Par 8-19 Page 8-381 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 cont act 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.
FIGURE 8-10. Point with minor irregularities.
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.
FIGURE 8-11. 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.
Adjustment of magneto point gaps must be correct for proper internal timing of a magneto. See applicable manufacturer’s publications for internal timing procedures.
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 contact poi nt assembly to ascertain that the cam follower is securely fastened.
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.
Further examination of the impulse coupling body may disclose cracks caused by exceedingly-tight flyweight axle rivets.
Check the magneto ventilators for proper functioning and obstructions. If drilled pl ugs are used, they should be in the lowest vent hole of the magneto to serve as a drain for condensation and oil.
Par 8-20 Page 8-382 (and 8-18) 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.
Before installing a new magneto, the correct “E” gap setting specified by the magneto manufacturer should be verified.
When setting or checking the magneto-to-engine timing, always turn the cra nkshaft steadily in the normal direction of rotation to eliminate any error caused by gear backlash.
Recheck magneto-to-engine timing after any pointgap adjustment, or after replacement of the breaker points.
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 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 8-383
SECTION 2. FUEL SYSTEMS
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..
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 the Chapter 9, Aircraft Systems and Components.
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.
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.
Alignment. Locate bends accurately so that the tubing is aligned with all support clamps and end fittings and is not drawn, pulled, or othe rwise 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.
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.
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.
Support clamps.
Place support clamps or brackets for metallic lines as follows.
Tube O.D.
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”
Locate clamps or brackets as close to bends as possible to reduce overhang. (See figure 8-12.)
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 heattreatable 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 fuel tank or cell vent line, fuel line, and sump drain attachment fittings closely.
Par 8-33 Page 8-384
FIGURE 8-12. Location of clamps at tube bends.
CAUTION: Purge de-fueled 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 CO2 as a purgative to assure the total deletion of fuel/air mixtures.
Integral Tanks. Examine the interior surfaces and seams for sealant deterioration and corrosion (especially in the sump area). Fo llow the manufacturer’s instructions for repair and cleaning procedures.
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. Ch eck 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 AC.
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 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 small quantity of the rinse water used last to wash the tank. If a heavy white precipitate is formed, the cleaning is insufficient and the washing should be repeated.
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 furt her inspection is required, follow the manufacturer’s instructions for cell removal, repair, and installation. Do not allow flexible fuel cells to dry out. Preserve th em in accordance with the manufacturer’s instructions.
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.
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 d ecreases 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,
Par 8-33 Page 8-385 the effects will occur sooner with a full fuel tank than with one partially filled.
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 openi ng recess drains, which may allow spilled fuel to accumu late 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.
Assure that filler opening markings are affixed to, or near, the filler opening; marked according to the applicable airworthiness requirements; and are complete and legible.
FUEL CROSS-FEED, FIREWALL SHUTOFF, AND TANK SELECTOR VALVES. Inspect these valves for leakage and proper operation as follows.
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.
External leakage from these units can result in a severe fire hazard, especially if the unit is located under the cabin floor or within a simila rly-confined area. Correct the cause of any fuel stains associated with fuel leakage.
Selector Handles. Check the operation of each handle or control to see that it indicates the actual position of the selector valve to the pl acard location. Movement of the selector handle should be smooth and free of binding. Assure that stops and detents have positive action and smooth operational feel. Worn or missing detents and stops can cause unreliable positioning of the fuel selector valve.
Worn Linkage. Inaccurate positioning of fuel selector valves can also be caused by worn mechanical linkage between the selector handle and the valve unit. An improper fuel valve position setting can seriously reduce engine power by restricting the available fuel flow. Check universal joints , pins, gears, splines, cams, levers, etc., for wear and excessive clearance which prevent the valve from positioning accurately or from obtaining fully “off” and “on” positions.
Assure that required placards are complete and legible. Replace those that are missing or cannot be read easily.
FUEL PUMPS. Inspect, repair, and overhaul boost pumps, emergency pumps, auxiliary pumps, and engine-driven pumps in accordance with the appropriate manufacturer’s instructions.
FUEL FILTERS, STRAINERS, AND DRAINS.
Check each strainer and filter element for contamination. Determine and correct the source of any contaminants found. Replace throw-away filter elements with the recommended type. Examine fuel strainer bowls to see that they are properly installed according to the direction of the fuel flow.
