On July 23, 2021, about 1510 Alaska daylight time, a float-equipped Piper PA-14 airplane, N4206H, sustained substantial damage when it’s rudder structurally failed in flight about 15 miles southeast of Anchorage, Alaska. The commercial pilot was uninjured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 flight. The pilot reported that while in level cruise flight, frequent left and right rudder inputs were required to keep the inclinometer centered, similar to a light turbulence encounter. The airplane then began to experience a constant yaw, with elevator adjustments required to maintain level flight. He stated that the oscillation then stopped and a “a very large right rudder input” was required to maintain the course heading. The pilot was able to maintain control of the airplane and land uneventfully.
Figure - Photo of the damaged rudder. Photo provided by FAA.
An initial examination of the rudder revealed that the vertical spar tube separated above the upper hinge point and the top portion of the rudder folded over the horizontal stabilizer tail brace wires. (See figure).
When the accident airplane’s rudder was examined, radial features with crack arrest lines were observed, consistent with fatigue from multiple origins at the outer diameter. The piece of post from the upper side of the fracture was further examined using a scanning electron microscope (SEM). Multiple fatigue origins were identified on the right side that originated from corrosion pits on the exterior surface. Fatigue fracture features were identified around approximately ¼ of the circumference on the right side, but the total extent of fatigue cracking could not be determined conclusively due to post-fracture contact damage on the surfaces.
The NTSB does not know the exact date of manufacture for this rudder, only the date of manufacture for the airplane. The composition of the rudder post was analyzed using an Olympus Vanta C-Series alloy analyzer. The material associated with the rudder post was identified as carbon steel, consistent with American Iron and Steel Institute (AISI) 1025 carbon steel originally specified for use in the rudder post.
According to engineering drawings, the rudder post for Piper part number 40622 was originally manufactured from 0.875-inch diameter, 0.035-inch wall thickness AISI 1025 carbon steel tube. In a Piper engineering change order (ECO) dated June 3, 1974, the specified tube material for the rudder post was changed to normalized AISI 4130 low-alloy steel. The ECO allowed for existing in-process and completed parts to be used to depletion. The material change was incorporated into the part number 40622 engineering drawing in June 1974.
The vintage single-engine Piper airplane model types from which these rudders were obtained were designed for static load conditions as required by the regulations in place at the time they were certified. However, in service, the loading conditions on many parts of these airplanes’ structures, including the rudders, are not static and contain dynamic alternating or repeated (fatigue) loads. It is well documented that fatigue failures in metal occur at stress levels well below the static strength stress levels.
Aircraft designed in accordance with modern regulations are required to account for fatigue loads. To assess the implications of this, the NTSB conducted a structural load analysis for three of the rudders with no scatter factor applied and found that the bending stresses on the rudder posts from certification gust and maneuver loads are significantly closer to the endurance limit for posts made of AISI 1025 carbon steel than they are for those made of AISI 4130 low-alloy steel.
FAA Advisory Circular 23-27, “Parts and Materials Substitution for Vintage Aircraft,” published in 2009, specifically advises that AISI 4130 low-alloy steel may be substituted for AISI 1020 or AISI 1025 carbon steel, including for structural posts on applicable aircraft. The advisory circular clarifies that this is because AISI 4130 low alloy steel is more widely available and generally has more desirable material properties, such as higher tensile ultimate strength, yield strength, and fatigue strength.