically all types of engines. Dense or the result of unusual loads not encountered in smooth chromium plating without roughened normal operation, surfaces, on the other hand, has not been found
c. A new replacement part of obsolete type to be generally satisfactory.
engine is not available, (1) Cylinder barrel pregrinding and chro-
employed will insure a first-quality weld in the type of material to be repaired and will insure military approved facilities engaged in doing are considered acceptable for all engines, and restoration of the original heat treat in heat- this work in accordance with military specifition on process details. treated parts. Refer to Chapter 2 for informa- Aviation Administration. cations are eligible for approval by the Federal
(2) Chromium plated cylinder barrels have
686. WELDING OF MINOR ENGINE PARTS. Many been required for some time to be identified in minor parts not subjected to high stresses may such a manner that the markings are visible be safely repaired by welding. Mounting lugs, with the cylinder installed. Military processed originally fabricated by welding are in this cowl lugs, cylinder fins, covers, and many parts cylinders are banded with orange enamel above the mounting flange. It has been the practice to category. The welded part should be suitably etch on either the flange edge or on the barrel stress-relieved after welding. skirt the processer's initials and cylinder oversize. Most plating facilities use the orange
687. METALLIZING. Metallizing internal parts of aircraft engines is not acceptable unless it is marks. band as well as the permanent identification proven to the Federal Aviation Administration (3) A current list of engine and maximum that the metallized part will not adversely af- permissible cylinder barrel oversize follows:
fect the airworthiness of the engine. Metalliz- Махітит ing the finned surfaces of steel cylinder barrels Engine manufacturer Enrine serles Oversize (in.)
with aluminum is acceptable, since many engines are originally manufactured in this man- Air Cooled Motors (Franklin). No oversize for sleeved cylinders.
Solid cylinders 0.020 Continental Motors - R-670, W-670, R94 0.020
688. PLATING. Restore plating on engine parts All others . 0.015 in accordance with the manufacturer's instruc- Jacobs All 0.020 tions. In general, chromium plating should not Kinner All 0.015 Pigman, LeBlond, All 0.025 be applied to highly stressed engine parts. Cer- Rearwin, Ken tain applications of this nature have been Hoyce.
found to be satisfactory; however, engineering Lycoming AlI 0.015 evaluation of the details for the processes used Menasco 0.010 should be obtained. (Table continues on next pare.)
Chap 14 Par 685
Page 272 AC 43.13-1А 1972 Pratt & Whitney R-2800B, C, CA, CB 0.025 691. ENGINE RUN-IN. After an aircraft engine *R-985 and R-1880 0.080 has been overhauled, it is recommended the Ranger All others 6-410 early yls. 6-890 - 6-410 late cyls. 6-440 0.020 0.010 0.020 in instructions be followed. Observe the manu- pertinent aircraft engine manufacturer's run- (L-440) series. facturer's recommendations concerning engine Warner All 0.015 temperatures and other criteria.
Wright All 0.020 Repair processes employed during overhaul *(The above oversize limits correspond to the manufacturer's re- often necessitate amending the manufacturquirements, except for PAW R-985 and R-1880 serles engines.)
(4) The following is a list of known agencies, and their identifying initials, performing cylinder barrel plating for the military service.
Agencies Initiale Hol-Chrome Corporation НСС San Antonio Air Materiel Area SAX Koppers Co. (American Hammered Piston er's run-in procedures. Follow the approved amended run-in procedures in such instances.
692. COMPRESSION TESTING OF AIRCRAFT EN- GINE CYLINDERS. The purpose of testing cylinder compression is to determine the internal condition of the combustion chamber by ascertaining if any appreciable leakage is occurring.
McQuay-Norris MQN VanDer Horst Corporation VDH Terry Industries TIX Lement Chromium LC Pennington Channel Chrome PCC Spar-Tan Engineering Company . SEC Superior Aero Chrome SAC
(5) Cylinder barrels which have been plated by an agency whose process is approved by the FAA and which have not been preground 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.
689. ENGINE ACCESSORIES. Overhaul and repair of engine accessories in accord with the engine and the accessory manufacturers' instructions are recommended.
