Piper PA-14 N4206H 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 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 “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.

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. Further examination of the accident airplane’s rudder post found radial features with crack arrest lines, 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. 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 1/4 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.

Analysis of the rudder post’s material composition found it to be consistent with Piper part number 40622. According to engineering drawings, part number 40622 rudder posts were originally manufactured from American Iron and Steel Institute (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.

As part of the NTSB’s investigation of a similar rudder failure on a Piper PA-12, the NTSB performed a rudder loads study that analyzed various structural loads and associated stresses on three other similarly fractured rudders; the posts from these rudders were also found to be consistent with part number 40622 and composed of AISI 1025 carbon steel. The structural load analysis 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.

In addition, corrosion, scratches, or surface roughness features were observed on each of the rudder posts examined as part of the NTSB’s study, and, as previously stated, examination of the post from this accident identified corrosion pits on the exterior surface. 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 ANC21LA064 for the full record.