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
Par 8-71 Page 8-393
FIGURE 8-22. Effect of improperly positioned exhaust pipe/muffler clamp.
high-altitude operation, the exhaust system pressure is maintained at, or near, sea level values. Due to the pressure differential, any leaks in the system will allow the exhaust gases to escape with a torch-like intensity that can severely damage adjacent structures. A common cause of turbo-supe rcharger malfunction is coke deposits (carbon buildup) in the waste gate unit causing erratic system operation. Exce ssive deposit buildups may cause the waste gate valve to stick in the closed position, causing an overboost condition. Coke deposit buildup in the turbo-supercharger itself will cause a gradual loss of power in flight and low deck pre ssure reading before takeoff. Experience has shown that periodic decoking, or removal of carbon deposits, is necessary to maintain peak efficiency. Clean, repair, overhaul, and adjust turbo-supercharger system components and controls in accordance with the applicable manufacturer’s instructions.
AUGMENTOR SYSTEMS. Inspect augmentor tubes periodically for proper alignment, security of attachment, and general
Par 8-71 Page 8-394
FIGURE 8-23. Primary inspection areas.
overall condition. Regardless of whether or not the augmentor tubes contain h eat exchanger surfaces, they should be inspected for cracks along with the rema inder of the exhaust system. Cracks can present a fire or CO hazard by allowing exhaust gases to enter nacelle, wing, or cabin areas.
8-53.8-70. [RESERVED.]
Par 8-71 Page 8-395 SECTION 4. REPAIR OF METAL PROPELLERS
GENERAL. Reject damaged blades with model numbers which are on the manufacturer’s list of blades that cannot be repaired. Follow the propeller manufacturer’s recommendations in all cases, and make repairs in accordance with latest techniques and best industry practices.
NOTE: Title 14 of the Code of Federal Regulations (14 CFR), part 65 does not allow an airframe and powerplant mechanic to perform major repairs to propellers.
STEEL BLADES. Due to the critical effects of surface injuries and their repair on the fatigue life of steel blades, all repairs must be made in accordance with the manufacturer’s instructions.
ALUMINUM PROPELLER REPAIRS. Aluminumalloy propellers and blades with dents, cuts, scars, scratches, nicks, leading-edge pitting, etc., may be repaired, provided the removal or treatment does not materially affect the strength, weight, or performance of the blade.
Remove these damages or otherwise treat as explained below, unless it is contrary to the manufacturer’s instructions or recommendations. More than one injury is not sufficient cause alone for rejection of a blade.
A reasonable number of repairs per blade may be made and not necessarily result in a dangerous condition, unless their location with respect to each other is such to form a continuous line of repairs that would materially weaken the blade. Suitable sandpaper or finecut files may be used for removing the necessary amount of metal. In each case, the area involved will be smoothly finished with #00 sandpaper or crocus cloth, and each blade from which any appreciable amount of metal has been removed will be properly balanced before it is used. Etch suspected cracks and all repairs. To avoid removal of an excessive amount of metal, local etching should be accomplished at intervals during the process of removing suspected cracks. Upon completion of the repair, carefully inspect the entire blade by etching or anodizing. Remove all effects of the etching process with fine emery paper. Blades identified by the manufacturer as being cold-worked (shotblasted or cold-rolled) may require peening after repair. Accomplish repair and peening operations on this type of blade in accordance with the manufacturer’s instructions. However, it is not permissible in any case to peen down the edges of any injury wherein the operation will lap metal over the injury.
Flaws in Edges. Round out nicks, scars, cuts, etc., occurring on the leading edge of aluminum-alloy blades as shown in figure 8-24 (view B). Blades that have the leading edges pitted from normal wear in service may be reworked by removing sufficient material to eliminate the pitting. In this case, remove the metal by starting a sufficient distance from the edge, as shown in figure 8-25, and working forward over the edge in such a way that the contour will remain substantially the same, avoiding abrupt changes in contour. Trailing edges of blades may be treated in substantially the same manner. On the thrust and camber face of blades, remove the metal around any dents, cuts, scars, scratches, nicks, longitudina l surface cracks, and pits to form shallow saucer-shaped depressions as shown in figure 8-24 (view C). Exercise care to remove the deepest point of the injury and also remove any raised metal around the edges of the injury as shown in figure 8-24 (view A). When repairing blades, figures 8-26 and 8-27 show the maximum reduction in width and thickness that is allowable below the minimum dimensions required by the blade drawing and blade manufacturing specification. Beyond the
CROSS - SECTION CROSS - SECTION
BEFORE REPAIR AFTER REPAIR —A
BEFORE REPAIR REPAIR AFTER C
SURFACE CRACK AFTER REPAIR -
BEFORE REPAIR CRACK WORKED OUT FIGURE 8-24. Method of repairing surface cracks, nicks, etc., on aluminum-alloy propellers.