690. CORROSION. Accomplish corrosion preventive measures for temporary and dead storage in accord with the instructions issued by the pertinent engine manufacturer. Avoid the use of strong solutions which contain strong caustic compounds and all solutions, polishes, cleaners, abrasives, etc., which might possibly promote corrosive action. (Refer to Chapter
a. Types of Compression Testers. The two basic types of compression testers currently in use are the direct compression and the differential pressure type testers. The optimum procedure would be to utilize both types of testers when checking the compression of aircraft cylinders.
in this respect, it is suggested that the direct compression method be used first and the findings substantiated with the differential pressure method. This provides a cross-reference to validate the readings obtained by each method 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, identify them to coincide with the cylinder.
Close examination of the plugs will reveal the actual operating conditions and aid in diagnosing problems within the cylinder. Paragraph 693d. of this section contains more information on this subject.
(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 ac-
(3) Before beginning a compression check, precautions should be taken to prevent the accidental starting of the engine.
b. Direct Compression Check. This type of compression test indicates the actual pressures Par 688 Chop 14
AC 43.12-1A Page 273 1972 within the cylinder. Although the particular defective component within the cylinder is difficult to determine with this method, the consistency of the readings for all cylinders is an indication of the condition of the engine as a whole. The following are suggested guidelines for performing a direct compression test.
(1) Thoroughly warm up the engine to operating temperatures and perform the test as soon as possible after shutdown.
(2) Remove the most accessible spark plug from each cylinder.
(3) Rotate the engine with the starter to expel any excess oil or loose carbon in the cylinders.
(4) If a complete set of compression testers is available, install one tester in each cylinder;
however, if only one tester is being used, check each cylinder in turn.
(5) Rotate the engine at least three complete revolutions using the engine starter and record the compression reading.
NOTE: It is recommended that an external power source be used, If possible, as a low battery will result in a slow engine-turning rate and lower readings. This will noticeably affect the validity of the second engine test on a twin-engine aircraft.
(6) Recheck any cylinder which shows an abnormal reading when compared with the others. Any cylinder having a reading approximately 15 p.s.i. lower than the others should be suspected of being defective.
(7) If a compression tester is suspected of being defective, replace it with one known to be accurate, and recheck the compression of the affected cylinders.
ment .040 orifice diameter,..250 inch long, 60° approach angle.
2. Engines in excess of 1,000 cubic inch displacement .060 orifice diameter, .250 inch long, 60° approach angle.
A typical schematic diagram of the differential pressure tester is shown in figure 14.1.
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.
(1) Performing the Check, The following procedures are listed to outline the principles in-
CYLINDER
ADAPTER AIR VALVE
CYLINDER
PRESSURE GUAGE
RESTRICTOR CYLINDER ORFICE
REGULATOR
PRESSURE GUAGE
c. Differential Pressure Compression Check, The differential pressure tester is designed to check the compression of aircraft engines by measur- PRESSURE REGULATOR ing 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 COMPRESSED should be sized for the particular engine as fol- AIR SOURCE lows: FIGURE 14.1.-Schematic of typical differential pres-
1. Engines up to 1,000 cubic inch displace- eure compression tester.
Chap 14 Par 692
volved, and are intended to supplement the manufacturer's instructions for the particular tester being utilized.
(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 p.s.i.) to the tester.
(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 80 p.s.i. on the regulator pressure gauge. At this time, the cylinder pressure gauge should also register 80 p.s.i.
(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 approximately 20 p.s.i.
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 rotate the crankshaft if the piston is not at TDC.
Continue rotating the engine against this pres- (TDC). Reaching TDC is indicated by a flat spot or sudden decrease in force required to tated 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.
(g) Open the air valve completely. Check the regulated pressure and adjust, if necessary, to 80 p.s.i.
(h) Observe the pressure indication on 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; how- Par 692 AC 43.13-TA 1972 ever, recheck the readings after operating the engine for at least 8 minutes to allow for sealing of the rings with oil.
i) 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,
693. 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 in 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 a measure of its ability to transfer heat to the cylinder head. The plug must operate hot enough to burn off the residue deposits which can cause fouling, yet remain cool enough to prevent a preignition condition from occurring.
