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)
Par 8-91 Page 8-402
SECTION 5. INSPECTION OF PROPELLERS
GENERAL. All propellers, regardless of the material from which they are made, should be regularly and carefully inspected for any possible defect. Any doubtful condition, such as looseness of parts, nicks, cracks, scratches, bruises, or loss of finish should be carefully investigated and the condition checked against repair and maintenance specifications for that particular type of propeller. Any propeller that has struck a foreign object during service should be promptly inspected for possible damage in accordance with the propeller manufacturer’s prescribed procedures and, if necessary, repaired according to the manufacturer’s instructions. If the propeller is damaged beyond the repair limits established by the propeller manufacturer, and a replacement is necessary, install the same make/model approved or alternate as specified in the equipment list, applicable FAA Aircraft Specification, Type Certificate Data Sheet (TCDS), or Supplemental Type Certificate (STC). A sample manufacturer’s propeller inspection checklist is shown in table 8-2. It shows the items to be inspected and the inspection intervals.
WOOD OR COMPOSITION PROPELLERS AND BLADES. Wood propellers are usually found on low-power, reciprocating engines while composition (Carbon fiber, Kevlar) propellers are used on high horsepower reciprocating and turbine engines. Due to the nature of wood, these propellers should be inspected frequently to assure airworthiness. Inspect for defects such as cracks, dents, warpage, glue failure, delamination defects in the finish, and charring of the wood between the propeller and the flange due to loose propeller mounting bolts. Composition propellers should be inspected in accordance with the propeller manufacturer’s instructions.
Fixed-pitch propellers are normally removed from the engine at engine overhaul periods. Whenever the propeller is removed, visua lly inspect the rear surface for any indication of cracks. When any defects are found, disassemble the metal hub from the propeller.
Inspect the hub bolts for wear and cracks at the head and threads, and if cracked or worn, replace with new equivalent bolts. Inspect for elongated bolt holes, enlarged hub bore, and for cracks inside the bore or anywhere on the propeller. Repair propellers found with any of these defects. If no defects are found, the propeller may be reinstalled on the engine. Before installation, touch up with varnish all places where the finish is worn thin, scratched, or nicked. Track and balance the propeller, and coat the hub bore and bolt holes with some moisture preventive such as asphalt varnish. In case the hub flange is integral with the crankshaft of the engine, final track the propeller after it is installed on the engine. In all cases where a separate metal hub is used, make a final balance and track with the hub installed on the propeller.
On new, fixed-pitch propeller installations, inspect the bolts for proper torque after the first flight and after the first 25 hours of flying. Thereafter, inspect and check the bolts for proper torque at least every 50 hours. No definite time interval can be specified, since a bolt’s proper torque is affected by changes in the wood caused by the moisture content of th e air where the airplane is flown and stored. During wet weather, some moisture is apt to enter the propeller wood through the holes drilled in the hub. The wood then swells, and because expansion is limited by the bolts extending between the two flanges, some of the wood fibers become crushed.
Later, when the propeller dries out during dry weather or due
Par 8-91 Page 8-403 TABLE 8-2. Sample manufacturer’s propeller inspection checklist.
Nature of Inspection Engine Operating Hours
PROPELLER GROUP
1. Inspect spinner and back plate for cracks....................................................................................
2. Inspect blades for nicks and cracks.............................................................................................
3. Check for grease and oil leaks.....................................................................................................
4. Lubricate propeller per Lubrication Chart.....................................................................................
5. Check spinner mounting Brackets for cracks...............................................................................
6. Check propeller mounting bolts and safety (Check torque if safety is broken).............................
7. Inspect hub parts for cracks and corrosion..................................................................................
8. Rotate blades of constant speed propeller and check for tightness in hub pilot tube....................
9. Remove constant speed propeller; remove sludge from propeller and crankshaft........................
10. Inspect complete propeller and spinner assembly for security, chafing, cracks, deterioration, wear and correct installation........................................................................................................
11. Check propeller air pressure (at least once a month)...................................................................
12. Overhaul propeller.......................................................................................................................
0 0 0 0
0 0 0 0 0 0 0 0
0 0
0 0 0 0 0 0 0 0 0
0 0
1000
0 0 0 0 0 0 0 0 0
0 0 0
to heat from the engine, a certain amount of propeller hub shrinkage takes place, and the wood no longer completely fills the space be tween the two hub flanges.
Consequently, the hub bolts become loose.
In-flight tip failures may be avoided by frequent inspections of the metal cap, leading edge strip, and surrounding areas. Inspect for such defects as looseness or slipping, separation of soldered joints, loose screws, loose rivets, breaks, cracks, eroded sections, and corrosion. Inspect for separation between the metal leading edge and the cap, wh ich would indicate the cap is moving outward in the direction of centrifugal force.
