Safety alert issued for aluminum propellers

The NTSB has issued a Safety Alert warning pilots about the importance of proper inspections and maintenance of aluminum propeller blades.

SA090, released in April 2024, notes that aluminum propeller blades can be susceptible to fatigue cracking and fracture if a small nick, pit, or corrosion on the surface or edge is not found and repaired during preflight inspection or maintenance.

“Such damage can concentrate stress from normal airplane operation loads, resulting in fatigue crack initiation and growth followed by propeller blade fracture,” the safety alert warns.

Fatigue cracking and fracture of a propeller blade can lead to airframe and engine damage and a possible loss of control, it continues.

The safety alert also notes that airplanes used for aerial application and coastal operations may be at higher risk for propeller blade damage because they can be exposed to chemicals or salt-laden moisture, which can cause corrosion that leads to a propeller blade fracture from fatigue cracking.

“Any airplane operating on an unimproved or backcountry airstrip is also at high risk for propeller blade damage because loose rocks, gravel, or debris on unimproved airstrips can create small nicks on aluminum propeller blades that can turn into large fatigue cracks,” according to the safety alert.

The NTSB released the Safety Alert after investigating several accidents and incidents where a failure to properly inspect and repair small damage to aluminum propeller blades resulted in propeller blade fatigue cracking and fractures.

Some of the accidents include:

The left-side aluminum propeller blade of a Beech 58 separated from the airplane in flight. The pilot was able to land at the nearest airport where an examination of the propeller blade fracture surface revealed signatures consistent with high-cycle fatigue.

It is likely that damage associated with the fatigue fracture origin was present during the last 100-hour maintenance inspection, about 28 flight hours before the incident, and during the subsequent preflight inspections, though the presence of black paint covering damage in the fatigue origin area could have made it more difficult to detect.

Though obscured by paint, the defect would have been detectable with careful visual inspection, which should have prompted further inspection and repair. (CEN22LA250)

In another accident, a portion of an airplane’s aluminum fixed pitch propeller blade separated during takeoff, and the pilot performed a 180° turn to land back to the runway.

An examination of the propeller blade revealed fatigue cracking initiating from corrosion pits on the camber or front side, near the propeller blade tip. Maintenance records indicate that the propeller had not undergone a manufacturer-recommended overhaul in the eight years since it was installed on the airplane in 2015.

If the recommended overhaul had been performed, the corrosion pits that led to fatigue crack initiation would likely have been detected and removed, preventing the blade separation. (CEN23LA218)

In a third accident, the aluminum propeller blade on a Cessna 185 fractured while the pilot was taxiing for takeoff, leading to excessive vibrations that substantially damaged the engine mount.

A post-accident examination of the fractured propeller blade revealed that a nick in the leading edge, likely from frequent operation on unpaved runways, led to fatigue cracking and fracture.

It is likely the nick was present during the preflight inspection and the pilot did not adequately inspect the propeller blade. Had the propeller blade been adequately inspected during the preflight inspections and repaired, the airplane likely would not have sustained substantial damage. (ANC17LA052)

What Can Pilots and Aircraft Owners Do?

The Safety Alert includes a number of suggestions, including:

• During preflight inspections, inspect all areas of the propeller blade for damage, such as nicks or corrosion. Include the back/face side of the blade and pay particular attention to the leading edge. Refer any findings to a qualified mechanic for inspection and repair before further flight.
• Following any propeller work, have a second person inspect the work. If any propeller blades need to be repaired, ask another mechanic to inspect the work if they are available or inspect the work yourself. Depending on the type of repair, another pilot may also be able to inspect the work.
• Consider increasing the frequency of propeller blade inspections by maintenance personnel for airplanes that are used for backcountry, aerial application, and coastal operations because they may be at higher risk for propeller blade damage.

What Can Maintenance Personnel Do?

“Always properly and comprehensively inspect aluminum propeller blades,” the Safety Alert recommends, adding some tips:

• Inspect all areas of the propeller blade, including the back/face side of the blade, paying particular attention to the leading edge.
• Inspect the propeller blades section by section; consider marking off sections of large propellers to ensure complete coverage.
• Use the best possible light and at least 10 times magnification for any questionable areas.
• Be aware of any cosmetic painting that may obscure underlying surface damage that needs to be repaired.
• If any propeller blades need to be repaired, ask another mechanic to inspect the work as well. Depending on the type of repair, a pilot may also be able to inspect the work.
• Increase frequency of propeller blade inspections for airplanes that are used for backcountry, aerial application, and coastal operations because they may be at higher risk for propeller blade damage.

You can find the full Safety Alert, which includes links to a number of other resources, at NTSB.gov.