The length of the nose core is the principal factor in establishing the plug's heat range.
"Hot" plugs have a long insulator nose thereby creating a long heat transfer path, whereas "cold" plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head. (See figure 14.2.)
HOT COLD FIGURE 14.2.-Spark plug heat ranges.
Chap 14
AC 43.13-1A 1972 REACH FIGURE 14.3.-Spark plug reach.
b. Reach, The spark plug reach (figure 14.8) is the threaded portion which is inserted into the spark plug bushing of the cylinder. A plug with the proper reach will insure that the electrode end inside the cylinder is in the best position to achieve ignition. Spark plug seizure or improper combustion within the cylinder will probably occur if a plug with the wrong reach is used.
c. Installation Procedures. When installing spark plugs, observe the following procedure:
(1) Visually inspect the plug for cleanliness and condition of the threads, ceramic, and electrodes.
NoTE: Never install a spark plug which has been
(2) Check the plug for the proper gap setting using a round wire feeler gauge as shown in figure 14.4. In the case of used plugs, procedures for cleaning and regapping are usually contained in the various manufacturers' manuals.
CORRECT
INCORRECT FIGURE 14.4.-Method of checking spark plug gap.
Chap 14 Page 275
(3) Check the plug and cylinder bushing to ascertain that only one gasket is used per spark plug. When a thermocouple-type gasket is used, no other gasket is required.
(4) Apply antiseize compound sparingly to the shell threads, but do not allow the compound to contact the electrodes as the material is conductive and will short out the plug. If desired, the use of antiseize compound may be eliminated on engines equipped with stainless steel spark plug bushings or inserts.
(5) Screw the plug into the cylinder head as far as possible by hand. If the plug will not turn easily to within 2 or 3 threads of the gasket, it may be necessary to clean the threads.
NOTE: Cleaning inserts with a tap is not recommended as permanent damage to the insert may result,
(6) Seat the proper socket securely on the spark plug and tighten to the torque limit specified by the engine manufacturer before proceeding to the next plug.
A loose spark plug will not transfer heat properly and, during engine operation, may overheat to the point where the nose ceramic will become a "hot spot" and cause preignition. However, avoid overtightening as damage to the plug and bushing may resuit.
(7) Connect the ignition lead after wiping clean with methylethylketone (MDK), acetone, or similar material. Insert the terminal assembly into the spark plug in a straight line.
(Care should be taken as improper techniques can damage the terminal sleeves.) Screw the connector nut into place until finger tight;
then tighten an additional one-quarter turn while holding the elbow in the proper position.
(8) Perform an engine runup after installing a new set of spark plugs. When the engine has reached normal operating temperatures, check the magnetos and spark plugs in accordance with the manufacturer's instructions.
d. Operational Problems. Whenever problems develop during engine operation which appear to be caused by the ignition system, it is recommended that the spark plugs and ignition harnesses be checked first before working on the magnetos. The following are the most common spark plug malfunctions and are relatively easy to identify:
Far 693
(1) Fouling,
(a) Carbon fouling (figure 14.68) is identified by the dull black, sooty deposits on the electrode end of the plug. Although the primary causes are excessive ground idling and rich idle mixtures, plugs with a cold heat range may also be a contributing factor.
(b) Lead fouling is characterized by hard, dark, cinderlike globules which gradually fill up the electrode cavity and short out the plug.
(See figure 14.5b). The primary cause for this condition is poor fuel vaporization combined with a high tetraethyl-lead content fuel. Plugs with a cold heat range may also contribute to this condition.
(c) Oil fouling is identified by a wet, black carbon deposit over the entire firing end of the plug as shown in figure 14.5c. This condition is fairly common on the lower plugs in horizontally opposed engines, and both plugs in the lower cylinders of radial engines. Oil fouling is normally caused by oil drainage past the piston rings after shutdown. However, when both spark plugs removed from the same cylinder are badly fouled with oil and carbon, some form of engine damage should be suspected, and the cylinder more closely inspected. Mild forms of oil fouling can usually be cleared up by slowly increasing power while running the engine until the deposits are burned off and the misfiring stops.