MAXIMUM THICKNESS OF BLADE
SECTION IS AT A POINT APPROX- IMATELY .3 OF CORD LENTH AS SHOWN.
3 OF CHORD LENGTH
CORRECT METHOD
ORIGINAL
REWORKED SECTION
DON NOT DESTROY MAXIMUM THICKNESS OF BLADE SECTION IF POSSIBLE.
NOTE - DAMAGED A - MAINTAIN ORIGINAL RADIUS. PORTION B - REWORK CONTOUR TO POINT OF MAXIMUM THICKNESS.
C - RADIUS IS TOO LARGE D - CONTOUR IS TOO BLUNT.
INCORRECT METHOD FIGURE 8-25. Correct and incorrect method of reworking leading edge of aluminum-alloy propellers.
Par 8-71 Page 8-396
Par 8-73 Page 8-397 90 percent blade radius point, the blade width and thickness may be modified as per the manufacturer’s instructions.
Shortening Blades. Shortening propeller blades is a major repair. When the removal or treatment of defects on the tip necessitates shortening a blade, shorten each blade used with it and keep such sets of blades together.
(See figure 8-26 for acceptable methods.) Mark the shortened blades to correspond with the manufacturer’s system of model designation to indicate propeller diameter. In making the repair, it is not permissible to reduce the propeller diameter below the minimum diameter limit shown on the pertinent specification or type certificate data sheet.
Straighten Propeller Blades. Never straighten a damaged propeller. Even partial straightening of blades to permit shipment to a certificated propeller repair facility may result in hidden damage not being detected and an unairworthy propeller being returned to service.
REPAIR LIMITS. The following limits are those listed in the blade manufacturing specification for aluminum-alloy blades and govern the width and thickness of new blades. These limits are to be used with the pertinent blade drawing to determine the minimum original blade dimensions to which the reduction of figure 8-27 and figure 8-28. may be applied.
When repairs reduce the width or thickness of the blade below these limits, reject the blade.
The face alignment or track of the propeller should fall within the limits recommended by the manufacturer for new propellers
FIGURE 8-26. Method of repairing damaged tip of aluminum-alloy propellers.
No repairs are permitted to the shanks (roots or hub ends) of aluminum-alloy, adjustable-pitch blades. The shanks must be within manufacturer’s limits.
The following two examples show how to determine the allowable repair limits on aluminum alloy blades.
Example 1. Determine the blade width repair allowable (∆w) and minimum blade width limit, (w1) for a blade having a diameter (d) of 10 ft.
6 in. The repair location
Par 8- Page 398 (r1) is 24 in. from the shank and the original, as manufactured, blade width ( w) at the repair location is 1.88 in.
Step 1. Calculate the blade radius (r)
r = d/2 = (10 ft 6 in)/2 = 126/2 = 63 in.
Step 2. Calculate percent of blade radius to repair (r%)
r% = r1/r x 100 = (24/63) x 100 = 38
a. Draw a vertical line at the value of r% = 38 on the horizontal axis.
b. Where the vertical line intersects the cu rve, draw a horizontal line to the right to intersect the vertical axis.
c. Read the percent reduction in width ( ∆w%) on the vertical axis at this intersection.
∆w% = 2.5 FIGURE 8-27. Example 1. Determine the repair width limits.
Step 3. Determine percent reduction in width (∆w%) from figure 8-27.
Step 4. Calculate the blade width repair allowable (∆w)
∆w =(∆w%) x ( w) x(0.01) = (2.5) x (1.88) x (0.01) =
0.05 in.
Step 5. Calculate the minimum blade width limit (w1) at the repair location
w1 = w - ∆w = 1.88 - 0.05 = 1.83 in.
Example 2. Determine the blade thickness repair allowable ( ∆t) and minimum blade thickness limit ( t1) for a blade having a diameter (d) of 10 ft. 6 in. The repair location
Par 8-74 Page 8-399 (r1) is 43 in. from the shank and the original, as manufactured, blade thickness ( t) at the repair location is 0.07 in.