This condition is often acco mpanied by discoloration and loose rivets. Inspect the tip for cracks by grasping it with the hand and slightly twisting about the longitudinal blade centerline and by slightly bending the tip backward and forward. If the leading edge and the cap have separated, carefully inspect for cracks at this point. Cracks usually start at the leading edge of the blade. A fine line appearing in the fabric or plastic may indicate a crack in the wood. Check the trailing edge of the propeller blades for bonding, separation, or damage.
Examine the wood close to the metal sleeve of wood blades for cracks extending outward on the blade.
These cracks sometimes occur at the threaded ends of the lag screws and may be an indication of internal cracking of the wood. Check the tightness of the lag screws, which attach the metal sleeve to the wood blade, in accordance with the manufacturer’s instructions. Inspect and protect the shank areas of composition blades next to the metal sleeve in the same manner as that used for wood blades.
METAL PROPELLERS AND BLADES. These propellers and blades are generally susceptible to fatigue failure resulting from the concentration of stresses at the bottoms of sharp nicks, cuts, and scratches. It is necessary, therefore, to frequently and carefully inspect them for such injuries.
Propeller manufacturers publish SB’s and instructions which prescribe the manner in which these inspections are to be accomplished. Additional information is also available in AC 20-37D, Aircraft Metal Propeller Maintenance.
Steel Blade Inspection. The inspection of steel blades may be accomplished by either visual, fluorescent penetrant (see chapter 5), or magnetic particle inspection. The visual inspection is easier if the steel blades are covered with engine oil or rust-preventive compound. The full length of the leading edge, especially near the tip, the full length of the trailing edge, the grooves and shoulders on the shank, and all
Par 8-93 Page 8-404 dents and scars should be examined with a magnifying glass to decide whether defects are scratches or cracks.
Aluminum Propellers and Blades. Carefully inspect aluminum propellers and blades for cracks and other flaws. A transverse crack or flaw of any size is cause for rejection. Multiple deep nicks and gouges on the leading edge and face of the blade is cause for rejection.
Use dye penetrant or fluorescent dye penetrant to confirm suspected cracks found in the propeller. Refer any unusual condition or app earance revealed by these inspections to the manufacturer.
Limitations.
Corrosion may be present on propeller blades in varying amounts. Before performing any inspection process, maintenance personnel must examine the specific type and extent of the corrosion. (See chapter 6, and/or refer to AC 43-4A, Corrosion Control For Aircraft.)
Corrosion, other than small areas (6 square inches or less) of light surface type corrosion, may require propeller removal and reconditioning by a qualified propeller repair facility. When intergranular corrosion is present, the repair can be properly accomplished only by an appropriately certificated propeller repair facility. Corrosion pitting under propeller blade decals should be removed as described in the propeller manufacturer’s SB’s and applicable airworthiness directives (AD).
Unauthorized straightening of blade, following a ground strike or other damage, can create conditions that lead to immediate blade failure. These unapproved major repairs may sometimes be detected by careful inspection of the leading edges and the flat face portion of the blade. Any deviation of the flat portion, such as bows or kinks, may indicate unauthorized straightening of the blade.
Sighting along the leading edge of a propeller blade for any signs of bending can provide evidence of unapproved blade straightening.
Blades should be examined for any discoloration that would indicate unauthorized heating. Blades that have been heated for any repair must be rejected, since only cold straightening is authorized. All blades showing evidence of unapproved repairs should be rejected. When bent propellers are shipped to an approved repair facility for inspection and repair, the propeller should never be straightened by field service personnel to facilitate shipping, because this procedure can conceal damage. Propeller tip damage will sometimes lead maintenance personnel to consider removing damaged material from the blade tips. However, propellers are often manufactured with a particular diameter to minimize vibration.
Unless the TCDS and both the engine and propeller manufacturers specifically permit shortening of the blades on a particular propeller, any shortening of the blades would probably create an unairworthy condition.
When conditions warrant, inspect the blade tips for evidence of shortening and, if necessary, measure the propeller diameter to determine if it has been changed by an unauthorized repair.
PROPELLER HUB.
Fixed Pitch.
Inspection procedures require removal of the propeller spinner for examination of the prop hub area. Cracks may be present in the hub area between or adjacent to bolt holes and along the hub pilot bore. Cracks in these areas cannot be repaired and require immediate scrapping of the propeller.
Propeller attach bolts should be examined for looseness or an unsafetied or cracked condition. Cracked or broken bolts are usually the result of overtorquing. Correct
Par 8-107 Page 8-405 torquing procedures require all bolt threads to be dry, clean, and free of any lubrication before torquing.
Controllable Pitch.
Inspect controllable pitch propellers frequently to determine that all parts are lubricated properly. It is especially recommended that all lubrication be accomplished in accordance with the propeller manufacturer’s instructions.