(2) Fused Electrodes. There are many different types of malfunctions which result in FIGURE 14.5a.-Typical carbon fouled spark plug.
Par 693 AC 43.13-1А 1972 FIGURE 14.5b.- Typical lead fouled spark plug.
FIGURE 14.5c.- Typical oil fouled spark plug.
fused spark plug electrodes; however, most are associated with preignition either as the cause or the effect. For this reason, any time a spark plug is found with the following defects, further investigation of the cylinder and piston should be conducted.
(a) Cracked Nose Ceramics. Occasionally, the ceramic nose core will crack, break away, and remain trapped behind the ground electrode. This piece of insulation material will then build up heat to the point where it will ignite the fuel/air mixture prematurely. The high temperatures and pressures encountered during this condition can cause damage to the cylinder and piston and ultimately lead to fusing and shorting out of the plug.
Chap 14
AC 43.13-1A 1972
(b) Copper Runout. Some erosion oricupping of the exposed center electrode copper core is normal and will gradually decrease with the service life of the plug until it practically ceases. This condition, depicted in figure
14.6a, is not a cause for rejection until the erosion has progressed to a point more than 8/82 inch below the tip of the center electrode.
The high temperatures and pressures associated with preignition can cause the condition shown in figure 14.6b. In this instance, the copper center electrode melted and flowed out, bridged the electrodes, and caused a shorted condition.
(3) Bridged Electrodes, Occasionally, free combustion chamber particles will settle on the electrodes of a spark plug and gradually bridge the electrode gap, resulting in a shorted plug. Small particles may be dislodged by slowly cycling the engine as described for the oil-fouled condition; however, the only remedy for more advanced cases is removal and re- FIGURE 14.6a.-Typical eroded spark plug.
FIGURE 14.6b.-Typical spark plug with copper runout.
Chap 14 Page 277 FIGURE 14.7.-Typical spark plug with bridged electrodes.
placement of the spark plug. This condition is shown in figure 14.7.
(4) Metal Deposits. Whenever metal spray is found on the electrodes of a spark plug, it is an indication that a failure of some part of the engine is in progress. The location of the cylinder in which the spray is found is important in diagnosing the problem, as various types of failures will cause the metal spray to appear differently. For example, if the metal spray is located evenly in every cylinder, the problem will be in the induction system, such as an impeller failure. If the metal spray is only found on the spark plugs in one cylinder, the problem is isolated to that cylinder and will generally be a piston failure. In view of the secondary damage which occurs whenever an engine part fails, any preliminary indication such as metal spray should be thoroughly investigated to establish the cause and correct it.
(5) Floshover. It is important that spark plug terminal contact springs and moisture seals be checked regularly for condition and cleanness to prevent "flashover" in the connector well. Foreign matter or moisture in the terminal connector well can reduce the insulation value of the connector to the point where ignition system voltages at higher power settings may flash over the connector well surface to ground and cause the plug to misfire. If moisture is the cause, hard starting can also result.
The cutaway spark plug shown in figure 14.8 Far 693
FIGURE 14.8.-Spark plug well flashover.
illustrates this malfunction. Any spark plug found with a dirty connector well may have this condition, and should be reconditioned before reuse.
e. Pre-reconditioning Inspection. All spark plugs should be inspected visually prior to reconditioning to eliminate any plug with obvious defects. A partial checklist of common defects includes:
(1) Ceramic chipped or cracked either at the nose core or in the connector well.
(2) Damaged or badly worn electrodes.
(3) Badly nicked, damaged, or corroded threads on shell or shielding barrel.
(4) Shielding barrel dented, bent, or cracked.
(5) Connector seat at the top of the shielding barrel badly nicked or corroded.