Check the operation of all drain devices to see that they operate properly and have positive shutoff action.
Par 8-45 Page 8-386 INDICATOR SYSTEMS. Inspect, service, and adjust the fuel indicator systems according to the manufacturer’s instructions. Determine that the required placards and instrument markings are complete and legible.
FUEL SYSTEM PRECAUTIONS. In servicing fuel systems, remember that fuel is flammable and that the danger of fire or explosion always exists. The following precautions should be taken:
Aircraft being serviced or having the fuel system repaired must be properly grounded.
Spilled fuel must be neutralized or removed as quickly as possible.
Par 8-45 Page 8-387 Open fuel lines must be capped.
Fire-extinguishing equipment must always be available.
Metal fuel tanks must not be welded or soldered unless they have been adequately purged of fuel fumes. Keeping a tank or cell filled with carbon dioxide will prevent explosion of fuel fumes.
Do not use Teflon tape on any fuel lines to avoid getting the tape between the flare and fitting, which can cause fluid leaks.
8-39.8-44. [RESERVED.]
Par 8-45 Page 8-388 SECTION 3. EXHAUST SYSTEMS
GENERAL. Any exhaust system failure should be regarded as a severe hazard. Depending upon the location and type of failure, it can result in carbon monoxide (CO) poisoning of crew and passengers, partial or complete engine power loss, or fire. Exhaust system failures generally reach a maximum rate of occurrence at 100 to 200 hours’ operating time, and over 50 percent of the failures occur within 400 hours.
MUFFLER/HEAT EXCHANGER FAILURES.
Approximately one-half of all exhaust system failures are traced to cracks or ruptures in the heat exchanger surfaces used for cabin and carburetor air heat sources.
Failures in the heat exchanger’s surface (usually the muffler’s outer wall) allow exhaust gases to escape directly into the cabin heat system. The failures are, for the most part, attributed to thermal and vibration fatigue cracking in the areas of st ress concentration; e.g., tailpipe and stack, inlet-attachment areas. (See figures 8-13 through 8-16.)
Failures of the spot welds which attach heat transfer pins, as shown in figure 8-14A, can result in exhaust gas leakage. In addition to the CO hazard, failure of heat exchanger surfaces can permit exhaust gases to be drawn into the engine induction system and cause engine overheating and power loss.
MANIFOLD/STACK FAILURES.
Exhaust manifold and stack failures are also usually fatigue-type failures which o ccur at welded or clamped joints; e.g., stack-to-flange , stack-to-manifold, muffler connections, or crossover pipe connections. Although these failures are primarily a fire hazard, they also present a CO problem. Exhaust gases can
FIGURE 8-13. Typical muffler wall fatigue failure at exhaust outlet. (A. Complete muffler assembly with heat shroud removed; B. Detail view of failure.)
enter the cabin via defectiv e or inadequate seals at firewall openings, wing stru t fittings, doors, and wing root openings. Manifold/stack failures, which account for approximately 20 percent of all exhaust system failures, reach a maximum rate of occurrence at about 100 hours’ operating time. Over 50 percent of the failures occur within 300 hours.
INTERNAL MUFFLER FAILURES. Internal failures (baffles, diffusers, etc.) can cause partial or complete engine power loss by restricting the flow of the exhaust gases. (See figures 8-17 through 8-20.)
Par 8- Sec 3 Page 389
FIGURE 8-14. Typical muffler wall failure. (A. Complete muffler assembly with heat shroud removed; B.
Detail view of failure; C. Cross section of failed muffler.)
As opposed to other failures, erosion and carbonizing caused by the extreme th ermal conditions are the primary causes of internal fa ilures. Engine after-firing and combustion of unburned fuel within the exhaust system are probable contributing factors.
In addition, local hot spot areas caused by uneven exhaust gas flow, result in burning, bulging, and rupture of the outer muffler wall. (See figure 8-14.) As might be expected, the time required for these failures to develop is longer than that for fatigue failures. Internal muffler failures account for nearly 20 percent of the total number of exhaust system failures.
The highest rate of internal muffler failures occurs between 500 and 750 hours of operating time. Engine power loss and excessive back-pressure caused by exhaust outlet blockage may be averted by the installation
FIGURE 8-17. Section of a muffler showing typical internal baffle failure.
FIGURE 8-15. Typical muffler wall fatigue failure. (A.
