Accident Details
On June 8, 2020, at approximately 0945 Alaska daylight time, a Piper PA-12, registration N3188M, sustained substantial damage during an instructional flight near Anchorage, Alaska. The airplane, operated under Title 14 Code of Federal Regulations Part 91, was conducting a training flight with a flight instructor and a pilot receiving instruction. Neither occupant was injured.
The flight departed from Lake Hood Airport (PALH) and proceeded to Twin Island Lake, about 8 nautical miles northwest of PALH, where a normal landing was performed. After departure, the airplane climbed to about 500 feet above ground level before turning onto a left crosswind traffic pattern leg. While on the crosswind leg, the airplane yawed abruptly to the right, and the pilot reported that the controls felt strange. The flight instructor took control and noted drastically diminished control about the vertical axis, along with a need for significant forward elevator pressure. The water rudders were deployed to aid directional control. Uncertain about the ability to return to Twin Island Lake due to poor directional control, the instructor elected to return to PALH, where emergency services were available, and conducted an uneventful landing.
Examination Findings
A post-accident examination of the rudder revealed that the vertical rudder post had fractured above the upper hinge point, causing the top portion of the rudder to fold over the horizontal stabilizer tail brace wires. The NTSB examined a total of five similarly fractured rudders, including the accident aircraft's rudder, one from a previous accident (ANC21LA064), two from a repair facility, and one received in response to an FAA Airworthiness Concern Sheet. All rudders were consistent with Piper part number 40622, featuring posts made of American Iron and Steel Institute (AISI) 1025 carbon steel, and had fractured above the upper hinge.
Further examination identified evidence of progressive fracture on each rudder post, with fatigue fracture features present on three of them. The fracture surfaces on the accident rudder post and one other were too damaged to conclusively determine the specific fracture mechanism. Corrosion, scratches, or surface roughness were found near the fractures on all rudder posts. Fracture origin areas were damaged, but corrosion pits or scratches were associated with fatigue origins on two of the posts examined.
Historical Context
A review of the NTSB accident database revealed two additional accidents involving in-flight structural failure of the rudder on single-engine Piper airplanes (SEA84LA220 and ANC21LA064). Piper part number 40622 rudder posts were originally manufactured from AISI 1025 carbon steel tube. In a Piper engineering change order dated June 3, 1974, the specified material was changed to normalized AISI 4130 low-alloy steel, with an allowance for using existing stock to depletion. The material change was incorporated into the engineering drawing in June 1974.
The vintage single-engine Piper airplane models from which these rudders were obtained were designed for static load conditions as per the certification regulations in effect at the time. However, in service, the loading conditions on many structural parts, including rudders, are dynamic and involve alternating (fatigue) loads.
A structural load analysis conducted by the NTSB on three of the rudders (without a scatter factor) indicated that the bending stresses from certification gust and maneuver loads are significantly closer to the endurance limit for AISI 1025 carbon steel posts than for AISI 4130 low-alloy steel posts. FAA Advisory Circular 23-27, published in 2009, advises that AISI 4130 low-alloy steel may be substituted for AISI 1020 or AISI 1025 carbon steel, including for structural posts, due to its more desirable material properties such as higher tensile strength, yield strength, and fatigue strength.