N78KLBeech 58 · 1990

On June 8, 2022, about 1100 central daylight time, a Beech 58 airplane, N78KL, sustained minor damage when it was involved in an incident near Evansville, Indiana. The pilot and four passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that no anomalies were noted during the preflight inspection of the airplane. The airplane departed from Kirk Field Airport (PGR), Paragould, Arkansas, with a destination of James M. Cox Dayton International Airport (DAY), Dayton, Ohio. When the airplane was about 1 hour into the flight, while at an altitude of about 9,000 ft mean sea level and an airspeed of about 180 knots, the pilot heard a “boom” noise. The airframe began vibrating, and the airplane yawed to the left. The pilot contacted approach control and reported that he needed to land the airplane immediately. The pilot was directed to divert to Evansville Regional Airport (EVV), Evansville, Indiana.

The pilot then reduced the power on the right engine and maneuvered the airplane for landing on runway 18 at EVV. The pilot was able to land the airplane without incident. After the pilot exited the airplane, he noticed that one of the aluminum propeller blades on the left engine had separated about mid-span. The separated blade segment was not recovered. No other damage was observed on the propeller, engine, or airframe.

The airplane had two Hartzell Propeller model HC-C4YF-2E/FC7063Q propellers that each had four blades. The propellers were installed on the airplane via Federal Aviation Administration (FAA) Supplemental Type Certificate SA1762SO on November 12, 2002.

The fractured blade was manufactured on September 6, 2000. Airplane registration records indicate that the airplane was sold on May 13, 2021. The airplane’s previous 100-hour inspection, completed before the airplane was sold, occurred on December 17, 2020, when the propeller had accumulated 1,097 hours of time since overhaul (TSO). The most recent 100hour inspection of the propeller occurred on January 10, 2022, when the propeller had accumulated 1,172 hours of TSO. The propeller had accumulated 1,200 hours of TSO at the time of the incident.

The fractured blade (design number FC7063Q and serial number J51843) was removed from its hub and was provided to the National Transportation Safety Board Materials Laboratory for further examination. The examination of the fracture surface revealed signatures consistent with high-cycle fatigue failure. The blade fractured was about 16.9 inches from the blade butt. The deice boot was severed at the fracture and was partially debonded on both the camber and face sides.

The fatigue origin area was coincident with an area of damage that was covered with a thick black paint that had a slightly different composition than the thinner layer of black paint on the rest of the blade. No evidence indicated any repair in the area of the damage.

The pilot reported that he was not aware of any paint work performed on the blades or any previous propeller blade impacts. The mechanic who performed the most recent 100-hour inspection reported that he performed a visual examination of the blades and recalled that only normal damage was observed on the blades, such as small nicks, which he dressed. He did not remember if he did any paint work on the blades.

FAA Advisory Circular 20-37E, Aircraft Propeller Maintenance, discusses propeller blade failures and states in part the following:

A propeller is one of the most highly stressed components on an aircraft. During normal operation, 10 to 25 tons of centrifugal force pull the blades from the hub while the blades are bending and flexing due to thrust and torque loads and engine, aerodynamic and gyroscopic vibratory loads. A properly maintained propeller is designed to perform normally under these loads, but when propeller components are damaged by corrosion, stone nicks, ground strikes, etc., an additional unintended stress concentration is imposed, and the design margin of safety may not be adequate. The result is excessive stress and the propeller may fail.

Additional causes of overstress conditions are exposure to overspeed conditions, other object strikes, unauthorized alterations, engine problems, worn engine vibration dampers, lightning strike, etc. Most mechanical damage takes the form of sharp-edged nicks and scratches created by the displacement of material from the blade surface and corrosion that forms pits and other defects in the blade surface. This small-scale damage tends to concentrate stress in the affected area and eventually, these high-stress areas may develop cracks. As a crack propagates, the stress becomes increasingly concentrated, increasing the crack growth rate. The growing crack may result in blade failure.

Many types of damage cause propellers to fail or become unairworthy. FAA data on propeller failures indicates that the majority of failures occur in the blade at the tip region, usually within several inches from the tip and often due to a crack initiator such as a pit, nick, or gouge. However, a blade failure can occur along any portion of a blade, including the mid-blade, shank, and hub, particularly when nicks, scratches, corrosion, and cracks are present. Therefore, during propeller inspection and routine maintenance, it is important to inspect the entire blade.

FAA Special Airworthiness Bulletin NE-08-22, Propeller Search Inspection (General Visual Inspection), discusses cosmetic repairs and states in part the following:

For exposed aluminum surfaces, an exposed defect can be inspected while a hidden defect cannot be inspected. A cosmetic repair that creates a hidden defect in an exposed surface is an unacceptable practice.