Complete inspection/servicing requires the removal of the spinner for examination and servicing of the propeller hub and blade clamp area. All inspections and servicing of the pitch control mechanism should follow the recommendations of the propeller, engine, and airframe manufacturers. Propellers must be in compliance with applicable AD’s and manufacturer’s SB’s.
The hub, blade clamps, and pitch change mechanisms should be inspected for corrosion from all sources, including rain, snow, and bird droppings that may have entered through the spinner openings. Examine the hub area for oil and grease leaks, missing grease-fitting caps, and leaking or missing grease fittings.
Propeller domes should be checked for leaks, both at the seals and on the fill valve (if so equipped). The dome valve may be leak-tested by appl ying soapy water over the fill valve end.
Domes should be serviced only with nitrogen or dry air in accordance with the manufacturer’s recommendations. When propeller domes are inspected and found filled with oil, the propeller should be removed and inspected/repaired by an appropriately-rated repair facility.
It is especially recommended that all lubrication be accomplished at the periods and
Par 8-107 Page 8-406 in the manner specified by the propeller manufacturer. On makes and models with a grease fitting on the hub, before greasing the hub remove the grease fitting opposite the one to which you are going to add grease. This will allow the excess grease and pressure to exit through the grease fitting hole rather than the hub seal.
Fiber-block, pitch-change mechanisms should be inspected for deterioration, fit, and the security of the pitch-clamp forks.
Certain models of full-feathering propellers use spring-loaded pins to retain the feathered blade position. Spring and pin units should be cleaned, inspected, and relubricated per the manufacturer’s recommendations and applicable AD’s.
Pitch change counterweights on blade clamps should be inspected for security, safety, and to ensure that adequate counterweight clearance exists within the spinner.
TACHOMETER INSPECTION. Due to the exceptionally high stresses that may be generated by particular propeller/engine combinations at certain engine revolutions per minute (RPM), many propeller and aircraft manufacturers have established areas of RPM restrictions and other restrictions on maximum RPM for some models. Some RPM limits do not exceed 3 percent of the maximum RPM permitted, and a slow-running tachometer can cause an engine to run past the maximum RPM limits. Since there are no post-manufacture accuracy requirements for engine tachometers, tachometer inaccuracy could lead to propeller failure, excessive vibration, or unscheduled maintenance. If the tachometer exceeds 2 percent (plus or minus) of the tested RPM, replace it.
8-96.8-106. [RESERVED.]
Par 8-107 Page 8-407 SECTION 6. PROPELLER TRACKING AND VIBRATION
GENERAL. To ensure smooth powerplant operations, first start with a properly-installed propeller. Each propeller should be checked for proper tracking (blades rotating in the same plane of rotation).
Manufacturer’s recommendations should in all cases be followed.
PROPELLER TRACKING CHECK. The following is a simple procedure that can be accomplished in less than 30 minutes:
Chock the aircraft so it cannot be moved.
Remove one spark plug from each cylinder. This will make the propeller easier and safer to turn.
Rotate one of the blades so it is pointing down.
Place a solid object (e.g. a heavy wooden block that is at least a couple of inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so that it just touches (see figure 8-30), or attach a pointer/indicator to the cowling itself.
Rotate the propeller slowly to see if the next blade “tracks” through the same point (touches the block/pointer). Each blade track should be within 1/16-inch (plus or minus) from the opposite blade’s track.
If the propeller is out of track, it may be due to one or more propeller blades being bent, a bent propeller flange, or propeller mounting bolts that are either over or under-torqued. An out-of-track propeller will cause vibration and stress to the airframe and engine, and may cause premature propeller failure.
VIBRATION. Although vibration can be caused by the propeller, there are numerous other possible sources of vibration which can make troubleshooting difficult.
If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates, throughout the entire RPM range, although the intensity of the vibration may vary with the RPM. If a vibration occurs only at one particular RPM or within a limited RPM range (e.g. 2200-2350 RPM), the vibration is not normally a propeller problem but a problem with a poor engine/propeller match.
If a propeller vibration is suspected but cannot be positively determined, if possible, the ideal troubleshooting method is to temporarily replace the propeller with one which is known to be airworthy and test fly the aircraft.
There are numerous a llowable tolerances in blade angles, balance, track, and blade width and thickness dimensions. These tolerances have been established through many years of experience. The degree to which these factors affect vibrati on is sometimes disputed and can involve significant repair bills, which may or may not cure a vibration problem. Reliance upon experienced, reputable propeller repair stations is the owner’s best method of dealing with these problems.
Blade shake is not the source of vibration problems.
Once the engine is running, centrifugal force holds the blades firmly (approximately 30-40,000 lbs.) against blade bearings.
Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180 º, and re-installed.