Par 8-15 Page 8-14
(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 anti-seize compound sparingly to the shell threads but do not allow the compound to contact the electrodes since the material is conductive and will short out the plug. If desired, the use of anti-seize 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 two or three 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.
CAUTION: A loose spark plug will not transfer heat properly and, during engine operation, may overheat to the point in which the nose ceramic will become a “hot spot” and cause pre-ignition. However, avoid over-tightening as damage to the plug and bushing may result.
(7) Connect the ignition lead after wiping clean with a dry, lint-free cloth. 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.
8-16. OPERATIONAL PROBLEMS. Whenever problems develop during engine operation and 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 more common spark plug malfunctions and are relatively easy to identify.
a. Fouling.
(1) Carbon fouling (see Figure 8-6) 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, a cold heat range may also be a contributing factor.
Par 8-16 Page 8-15 FIGURE 8-6. TYPICAL CARBON-FOULED SPARK PLUG (
2) Lead fouling is characterized by hard, dark, cinder-like globules which gradually fill up the electrode cavity and short out the plug (see Figure 8-6a). The primary cause for this condition is poor fuel vaporization combined with a high tetraethyllead content fuel. A cold heat range may also contribute to this condition.
FIGURE 8-6a. TYPICAL LEAD-FOULED SPARK PLUG
Par 8-16 Page 8-16
(3) Oil fouling is identified by a wet, black carbon deposit over the entire firing end of the plug as shown in Figure 8-6b. 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.
FIGURE 8-6b. TYPICAL OIL-FOULED SPARK PLUG
b. F used Electrodes. There are many different types of malfunctions which result in fused spark plug electrodes; however, most are associated with pre-ignition as either 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.
(1) 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 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 (see Figure 8-6c).
Par 8-16 Page 8-17 FIGURE 8-6c. TYPICAL SPARK PLUG WITH CRACKED CORE NOSE (
2) Corrosive gases formed by combustion and the high voltage spark have eroded the electrodes. Spark plugs in this condition require more voltage to fire—often more than the ignition system can produce (see Figure 8-6d).
FIGURE 8-6d. TYPICAL WORN OUT SPARK PLUG
Par 8-16 Page 8-18
c. 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 replacement of the spark plug. This condition is shown in Figure 8-6e.
FIGURE 8-6e. TYPICAL SPARK PLUG WITH BRIDGED ELECTRODES
d. Met al 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 found only 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 and correct the cause.
e. Flashover. It is important that spark plug terminal contact springs and moisture seals be checked regularly for condition and cleanliness 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 the 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 8-7 illustrates this malfunction.
Any spark plug found with a dirty connector well may have this condition and should be reconditioned before reuse.
Par 8-16 Page 8-19 FIGURE 8-7. SPARK PLUG WELL FLASHOVER 8-
17. SPARK PLUG PRE-RECONDITIONING INSPECTION. All spark plugs should be inspected visually before reconditioning to eliminate any plug with obvious defects. A partial checklist of common defects includes:
a. Chipped or cracked ceramic either at the nose core or in the connector well.
b. Damaged or badly worn electrodes.
c. Badly nicked, damaged, or corroded threads on shell or shielding barrel.
d. Dented, bent, or cracked shielding barrel.
e. Connector seat at the top of the shielding barrel badly nicked or corroded.
8-18. IGNITION HARNESSES INSPECTION. 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 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 cracks, breaks, or evidence of old age. Make sure they are clean.
Par 8-18 Page 8-20
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 8-8, to prevent overstressing the lead.
FIGURE 8-8. TYPICAL METHOD OF CLAMPING LEADS 8-
19. 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.
a. 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 indicate a bad oil seal or bushing at the drive end. This condition 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 8-9 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 8-10), 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 (see Figure 8-11), reject the contact assembly.
FIGURE 8-9. NORMAL CONTACT POINT
NORMAL POINT IS SMOOTH AND FLAT. SURFACE HAS DULL GRAY "SANDBLASTED" APPEARANCE.
FIGURE 8-10. POINT WITH MINOR IRREGULARITIES
MINOR IRREGULARITIES - SMOOTH ROLLING
HILLS AND DALES WITHOUT ANY DEEP PITS OR HIGH PEAKS. THIS IS A NORMAL CONDITION OR POINT OF WEAR.
FIGURE 8-11. POINT WITH WELL-DEFINED MOUND 9/8/98
WEEL-DEFINED MOUND EXTENDING NOTICEABLY ABOVE SURROUNDING SURFACE. THE MATING
POINT HAS A PIT OR HOLE CORRESPONDING TO THE MOUND SEEN HERE.
h. 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.
Par 8-19 Page 8-21
Par 8-19 Page 8-22 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.
i. Adjustment of magneto point gaps must be correct for proper internal timing of a magneto. See the applicable manufacturer’s publications for internal timing procedures.
j. Check the breaker cam to assure cleanness and smoothness. Check the cam screw for tightness. If new points have been installed, blot a little oil on the cam. In addition, check the contact point assembly to ascertain that the cam follower is securely fastened.
k. If the impulse coupling is accessible, inspect for excessive wear on the contact edges of the body and flyweights. In addition, check the flyweights for looseness on the axles.
l. Further examination of the impulse coupling body may disclose cracks caused by exceedingly tight flyweight axle rivets.
m. Check the magneto ventilators 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.
8-20. 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.
a. Before installing a new magneto, the correct “E” gap setting specified by the magneto manufacturer should be verified.
b. When setting or checking the magneto-to-engine 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.
e. 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 shaft, if used for timing, must be determined by multiplying the crankshaft timing angle in degrees before top center (BTC) by the propeller gear ratio.
8-21.–8-29. [RESERVED.]
