Part V - Standard 571 Appendix D - Field Repair of Aircraft Propellers
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Canadian Aviation Regulations (CARs) 2017-2
Content last revised: 2007/12/30
1. Blade Repairs
All repairs shall be made in accordance with the manufacturer's recommendations. This section provides guidance on the field repair of propeller blades where manufacturer's repair manuals do not specify the level of work which can be performed in the field.
In cases of conflict, the current manufacturer's recommended repair practices shall take precedence. As a general guide, permissible field repairs are restricted to the following:
(a) Wooden Propellers
Small cracks that are parallel to the grain can be filled with glue, and sanded flush with the propeller surface. Dents or scars can be filled with a mixture of glue and fine sawdust provided that they are surface defects only (i.e. no more than 5 per cent of blade thickness).
Narrow slivers of up to 1/8 inch wide broken from the trailing edge can be repaired by sandpapering a new edge, removing the least amount of material necessary to achieve a smooth contour. Both blades shall be altered by the same amount.
Following repair the propeller shall be re-protected with an even coat of varnish. These repairs add nothing to the strength of the propeller. They are intended to preserve the aerodynamic shape and prevent the ingress of moisture. Any repairs other than those described are beyond the scope of the AME in the field.
(b) Aluminum-alloy Propellers
The most common error in blade repairs is failure to remove sufficient material. This can lead to the damage being blended out over too small an area, causing unacceptably high local stress concentrations. In the worst cases the original fatigue crack may still be present after blending, being concealed by the smearing action of the blending file.
Correct repair procedure involves not only the removal of all damage, but also the elimination of local stress concentrations. When this has been achieved, the finished repair is assessed against the blade repair limits to determine if the blade may remain in service. Field repair limits for aluminum-alloy blades are illustrated in figure 1 of this appendix.
Aluminum-alloy propeller blades with dents, scratches, nicks, leading-edge pitting, etc., can be repaired as described below unless contrary to manufacturer's recommendations. Multiple repairs can be made provided their location with respect to each other is not such as to form a continuous line across the blade. Sharp riffler or fine-cut files can be used for removing material. The use of blunt or worn files to blend out nicks will smear metal into cracks, rendering them difficult to detect. Finally, the area should be smoothed with fine crocus cloth.
Blend out nicks, scars, cuts, lightning burns, etc. on the leading edge of aluminum-alloy blades as shown in figure 2 of this appendix. The finished repair must be blended over a distance at least 10 times the depth of damage. In addition to blending the planform, care should be taken to restore the original cross-sectional profile.
Blades with the leading edges pitted from normal wear in service can be reworked by removing sufficient material to eliminate the pitting. In this case, remove the metal by starting well back from the edge, as shown in figure 3 of this appendix, and working forward over the edge in such a way that the contour will remain substantially the same, avoiding abrupt changes in contour or blunt edges. Trailing edges of blades can be treated in a similar manner.
On the thrust and camber faces of blades, remove the metal around any dents, scratches, cracks and burns to form shallow saucer-shaped depressions as shown in figure 4 of this appendix. Take care to remove the deepest point of the damage. If, prior to commencement of the work, two lines are drawn on the blade with a soft crayon or marker, in such a way that they intersect at the deepest point, this will assist in locating the point as the work progresses. (Do not use a lead pencil for this purpose, as it can cause corrosion due to electrolytic action).
As indicated in the example, the finished repair should have its longest axis in a spanwise direction (root to tip) irrespective of the alignment of the original damage. The reworked area must extend over a distance of at least 10 times the depth in a chordwise direction, and 30 times the depth in a spanwise direction.
When the treatment of defects on a blade tip necessitates shortening of the blade, each blade on the propeller shall be shortened by the same amount. The use of a template will assist in keeping the planform of each blade identical. Remove sufficient material from the cambered face of the blade to maintain the original shape of the blade cross-section. Take care not to reduce the blade diameter below the minimum permitted for the particular installation. On certain installations, no reduction is permitted; however, the maximum reduction in blade length is normally 2%. Details of any change in blade diameter must be entered in the leading particulars section in the front of the propeller log.
On completion of the blending out of any damage, inspect the area with a x5 or x10 magnifying glass. If certain that no cracks remain, remove a further .002" by polishing with fine crocus paper as an additional stress relieving measure. In the case of lightning burns, remove .020" to ensure that no heat damaged material remains. (Some manufacturers can also specify a local hardness check following lightning strike).
The finished repair can then be checked by depth gauge or callipers to ensure that the final dimensions are within limits. The maximum field repair limits are specified in figure 1 of this appendix, which applies provided that the minimum approved blade dimensions are observed.
As indicated in figure 1 of this appendix, more material can be removed near the tip than near the root, as the forces acting on the blade are lower in this area. It should not be assumed, however, that repairs near the tip are not critical. In fact, the majority of failures take place in the outer portion of the blade. Great care must be taken to remove stress raisers at any point on the blade.
Finally, the entire area shall be subjected to a close examination for cracks prior to the application of protective coating. The use of an etch and penetrant dye process, to detect residual cracks following blending out of damage, is strongly recommended. No straightening of bent blades, welding, or heat treatment, is permitted in the field. If it should be necessary to straighten a propeller to facilitate its packing for return to an approved overhaul organisation, an accurate drawing of the bend prior to straightening shall accompany the propeller. No peening is permitted, except where specifically recommended by the manufacturer. The flattening of rough areas by rolling or peening is prohibited under all circumstances.
(c) Steel And Composite Propeller Blades
No general comments can be made about the repair of steel or composite blades, as repair schemes vary widely.
All repairs are to be carried out in accordance with the propeller manufacturer's recommendations or methods.
2. Field Repair Standards Aluminum Blades
The field repair limits shown below can be applied, provided the tolerances of the appropriate blade specification are not exceeded. Repairs which exceed these limits may only be performed by an approved propeller overhaul organization.
- Figure 1 - Field Repair Limits
- Figure 2 - Local Rework Face or Edge
- Figure 3 - Rework-Leading Edge
- Figure 4 - Rework Blade Face
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