The NTSB has asked the FAA to Issue an Airworthiness Directive that describes the safety risk associated with the continued use of some rudder posts made of 1025 carbon steel in some vintage PA-12s, PA-14s, and PA-18s.
The recommendation from the NTSB relates to Piper part number 40622 rudders equipped with a post made of American Iron and Steel Institute (AISI) 1025 carbon steel. The safety agency wants the FAA to issue an AD that “requires owners and operators to address the unsafe condition, such as by replacing them with rudders equipped with a post made of AISI 4130 low-alloy steel or its equivalent.”
The recommendation stems from two accidents in Alaska.
In the first accident, on June 8, 2020, a float-equipped Piper PA-12 sustained substantial damage when its rudder structurally failed in flight about eight miles north of Anchorage. The flight instructor and private pilot receiving instruction were not injured.
While on a left crosswind leg after takeoff from a lake, the airplane yawed abruptly to the right and the pilot told investigators 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. In an effort to aid in directional control, the water rudders were deployed.
Uncertain that he could make a 180° turn due to the poor directional control, the flight instructor elected to return to the airport from which they had departed and landed without further incident.
The second accident happened a year later, on July 23, 2021, when a float-equipped Piper PA-14 sustained substantial damage when its rudder structurally failed in flight about 15 miles southeast of Anchorage.
The commercial pilot, who was not injured, 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. He was able to land the airplane without further incident.
Through examination of the rudders involved in these accidents, the NTSB determined the rudders were consistent with Piper part number 40622 and
the rudder posts were made of AISI 1025 carbon steel.
The NTSB has also determined that in both accidents, the airplane’s rudder post fractured above the upper hinge, and the top portion of the rudder folded over the upper tail brace wires (see figures 1 and 2).
Concerned that the structural failure of these rudder posts was indicative of a wider safety issue, the NTSB examined three additional similarly fractured rudders. The NTSB obtained two rudders from a repair facility, and the third was provided in response to an airworthiness concern sheet the FAA issued in September 2020 (the rudder had been found and reported during an inspection). [You can see the airworthiness concern sheet in the docket of the first accident.]
The NTSB found that the three additional rudders were also consistent with Piper part number 40622, had posts made of AISI 1025 steel, and had fractured above the upper hinge.
NTSB officials also noted that all five of the rudders it evaluated had an aftermarket beacon or strobe installed on the top of the rudder post.
“The additional surface area and mass of a beacon or strobe would likely increase the stresses even further,” officials said.
The NTSB concluded that structural failures of Piper part number 40622 rudders equipped with a post made of AISI 1025 carbon steel “indicate a serious hazard to flight safety that warrants action.”
That led to the recommendation to the FAA to issue an Airworthiness Directive that requires owners “to address the unsafe condition, such as by replacing them with rudders equipped with a post made of AISI 4130 low-alloy steel or its equivalent.”
The NTSB also encourages “all owners, mechanics, and operators to read our Aviation Investigation Report and be aware that posts made of AISI 1025 carbon steel in Piper Aircraft part number 40622 rudders are susceptible to fatigue cracking under normal service conditions. Recently documented structural failures of these rudders indicate a serious hazard to flight safety that warrants action.”