Par 8-30 Page 8-23 SECTION 2. FUEL SYSTEMS
8-30. GENERAL. Maintain, service, and adjust aircraft fuel systems and fuel system components in accordance with the applicable manufacturer’s maintenance instructions. Certain general fuel system maintenance principles are outlined in the following paragraphs.
8-31. FUEL LINES AND FITTINGS. When fuel system lines are to be replaced or repaired, consider the following fundamentals in addition to the applicable airworthiness requirements.
Additional inspection and repair practices for aircraft tubing systems may be found in Chapter 9, Aircraft Systems and Components.
a. Compatibility of Fittings. All fittings are to be compatible with their mating parts.
Although various types of fittings appear to be interchangeable, in many cases they have different thread pitch or minor design differences which prevent proper mating and may cause 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 should wiring be supported 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.
Integrally bonded and cushioned line support clamps are preferred to other clamping and bonding methods.
e. Support of Line Units. To prevent possible failure, all fittings heavy enough to cause the line to sag should be supported by means other than the tubing.
f. Support Clamps.
(1) Place support clamps or brackets for metallic lines as follows:
Tube OD Approximate Distance Between Supports 1/8”–3/16” 9” 1/4”–5/16” 12” 3/8”–1/2” 16” 5/8”–3/4” 22” 1”–1 1/4” 30” 1 1/2”–2” 40”
Par 8-31 Page 8-24
(2) Locate clamps or brackets as close to bends as possible to reduce overhang (see Figure 8-12).
FIGURE 8-12. LOCATION OF CLAMPS AT TUBE BENDS 8-
32. FUEL TANKS AND CELLS. Welded or riveted fuel tanks that are made of commercially pure aluminum, 3003, 5052, or similar alloys may be repaired by welding. Tanks made from heat-treatable aluminum alloys are generally assembled by riveting. In case it is necessary to rivet a new piece in a tank, use the same material as used in the tank undergoing repair and seal the seams with a compound that is insoluble in gasoline. Special sealing compounds are available and should be used in the repair of tanks. Inspect fuel tanks and cells for general condition, security of attachment, and evidence of leakage. Examine the fuel tank or cell vent line, fuel line, and sump drain attachment fittings closely.
CAUTION: Purge defueled tanks of explosive fuel/air mixtures in accordance with the manufacturer’s service instructions. In the absence of such instructions, utilize an inert gas, such as carbon dioxide (CO2), as a purgative to assure the total deletion of fuel/air mixtures.
a. Integral Tanks. Examine the interior surfaces and seams for sealant deterioration and corrosion (especially in the sump area). Follow the manufacturer’s instructions for repair and cleaning procedures.
b. Internal Metal Tanks. Check the exterior for corrosion and chafing. Dents or other distortion, such as a partially collapsed tank caused by an obstructed fuel tank vent, can adversely affect fuel quantity gauge accuracy and tank capacity. Check the interior surfaces for corrosion. Pay particular attention to the sump area, especially for those of which sumps are made of cast material. Repairs to the tank may be accomplished in accordance with the practices outlined in the Chapter 4, Metal Structure, Welding, and Brazing of this advisory circular (AC).
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c. Removal of Flux After Welding. It is especially important, after repair by welding, to completely remove all flux in order to avoid possible corrosion. Promptly upon completion of welding, wash the inside and outside of the tank with liberal quantities of hot water and then drain. Next, immerse the tank in either a 5-percent nitric or 5-percent sulfuric acid solution. If the tank cannot be immersed, fill the tank with either solution and wash the outside with the same solution. Permit the acid to remain in contact with the weld for about one hour and then rinse thoroughly with clean water. Test the efficiency of the cleaning operation by applying some acidified 5-percent silver nitrate solution to a small quantity of the rinse water used last to wash the tank. If a heavy white precipitate forms, the cleaning is insufficient, and the washing should be repeated.
d. Flexible Fuel Cells. Inspect the interior for checking, cracking, porosity, or other signs of deterioration. Make sure the cell retaining fasteners are properly positioned. If repair or further inspection is required, follow the manufacturer’s instructions for cell removal, repair, and installation. Do not allow flexible fuel cells to dry out. Preserve them in accordance with the manufacturer’s instructions.
8-33. FUEL TANK CAPS, VENTS, AND OVERFLOW LINES. Inspect the fuel tank caps to determine they are the correct type and size for the installation and that O-rings are in good condition.
a. Unvented caps, substituted for vented caps, will cause fuel starvation and possible collapse of the fuel tank or cell. Malfunctioning of this type occurs when the pressure within the tank decreases as the fuel is withdrawn. Eventually, a point is reached where the fuel will no longer flow and/or the outside atmospheric pressure collapses the tank. Thus, the effects will occur sooner with a full fuel tank than with one partially filled.
b. Check tank vents and overflow lines thoroughly for condition, obstructions, correct installation, and proper operation of any check valves and ice protection units. Pay particular attention to the location of the tank vents when such information is provided in the manufacturer’s service instructions. Inspect for cracked or deteriorated filler opening recess drains, which may allow spilled fuel to accumulate within the wing or fuselage. One method of inspection is to plug the fuel line at the outlet and observe fuel placed in the filler opening recess.
If drainage takes place, investigate condition of the line and purge any excess fuel from the wing.
c. Assure that filler opening markings are affixed to, or near, the filler opening; are marked according to the applicable airworthiness requirements; and are complete and legible.
8-34. FUEL CROSS-FEED, FIREWALL SHUTOFF, AND TANK SELECTOR VALVES.
Inspect these valves for leakage and proper operation as follows.
a. Internal leakage can be checked by placing the appropriate valve in the “off” position, draining the fuel strainer bowl, and observing if fuel continues to flow into it. Check all valves located downstream of boost pumps with the pump(s) operating. Do not operate the pump(s) longer than necessary.