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).
Par 8-32 Page 8-25
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.
Par 8-34 Page 8-26
b. 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 similarly confined area. Correct the cause of any fuel stains associated with fuel leakage.
c. Selector Handles. Check the operation of each handle or control to see that it indicates the actual position of the selector valve to the placard 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.
d. 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.
e. Assure that required placards are complete and legible. Replace those that are missing or cannot be read easily.
8-35. FUEL PUMPS. Inspect, repair, and overhaul boost pumps, emergency pumps, auxiliary pumps, and engine-driven pumps in accordance with the appropriate manufacturer’s instructions.
8-36. FUEL FILTERS, STRAINERS, AND DRAINS. Check each strainer and filter element for contamination. Determine and correct the source of any contaminants found. Replace throwaway 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.
8-37. 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.
8-38. 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:
a. Aircraft being serviced or having the fuel system repaired must be properly grounded.
b. Spilled fuel must be neutralized or removed as quickly as possible.
c. Open fuel lines must be capped.
d. Fire-extinguishing equipment must always be available.
e. 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 CO2 will prevent explosion of fuel fumes.
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f. 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 8-28 SECTION 3. EXHAUST SYSTEMS
8-45. 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.
8-46. 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.
a. 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 stress concentration,
e.g., tailpipe and stack, inlet-attachment areas (see Figures 8-13 through 8-16).
FIGURE 8-13. TYPICAL MUFFLER WALL FATIGUE FAILURE AT EXHAUST OUTLET. (A. COMPLETE MUFFLER ASSEMBLY WITH HEAT SHROUD REMOVED; B. DETAIL VIEW OF FAILURE)
Par 8-46 Page 8-29 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)
Par 8-46 Page 8-30 FIGURE 8-15. TYPICAL MUFFLER WALL FATIGUE FAILURE. (A. COMPLETE MUFFLER ASSEMBLY WITH HEAT SHROUD PARTIALLY REMOVED;
B. DETAILED VIEW OF FAILURE) F IGURE 8-16. TYPICAL FATIGUE FAILURE OF MUFFLER END PLATE AT STACK INLET
b. F ailures of the spot welds which attach heat transfer pins, as shown in Figure 8-14, image A, 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.
8-47. MANIFOLD/STACK FAILURES. Exhaust manifold and stack failures are also usually fatigue-type failures, which occur 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 enter the cabin via
Par 8-47 Page 8-31 defective or inadequate seals at firewall openings, wing strut 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.
8-48. 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). As opposed to other failures, erosion and carbonizing caused by the extreme thermal conditions are the primary causes of internal failures. Engine after-firing and combustion of unburned fuel within the exhaust system are probable contributing factors.
FIGURE 8-17. SECTION OF A MUFFLER SHOWING TYPICAL INTERNAL BAFFLE
FAILURE FI GURE 8-18. LOOSE PIECES OF A FAILED INTERNAL BAFFLE
Par 8-48 Page 8-32 FIGURE 8-19. FAILED INTERNAL BAFFLE PARTIALLY OBSTRUCTING THE
MUFFLER OUTLET F IGURE 8-20. FAILED INTERNAL BAFFLE COMPLETELY OBSTRUCTING THE
MUFFLER OUTLET
a.
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.
b. The highest rate of internal muffler failures occurs between 500 and 750 hours of operating time. Engine power loss and excessive backpressure caused by exhaust outlet blockage may be averted by the installation 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.
Par 8-48 Page 8-33 FIGURE 8-21a. EXAMPLE OF EXHAUST OUTLET GUARD FI GURE 8-21b. EXAMPLE OF EXHAUST OUTLET GUARD INSTALLED
c. Fo rm 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.
8-49. 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.
a. 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.
b. Remove or loosen all exhaust shields, carburetor and cabin heater muffs, shrouds, heat blankets, etc., required to permit inspection of the complete system.
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c. Perform necessary cleaning operations and inspect all external surfaces of the exhaust system for cracks, dents, and missing parts. Pay particular attention to welds, clamps, supports and support attachment lugs, bracing, slip joints, stack flanges, gaskets, flexible couplings, 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.
FIGURE 8-22. EFFECT OF IMPROPERLY POSITIONED EXHAUST PIPE/MUFFLER CLAMP
Par 8-49 Page 8-35 FIGURE 8-23. PRIMARY INSPECTION AREAS
d. Sho uld a component be inaccessible for a thorough visual inspection or hidden by nonremovable 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.
8-50. REPAIRS. It is generally recommended that exhaust stacks, mufflers, tailpipes, 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 of this AC. Retain the original contours and make sure that the completed repair has not warped or otherwise affected the alignment of the