N3188MPiper PA-12 · 1947

On June 8, 2020, about 0945 Alaska daylight time, a Piper PA-12, N3188M, was substantially damaged when it was involved in an accident near Anchorage, Alaska. The flight instructor and pilot receiving instruction were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.

According to the flight instructor, they departed Lake Hood Airport (PALH) and proceeded to Twin Island Lake (about 8 nm northwest of PALH), where, upon arrival, they conducted a normal landing. After departing, they climbed to about 500 ft above ground level (agl) before turning to a left crosswind traffic pattern leg. While on the left crosswind leg, the airplane yawed abruptly to the right and the pilot indicated that the controls felt strange. The flight instructor assumed control of the airplane and noticed drastically diminished control about the vertical axis. In addition, a significant downward elevator pressure (forward control yoke) was required. In an effort to aid in directional control the water rudders were deployed. Uncertain that he could make a 180° and return to Twin Island Lake due to the poor directional control, he elected to return to PALH where emergency services were available and conducted an uneventful landing.

An examination of the rudder revealed that the vertical rudder post fractured above the upper hinge point and the top portion of the rudder folded over the horizontal stabilizer tail brace wires (see figure).

Figure - Photo of the damaged rudder. Photo provided by FAA.

A review of the National Transportation Safety Board (NTSB) accident database found two additional accidents that involved an in-flight structural failure of the rudder on single engine Piper airplanes, NTSB accident numbers SEA84LA220 and ANC21LA064. In addition, three additional rudders with similar structural failures were discovered during the investigation and examined.

In total, five similarly fractured rudders were examined by the NTSB through the course of this investigation; this accident airplane’s rudder, a rudder obtained from ANC21LA064, two rudders that were provided by a repair facility and a fifth that was received in response to an FAA “Airworthiness Concern Sheet.” The NTSB found that all the rudders examined were consistent with Piper part number 40622, had posts made of American Iron and Steel Institute (AISI) 1025 carbon steel, and had fractured above the upper hinge.

Further NTSB examination identified evidence of a progressive fracture mechanism on each of the rudder posts, including fatigue fracture features on three of the rudder posts. The fracture surfaces on the accident rudder post and the remaining rudder post were too damaged to conclusively identify the specific fracture mechanism. Corrosion, scratches, or surface roughness features were found on each of the rudder posts near the fractures. Fracture origin areas were damaged on all the rudders, but corrosion pits or scratches on the exterior surface were associated with the fatigue origin areas on two of the examined rudder posts.

Piper part number 40622 rudder posts were originally manufactured from 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 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.

Aircraft designed in accordance with modern regulations are required to account for fatigue loads. To assess the implications of fatigue loads on the subject rudder posts, 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 rudder 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 lowalloy steel is more widely available and generally has more desirable material properties, such as higher tensile ultimate strength, yield strength, and fatigue strength.