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The NTSB issued a safety alert to operators and pilots, stressing the need for proper inspections and maintenance to prevent fractures on aluminum propeller blades. One of the most crucial aspects of aircraft ownership and operation is proper and timely maintenance. Routine inspections and maintenance can keep your aircraft in tip-top shape, and prevent problems down the road.
Aluminum propeller blades are susceptible to fatigue cracking and can fracture if a small nick or corrosion on the surface or edge is not found and fixed during the preflight inspection or maintenance. Damage like this can concentrate stress from normal airplane operation loads and lead to fatigue cracking and propeller blade fracture. Fatigue cracking and fracture of the propeller blade can result in airframe and engine damage and possible loss of control. Planes used for aerial application and coastal operations are at higher risk for propeller damage due to more frequent exposure to chemicals or salt-laden moisture, causing corrosion that leads to a propeller blade fracturing due to fatigue cracking. Planes flying from unimproved or backcountry airstrips are also at higher risk for prop blade damage due to the loose rocks, gravel or debris that can cause small nicks in the propeller that turn into fatigue cracks. Without regular inspections and proper adherence to manufacturer-recommended overhaul schedules, aluminum propeller blades can have undetected fatigue cracks and blade separation, which can have catastrophic consequences.Read the Safety Alert here
Since the early days of aviation, planes have evolved with modern technology. Along with advanced avionics, faster speeds, better engines and improved safety, propellers have also seen improvements and changes over the decades. Not only has the design of propellers changed, but the materials they are made with. Early propellers were made from wood and were fixed pitch, limiting the performance capabilities of the aircraft, according to Hartzell Propeller. Now, propellers can be made from wood, aluminum or composites. Aluminum had been used in aircraft design since the Wright Brothers were flying, but it was not until after WWII that aluminum propellers became common enough for large-scale production, according to Hartzell Propeller, which began producing aluminum props in 1948.Prop fracture on a Beecraft Baron 58NTSB points out past prop issues
While aluminum props have benefits, like their light weight, the NTSB points out the negative effect of improperly caring for aluminum props. The NTSB investigated numerous crashes in which a failure to properly inspect and repair damage to aluminum props resulted in fatigue cracking and fractures. In 2017, a pilot was taxiing a Cessna A185 Skywagon for takeoff when its McCauley aluminum prop blade fractured, leading to excessive vibrations that substantially damaged the engine mount. The NTSB found a fatigue cracking stemming from a nick in the leading edge, likely resulting from backcountry operations. The agency reported that had the blade been adequately inspected during the preflight inspections and repaired, the plane would not have been substantially damaged.
In 2022, about an hour into a cross-country flight, a pilot heard a "boom" noise on his Beechcraft Baron 58. The plane began to vibrate and yawed to the left. The pilot was able to land the plane without incident and upon exiting, he noticed one of the aluminum prop blades on the left engine had separated about midspan, resulting in minor damage. Post-accident examination revealed signatures consistent with high-cycle fatigue. The NTSB said it was likely that the damage associated with the fatigue fracture origin was present during the last 100-hour maintenance inspection, about 28 hours before the flight, as well as during preflight inspections. The black paint covering the damage made it difficult to detect, but the NTSB said the defect would still have been noticed with careful visual inspection.
In 2023, during takeoff, a Piper J-3 Cub began to shake violently. The pilot closed the throttle and issued an emergency transmission, turning back to land on the departure runway. The pilot landed without incident, and upon exiting the plane, noticed that about five inches of the outboard portion of one of the aluminum prop blades had separated, resulting in minor damage to the propeller. The post-incident examination revealed fractures consistent with fatigue cracking initiating at the midpoint of the cambered face of the prop blade. The propeller had been overhauled on Jan. 28, 2015 and the total time since new was unknown. The prop was installed on the plane on Aug. 18, 2015 and maintenance records did not show any overhaul work performed on the prop since its installation on the Cub. The propeller manufacturer recommends the prop be overhauled at 2,000 hours or 72 calendar months, whichever comes first. The NTSB said that if the recommended overhaul had been performed the corrosion pits that led to fatigue cracking would have been detected and removed, preventing the blade separation.NTSB recommendations
The NTSB recommends that owners, operators and pilots inspect all areas of the propeller blade for damage like nicks or corrosion during the preflight inspections, including the back/face side of the blade, paying particular attention to the leading edge. Any findings should be referred to a qualified mechanic for inspection and repair before further flight. After propeller work, the NTSB suggests having a second person inspect the work, or inspect the work yourself. The NTSB also suggests possibly increasing the frequency of propeller blade inspections by maintenance personnel for aircraft used for backcountry, aerial application and coastal operations due to the higher risk for propeller blade damage. Operations that are not required to follow the manufacturer's recommended overhaul times, like public aircraft, Part 91 and Part 137 operations, should maintain the propeller based on the manufacturer's instructions for continued airworthiness and follow the recommended aluminum propeller blade overhaul schedules.
Maintenance personnel should carefully inspect the aluminum prop blades, following the maintenance manual and using appropriate checklists. The NTSB recommends inspecting all areas of the blade, inspecting section by section, using the best light possible and at least 10 times magnification for questionable areas. Maintenance personnel should be aware of any cosmetic painting that could obscure the underlying surface damage, avoid distractions while performing inspections, ask another mechanic to inspect the work as well before repairs and add to the prop logbook any work performed to the blades. Visible dye can be used to help identify cracks. Service documentation should be carefully maintained with any inspection or repair.
Proper inspections and maintenance can prevent damage and incidents while flying. Propellers braking off midflight can result in loss of control of the aircraft, which if the pilot is unable to regain control, can have serious consequences. The NSTB urges vigilance and proper care of aircraft to keep flying safely and smoothly.