694. IGNITION HARNESSES. Aircraft quality ignition harness is usually made of either medium or high temperature wire. The type used will depend upon the manufacturing specification for the particular engine. In addition to the applicable manufacturer's maintenance and repair procedures, the following is a quick-reference checklist for isolating some of the malfunctions inherent to ignition harnesses.
a. Carefully inspect the lead conduit or shielding. A few broken strands will not affect serviceability, but if the insulation in general looks worn, replace the lead.
b. When replacing a lead, if the dressing procedure is not accomplished properly, strands of shielding may be forced through the conductor insulation. If this occurs, a short Par 693 AC 43.13-1A 1972 will exist in the conductor; therefore, it is essential this task be performed properly.
c. The high temperature coating used on some lightweight harnesses is provided for vibration abrasion resistance and moisture protection. Slight flaking or peeling of this coating is not serious, and a harness assembly need not be removed from service because of this condition.
d. Check the spark plug contact springs for breaks, corrosion, or deformation. If possible, check the lead continuity from the distributor block to the contact spring.
e. Check the insulators at the spark plug end of the lead for cracka, breaks, or evidence of old age. Make sure they are clean.
f. Check to see that the leads are positioned as far away from the exhaust manifold as possible and are supported to prevent any whipping action.
g. When lightweight harnesses are used and the conduit enters the spark plug at a severe angle, use clamps as shown in figure 14.9 to prevent overstressing the lead.
695. MAGNETO INSPECTION. Whenever ignition problems develop and it is determined that the magneto is the cause of the difficulty, the following are a few simple inspection procedures which may locate the malfunction quickly.
However, conduct any internal inspection or repair of a magneto in accordance with the manufacturer's maintenance and overhaul manuals.
o. Inspect the distributor block contact springs. If broken or corroded, replace.
b. Inspect the felt oil washer if applicable.
It should be saturated with oil. If it is dry, check for a worn bushing.
c. Inspect the distributor block for cracks or a burned area. The wax coating on the block should not be removed. Do not use any solvents for cleaning.
d. Look for excess oil in the breaker compartment. If oil is present, it may mean a bad oil seal or bushing at the drive end. This condi- Chap 14
AC 43.13-1A 1972 FIGURE 14.9.-Typical method of clamping leads.
tion could require complete overhaul, as too much oil may foul and cause excessive burning of the contact points.
e. Look for frayed insulation on the leads in the breaker compartment of the magneto.
See that all terminals are secure. Be sure that wires are properly positioned.
f. Inspect the capacitor visually for general condition, and check the mounting bracket for cracks or looseness. If possible, check the capacitor for leakage, capacity, and series resistance.
g. Examine the points for excessive wear or burning. Discard points which have deep pits or excessively burned areas. Desired contact surfaces have a dull gray, sandblasted (almost rough) or frosted appearance over the area where electrical contact is made. Figure 14.10 shows how the normal contact point will look when surfaces are separated for inspection.
Minor irregularities or roughness of point surfaces are not harmful (see figure 14.11); neither are small pits or mounds, if not too pronounced. If there is a possibility of the pit becoming deep enough to penetrate the pad (figure 14.12), reject the contact assembly.
Generally, no attempt should be made to dress or stone contact point assemblies; however, if provided, procedures and limits contained in the manufacturer's manuals may be followed.
Chap 14 Page 279
NORMAL POINT IS SMOOTH
ANAY SANDBLASTE HAS DULL GRAY
APPEARANCE FIGURE 14.10.-Normal contact point.
MINOR IRREGULARITIES - SMOOTH ROLLING HILLS
AND DALES WITHOUT ANY DEEP PITS OR HIGH PEAKS. THIS IS A NORMAL CONDITION OF POINT FIGURE 14.11.-Point with minor irregularities.
WELL DEFINED MOUND EXTENDING NOTICEABLY ABOVE SURROUNDING SURFACE. THE MATING
POINT HAS A PIT OR HOLE CORRESPONDING TO THE MOUND SEEN HERE.
FIGURE 14.12.-Point with well-defined mound.
