Piper PA 12 N3188M accident near Anchorage, Alaska

Probable cause

The fatigue fracture of the American Iron and Steel Institute (AISI) 1025 carbon steel rudder post, which resulted in structural failure of the rudder. Contributing to the fatigue failure was the post’s susceptibility to fatigue cracking under normal service conditions.

NTSB analysis

The flight instructor reported that after conducting a landing and departure at a remote lake in the float-equipped airplane, 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 receiving instruction 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, significant downward elevator pressure (forward control yoke) was required. The flight instructor decided to return to their destination airport, 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. As part of the NTSB’s investigation, examination of four other similarly fractured rudder posts revealed that they had all fractured above the upper hinge. Each of the rudder posts exhibited evidence of a progressive fracture mechanism. Although the fracture surfaces of the accident rudder post were obliterated by post-fracture contact, fatigue fracture features were observed on three of the other rudder posts.

Analysis of the material composition of all of the posts found it to be consistent with Piper part number 40622 posts made of American Iron and Steel Institute (AISI) 1025 carbon steel. According to engineering drawings, rudder posts were originally manufactured from AISI 1025 carbon steel. A Piper engineering change order specified a change to AISI 4130 low-alloy steel that was incorporated into the part number 40622 engineering drawing in June 1974. According to Advisory Circular 23-27, Parts and Materials Substitution for Vintage Aircraft, AISI 4130 lowalloy steel generally has more desirable material properties, such as higher tensile ultimate strength, yield strength, and fatigue strength.

A structural load analysis of three of the rudders found that, with no scatter factor applied, the bending stress from certification gust and maneuver loads is significantly closer to the endurance limit of AISI 1025 carbon steel than to the endurance limit of AISI 4130 low-alloy steel. These results indicate rudder posts constructed of AISI 1025 steel are more susceptible to fatigue failure during normal service than those constructed of AISI 4130 steel.

In addition, corrosion, scratches, or surface roughness features were observed on each of the rudder posts examined. These types of surface anomalies can occur over time during normal operations and can weaken metal material where they occur. Because AISI 1025 carbon steel has lower overall and fatigue strength than AISI 4130 low-alloy steel, the surface anomalies likely made the affected rudder posts more susceptible to fatigue failure during normal service conditions than they would have been if manufactured with 4130 low-alloy steel.

Thus, the rudder failure on the accident airplane was due to the fatigue fracture of the AISI 1025 carbon steel rudder post. Contributing to the fatigue failure was the post’s susceptibility to fatigue cracking under normal service conditions.

Text from the NTSB. Open the NTSB docket for ANC20LA059 for the full record.