Incident Overview
On July 23, 2021, at approximately 1510 Alaska daylight time, a float-equipped Piper PA-14 (registration N4206H) sustained substantial damage when its rudder structurally failed in flight. The aircraft was operating about 15 miles southeast of Anchorage, Alaska, under Title 14 Code of Federal Regulations Part 91. The commercial pilot was uninjured.
Pilot Report
The pilot reported that during level cruise, frequent left and right rudder inputs were needed to keep the inclinometer centered, similar to light turbulence. Subsequently, the airplane experienced a constant yaw requiring elevator adjustments. The oscillation ceased, but a very large right rudder input was necessary to maintain course heading. The pilot maintained control and landed without further incident.
Examination of the Rudder
Initial examination revealed that the rudder's vertical spar tube had separated above the upper hinge point, and the top portion of the rudder folded over the horizontal stabilizer tail brace wires. The fracture surfaces exhibited radial features and crack arrest lines consistent with fatigue from multiple origins at the outer diameter. Scanning electron microscope (SEM) analysis identified fatigue origins on the right side, which originated from corrosion pits on the exterior surface. Fatigue fracture features extended approximately one-quarter of the circumference on the right side; the total extent of fatigue cracking could not be determined conclusively due to post-fracture contact damage.
Material and Historical Context
The exact manufacture date of the rudder is unknown, but the aircraft's manufacture date is known. Material composition analysis identified the rudder post as AISI 1025 carbon steel. Engineering drawings specified the original rudder post (Piper part number 40622) as 0.875-inch diameter, 0.035-inch wall thickness AISI 1025 carbon steel tube. A Piper engineering change order dated June 3, 1974, changed the specified material to normalized AISI 4130 low-alloy steel, allowing existing in-process and completed parts to be used to depletion.
The vintage single-engine Piper models were designed for static load conditions per regulations at certification. However, in-service loads include dynamic alternating loads that can cause fatigue. Modern aircraft regulations require accounting for fatigue loads. The NTSB conducted a structural load analysis on three rudders (without scatter factor) and found that bending stresses from certification gust and maneuver loads were significantly closer to the endurance limit for AISI 1025 posts than for AISI 4130 posts.
FAA Advisory Circular 23-27 (2009) notes that AISI 4130 may be substituted for AISI 1020 or AISI 1025 carbon steel, including structural posts, due to better material properties such as higher tensile ultimate strength, yield strength, and fatigue strength.