Step 1. Calculate the blade radius (r)
r = d/2 = (10 ft 6 in)/2 = 126/2 = 63 in.
Step 2. Calculate percent of blade radius to repair (r%)
r% = r/r x 100 = (43/63) x 100 = 68
Step 3. Determine percent reduction in thickness (∆t%) from figure 8-28.
Step 4. Calculate the blade thickness repair allowable (∆t)
∆t = (∆t%) x (t) (0.01) = (4.0) x (0.07) x (0.01) = 0.003 in.
Step 5. Calculate the minimum blade thickness limit (t1) at the repair location
t1 = t - ∆t = 0.07 - 0.003 = 0.067 in.
STEEL HUBS AND HUB PARTS. Repairs to steel hubs and parts must be accomplished only in accordance with the manufacturer’s recommendations. Welding and remachining is permissible only when covered by manufacturer’s service bulletins (SB).
PROPELLER HUB AND FLANGE REPAIR. When the fixed-pitch propeller bolt holes in a hub or crankshaft become damaged or oversized, it is permissible to make repairs by using methods (A) or (B) in figure 8-29, or by use of aircraft standard bolts 1/16-inch larger than the original bolts. Make the repairs in accordance with the recommendations of the propeller metal hub manufacturer or the engine manufacturer, as applicable. Obtain from the engine or propeller hub manufacturer suitable flange bushings with threaded or smooth bores, as illustrated in methods (A) or (B) of figure 8-29. Drill the flange and insert the bushings as recommended by the propeller to accommodate the bushings, and protect the holes with 2 coats of aluminum paint or other high moisture-resistant coating. Use bolts of the same size as those originally used. Any of the following combinations may be used:
(1) drilled head bolt and castellated nut,
(2) drilled head bolt and threaded bushing, or
(3) undrilled bolt and self-locking nut. Where it is desirable to use oversized bolts, obtain suitable aircraft-standard bolts 1/16-inch larger than the original bolts. Enlarge the crankshaft propeller flange holes and the propeller hub holes sufficiently to accommodate the new bolts without more than 0.005-inch clearance.
Such reboring will be permitted only once.
Further repairs of bolt holes may be in accordance with the methods listed in (A) or (B) of figure 8-29.
NOTE: Method (A) or (B) is preferred over the oversized bolt method, because a propeller hub flange re-drilled in accordance with this latter
Par 8-74 Page 8-400
a. Draw a vertical line at the value of r% = 68 on the horizontal axis.
b. Where the vertical line intersects the curve, draw a horizontal line to the right to intersect the vertical axis.
c. Read the percent reduction in thickness (∆t%) on the vertical axis intersection ∆t% = 4.0 FIGURE 8-28. Example 2. Determine the repair thickness limits.
method will always require the re-drilling of all new propellers subsequently used with the re-drilled flange.
CONTROL SYSTEMS. Components used to control the operation of certificated propellers should be inspected, repaired, assembled, and/or tested in accordance with the manufacturer’s recommendations. Only those repairs which are covered by the manufacturer’s recommendations should be made, and only those replacement parts which are approved under 14 CFR, part 21 should be used.
DEICING SYSTEMS. Components used in propeller deicing systems should be inspected, repaired, assembled, and/or tested in accordance with the manufacturer’s recommendations. Only those repairs which are covered by the manufacturer’s recommendations should be made, and only those replacement parts which are approved under 14 CFR, part 21 should be used.
WOOD HUB OF PROPELLER WOOD HUB OF PROPELLER -D /2"
(2) •(1) D = 1/4" LARGER
THAN BOLT SIZE
METHOD (A) METHOD (B)
REPAIR OF DAMAGED OR ELONGATED BOLT HOLES REPAIR OF ELONGATED BOLT HOLES IN PROPELLER HUB FLANGES. IN PROPELLER.
(1) DRILLED BOLT WITH CASTELATED NUT OR UNDRILLED BOLT WITH SELF- LOCKING NUT.
(2) BOLT WITH HEAD DRILLED FOR SAFETY WIRING.
NOTE: THESE REPAIRS ARE PERMITTED ONLY ON THE DRIVING FLANGE OF THE PROPELLER HUB AND THE ADJACENT FACE OF THE PROPELLER.
FIGURE 8-29. Repair of fixed-pitch hub and propeller with elongated or damaged bolt holes.
8-79.-8-90. [RESERVED.] Par 8-78 Page 8-401 (and 8-36)