Complete muffler assembly with heat shroud partially removed; B. Detailed view of failure.)
FIGURE 8-16. Typical fatigue failure of muffler end plate at stack inlet. FIGURE 8-18. Loose pieces of a failed internal baffle.
Par 8-48 Page 8-390
Par 8-48 Page 8-391 of an exhaust outlet guard as shown in figures 8-21a and 8-21b. The outlet guard may be fabricated from a 3/16-inch stainless steel welding rod.
Form the rod into two “U” shaped segments, approximately 3 inches long and weld onto the exhaust tail pipe as shown in figure 8-21b so that the guard will extend 2 inches inside the exhaust muffler outlet port.
Installation of an exhaust outlet guard does not negate the importance of thorough inspection of the internal parts of the muffler or the necessity of replacing defective mufflers.
INSPECTION. Inspect exhaust systems frequently to ascertain complete system integrity.
CAUTION: Marking of exhaust system parts. Never use lead pencils, carbon based pencils, etc., to mark exhaust system parts.
Carbon deposited by those tools will cause cracks from heat concentration and carbonization of the metal. If exhaust system parts must be marked, use chalk, Prussian blue, India ink, or a grease pencil that is carbon-free.
Before any cleaning operation, remove the cowling as required to expose the complete exhaust system.
Examine cowling and nacelle areas adjacent to exhaust system components for telltale signs of exhaust gas soot indicating possible leakage points. Check to make sure no portion of the exhaust system is being chafed by cowling, engine control cables, or other components.
The exhaust system often operates at red-hot temperatures of 1,000 °F or more; therefore, parts such as ignition leads, hoses, fuel lines, and flexible air ducts, should be protected from radiation and convection heating by heat shields or adequate clearance.
Remove or loosen all exhaust shields, carburetor and cabin heater muffs, shrouds, heat blankets, etc., required to permit inspection of the complete system.
Perform necessary cleaning operations and inspect all external surfaces of the e xhaust system for cracks, dents, and missing parts. Pay particular attention to welds, clamps, supports and support attachment lugs, bracing, slip joints, stack fl anges, gaskets, flexible couplings, and etc. (See figures 8-22 and 8-23.)
Examine the heel of each bend, areas adjacent to welds, any dented areas, and low spots in the system for thinning and pitting due to internal erosion by combustion products or accumulated moisture. An ice pick (or similar pointed instrument) is useful in probing suspected areas. Disassemble the system as necessary to inspect internal baffles or diffusers.
Should a component be inaccessible for a thorough visual inspection or hidden by non-removable parts, remove the component and check for possible leaks by plugging its openings, applying approximately 2 psi internal pressure, and submerging it in water. Any leaks will cause bubbles that can be readily detected.
Dry thoroughly before reinstallation.
REPAIRS. It is generally recommended that exhaust stacks, mufflers, tailpipes, and etc., be replaced with new or reconditioned components rather than repaired. Welded repairs to exhaust systems are complicated by the difficulty of accurately identifying the base metal so that the proper repair materials can be selected. Changes in composition and grain structure of the original base metal further complicates the repair. However, when welded repairs are necessary, follow the general procedures outlined in Chapter 4; Metal Structure, Welding, and Brazing; of this AC.
Retain the original contours and make sure that
Par 8-50 Page 8-392
FIGURE 8-19. Failed internal baffle partially obstructing the muffler outlet.
FIGURE 8-20. Failed internal baffle completely obstructing the muffler outlet.
the completed repair has not warped or otherwise affected the alignment of the exhaust system. Repairs or sloppy weld beads, which protrude internally, are not acceptable since they cause local hotspots and may restrict exhaust gas flow. All repairs must meet the manufacturer’s specifications. When repairing or replacing exhaust system components, be sure that the proper hardware and clamps are used. Do not substitute steel or low-
FIGURE 8-21a. Example of exhaust outlet guard.
FIGURE 8-21b. Example of exhaust outlet guard installed.
temperature self-locking nuts for brass or special hightemperature locknuts used by the manufacturer. Never reuse old gaskets or old star lock washers. When disassembly is necessary re place gaskets with new ones of the same type provided by the manufacturer.
TURBO-SUPERCHARGER. When a turbosupercharger is included, the exhaust system operates under greatly-increased pressure and temperature conditions. Extra precautions should be taken in the exhaust system’s care and maintenance. During