Dating back to our first preflight we have all been instructed to look for propeller imperfections such as nicks, dings, dents and even significant scratches. A slight nick is an easy item to rationalize — if the nick covers less than 1% of the blade sectional area, it’s a beautiful weekend to fly and the decrease in strength seems insignificant. But is it really insignificant? Nothing bears weight or strain without bending or stretching. Propeller failure can create an out of balance rotation great enough to break the engine loose from the frame, causing the airplane to become uncontrollable. The stresses created in a prop blade are far greater than you may realize. At take off RPM the centrifugal force of the turning prop subjects it to loads that can be more than 10 times the weight of the aircraft. Imagine 10 airplanes hanging above your head by that small piece of aluminum. Would you then ignore that nick? Fortunately, the prop blade is somewhat over engineered, which provides a margin of safety. However, the geometry of stress concentration can multiply the strain around the base of the imperfection many times. Imagine lines drawn parallel to the leading edge of the prop from hub to tip. Now put an exaggerated nick in the leading edge. Visualize as the prop stretches outward with increased workload how the once parallel lines become compressed at the base of the nick, multiplying stress at that point. The change in direction of stress lines also raises opposing stress factors with the angle of attack compression. Now add the bending of the prop as the airplane moves through the air and it’s easy to see how the metal around the imperfection maybe taking a triple hit. The only course of action with prop failure is to shut down the engine and glide to a landing site.
Aluminum is a fairly forgiving prop material, however, when stressed beyond a certain point even a small number of times it undergoes permanent changes in its molecular structure that can weaken it, leaving it more susceptible to cracking.
Flying an airplane until a qualified person can “dress out” a nick is always a judgment call. The more you listen, read and learn about airplanes, the more comfortable and enjoyable flying becomes.
Reference: Aviation Safety October 2003 by Roger Long, CFI flight training
These safety tips are provided by the WCFC Safety Committee. They are intended to stimulate thought and discussion about flight safety and do not necessarily represent club policy nor are they intended to replace instruction from a qualified instructor.