CAUTION When inspecting the contact points for condition, do not open further than absolutely necessary. Excess tension on the spring will weaken it and adversely affect the performance of the magneto.
h. Adjustment of magneto point gap must be correct for proper internal timing of mag- Par 695
neto. (See applicable manufacturer's publications for internal timing procedures.)
I. Check breaker cam to assure cleanness and smoothness. Check cam screw for tightness. If new points have been installed, blot a little oil on the cam. In addition, check contact point aasembly to ascertain that the cam follower is securely fastened.
I. If the impulse coupling ia accessible, inspect for excessive wear on the contact edges of the body and flyweights. In addition, check the flyweights for looseness on the axles.
k. Further examination of the impulse coupling body may disclose cracks caused by exceedingly tight flyweight axle rivets.
I. Check the magneto ventilatora for proper functioning and obstructions. If drilled plugs are used, they should be in the lowest vent hole of the magneto to serve as a drain for condensation and oil.
696. MAGNETO-TO-ENGINE TIMING. While the actual process of timing magnetos to an engine is covered in the engine manufacturer's technical manuals, the following general procedures may be applied:
Par 695 AC 43.12-1A 1972
a. Before installing a new magneto, the correct "p" gap setting specified by the magneto manufacturer should be verifled.
b. When setting or checking the magneto-toengine timing, always turn the crankshaft steadily in the normal direction of rotation to eliminate any error caused by gear backlash.
c. Recheck magneto-to-engine timing after any point gap adjustment, or after replacement of the breaker points.
d. Never advance the magneto timing beyond the engine timing specification recommended by the engine manufacturer.
•. The possibility of a timing error exists if a timing indicator which attaches to the propeller shaft or spinner of geared engines is used. Engine timing specifications are always given in degrees of crankshaft travel, and cannot be applied directly to geared propeller shafts because of the gear ratio. Therefore, the correct position of the propeller shait, if used for timing, must be determined by multiplying the crankshaft timing angle in degrees before top center (BTC) by the propeller gear ratio.
697.-707. RESERVED.
Chap 14
AC 43.13-1A Pae 281 1972 Section 2. FUEL SYSTEMS
708. GENERAL. Maintain, service, and adjust Integrally bonded and cushioned line support aircraft fuel systems and fuel system compo- clamps are preferred to other clamping and nents in accordance with the applicable manu- bonding methods.
facturer's maintenance instructions. Certain e. Support of Line Units. To prevent possible general fuel system maintenance principles are failure, all fittings heavy enough to cause the outlined below. line to sag should be supported by means other than the tubing.
709. FUEL LINES AND FITTINGS. When fuel 8y8- f. Support Clamps.
tem lines are to be replaced or repaired, con- (1) Place support clamps or brackets for sider the following fundamentals in addition to metallic lines as follows:
the applicable airworthiness requirements. Ad- Approximate ditional inspection and repair practices for air- Tube O.D. distance between eupporte craft tubing systems may be found in paragraph 393. 6"-32" 16" 12"
a. Compatibility of Fillings. All fittings are to 22" be compatible with their mating parts. Al- though various types of fittings appear to be 1"-1¼" 1½"-2" 30" 40" interchangeable in many cases they have dif- (2) Locate clamps or brackets as close to ferent thread pitch or minor design differences bends as possible to reduce overhang (see which prevent proper mating and may cause figure 14.18).
the joint to leak or fail.
b. Routing. Make sure that the line does not chafe against control cables, airframe structure, etc., or come in contact with electrical wiring or conduit. Where physical separation of the fuel lines from electrical wiring or conduit is impracticable, locate the fuel line below the wiring and clamp it securely to the airframe structure. In no case may wiring be sup- OVERHANG ported by the fuel line.
c. Alignment. Locate bends accurately so that the tubing is aligned with all support clamps and end fittings and is not drawn, pulled, or otherwise forced into place by them. Never install a straight length of tubing between two rigidly mounted fittings. Always incorporate at least one bend between such fittings to absorb strain caused by vibration and temperature changes.
d. Bonding. Bond metallic fuel lines at each point where they are clamped to the structure. FIGURE 14.18.- Location of clamps at tube bends.
Chap 14 Par 708