Aircraft Maintenance: Know your propeller repair limits

Most propeller issues fall into two categories: corrosion and physical damage.

Corrosion

One of the most insidious causes of propeller damage is corrosion. This includes external corrosion that is visible on the blades, as well as internal corrosion that literally eats away at the components within the hub of a variable-pitch propeller. Regardless of its location, corrosion reduces the structural integrity of the propeller as well as its performance.

Pitting is a very specific and serious form of corrosion. Pitting corrosion consists of small, visible corrosion cavities extending inward from the metal surface of the propeller. Pits can grow on the surface, but are more commonly found where moisture is trapped, such as under decals left in place after propeller balancing or improperly installed de-icing boots. Any sign of pitting is serious and should be evaluated quickly in an effort to save the propeller from the junk heap.

Intergranular corrosion is a form of corrosion internal to the metal structure itself. It is less common and more likely to be caused by a problem in the metal casting, but it too can be caused by trapped moisture under decals or around bolt holes.

Physical damage

The most common forms of physical propeller damage are nicks, dings, and cracks on both the blades and the hub of the propeller. A propeller shop has the tools to do much more detailed inspections of propellers for cracks, including ultrasonic, eddy current, dye penetrant, and magnetic particle inspections. However, routine inspections for damage visible to the naked eye are a crucial starting point.

According to the FAA’s Advisory Circular AC 20-37E, "Limited minor repairs may be made on propellers by appropriately rated maintenance technicians either on the aircraft or when the propeller is removed. Minor dents, cuts, scars, scratches, and nicks may be removed providing their removal does not weaken the blade, substantially change weight or balance, or otherwise impair its performance."

It’s important to always use the propeller manufacturer’s maintenance manuals for proper maintenance procedures and limits when working on a propeller. That said, the FAA provides some guidelines for minor repairs in AC 20-37E:

For nicks, dents, pits, and cuts in the leading or trailing edges of blades, ensure that the bottom of the damage is removed first by rounding out and fairing in the repair only slightly deeper than the damage. Initial removal of material should be done using a fine cut file. All traces of file marks in the repaired area should be removed with number 240 emery cloth followed by polishing with number 320 emery cloth, then finished with crocus cloth or 600 grit emery cloth, and then visually inspected. An individual edge repair should not exceed a depth of 3/16-inch.  The repair length should be 10 times longer than the depth of the repair.

For gouges, cuts, and small dents on blade faces, ensure that the bottom of the damage is removed first by rounding out and fairing in the repair to form a saucer-shaped depression only slightly deeper than the damage. The initial repair should be accomplished by filing with a fine cut file parallel to the damage and finishing with 240 and 320 emery abrasive cloth, as in the manner of damage removal from blade-leading edges. Final polishing of the repair should be done with crocus cloth or 600 grit emery cloth. An individual repair should not exceed 1/16-inch in depth and the surface radius of curvature of the repair must not be less than 3/8-inch. Repair width should be 30 times the repair depth.

Of particular note in these repair guidelines are the rules regarding the transitions and fairing out of the repairs. For leading edge repairs, the total repair length is 10 times longer than the depth of the repair.  This is very important. It means that if you need to down 1/8-inch (0.125 inch) to get to the base of a nick, you will need to smooth out the repair over the distance of 1.25 inches. For blade face repairs, it’s 30 times. For a 1/16-inch deep repair (0.0625 of an inch), that’s a 1.875-inch disk of repair area. Those are pretty big dimensions; more than one might assume without proper training. So, it pays to know proper maintenance practices such as this to ensure that repairs are made properly.

We’ve just scratched the surface of propeller inspection and repair. Next time we will delve deeper and look at what happens when it’s time to get the prop shop involved for some real maintenance, repairs, and overhauls. Until then…happy flying!