CEN23LA218 · Piper J3C 65

The pilot reported that no anomalies were noted during the preflight inspection of the airplane. During the takeoff, about 650 ft agl, the airplane began to shake “very violently.” The pilot closed the throttle, issued an emergency transmission to the air traffic control tower, and executed a 180° turn to the left to land back on the departure runway. About halfway through turn, the pilot turned off the engine as he felt the airplane could successfully make the landing. The pilot was able to land the airplane on the runway without further incident. After the pilot exited the airplane, he noticed that about 5 inches of the outboard portion of one of the aluminum propeller blades had separated. The airplane sustained minor damage to the propeller. Postincident examination revealed features that were consistent with fatigue cracking initiating at the midpoint of the cambered face of the propeller blade. These initiation sites exhibited corrosion pits consistent with those found on the cambered face of the propeller, which had been present underneath the paint and primer. On closer examination, these pits exhibited higher amounts of chlorine than the rest of the blade surfaces. It is unclear as to the origin of the pitting corrosion, which was likely due to chlorine species. Chlorine is a common element known to cause pitting of aluminum alloys in service. Many chemicals, locales, and substances can impart chlorine (as well as sulfur, phosphorus, and alkali metals) onto metal parts. These constituents can diffuse through a variety of coatings and materials, though their effectiveness resisting potentially aggressive chemicals in this case is unknown. The propeller was overhauled on January 28, 2015, and the total time since new was listed as “unknown.” The propeller was installed on the airplane on August 18, 2015. The maintenance records did not show any overhaul work performed on the propeller since it was installed on the airplane. According to the propeller manufacturer, this propeller is to be overhauled at 2,000 hours or 72 calendar months, whichever occurs first. The Federal Aviation Administration (FAA) does not mandate that propellers be overhauled for Title 14 Code of Federal Regulations (CFR) Part 91 operations. Cracks in propellers can grow to fracture in just a few flights once started. At overhaul, the paint, primer, and any coatings would likely be removed, and the surfaces refinished. These processes would likely remove surface stress concentrators like pitting and other imperfections, along with detecting any visible cracks. With the blade being 2.5 years outside of a recommended overhaul, the chances of cracks initiating would be higher.

Verbatim NTSB analysis and probable cause from the NTSB dataset