NTSB Wants Cub Rudder Post Replacement AD


The NTSB wants the FAA to issue an AD for old Pipers after two incidents in which the rudder posts snapped and the top section of the rudder folded into a roughly horizontal position. The failure obviously had a big effect on the controllability of the aircraft involved, a PA-12 Super Cruiser and a PA-14 Family Cruiser. The NTSB has recommended the FAA require replacement of the original rudder posts with a stronger and more resilient piece of pipe. Most Pipers of that era have been extensively repaired, rebuilt and modified over the last 70 to 80 years but many still have the original rudder post, which is part number 40622. The same part was used in most of the early model Pipers so the AD could include thousands of airplanes.

In the first Alaska incident on June 8 of 2020, an instructor and the owner of the PA-12 on floats had just taken off when the pilot told the instructor the controls “felt strange.” The instructor took over and didn’t feel safe trying to turn around so wrestled the balky airplane to a safe landing at the airport from which they’d taken off earlier. A bit more than a year later the pilot of a PA-14 on floats reported he needed “a very large right rudder input” to keep the plane straight and level and he, too, was able to land safely.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.

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  1. Lemme see now … two Alaskan floatplanes (probably high time?) have a problem so thousands of other airplanes kept on land and — many — in hangars have to rebuild their rudders. Give us all a break, NTSB. This is an SAIB, at best !!

    • Maybe they should just ban all floats. No more planes on the water. Seems like floats are the common denominator.

    • If you have a plane that requires that AD, and you don’t do it, I suggest reading your insurance policy very, very carefully.
      And as others will mention, this isn’t anything near like a wing spar attachment AD.

  2. Is it that the wheel is attached to this post that causes the fatigue? Is there similar fatigue on the same post on the vertical fin? Do those typically get replaced? Wish an A&P familiar with this would chime in.

  3. Fortunately, making assumptions, this should be an easy AD issue to remove, inspect, and repair if the AD requires. A Cub rudder can be off by the time you finish your cup of coffee. And the inspection of that right in front of you. Not encouraging this, however, I can still picture my Dad with an Aeronca rudder on my Mom’s kitchen table as he striped and recovered. He was the worlds’ finest repairman, handcrafted space components for Hughes during the ’50s and ’60s. Just didn’t care to deal with feds for an A&P piece of paper (license). Not encouraging though.

  4. Former aircraft structural integrity engineer here.

    The tailwheel attaching to this part (if it even does) would not cause this particular problem. This failure appears to be at or just above the upper hinge attachment. Almost the only load seen by this component would be due to use of the control surface (i.e. using the rudder…as a rudder). The rudder post will be put in sideways bending any time the rudder is deflected with significant airflow over it, with the highest loads being in large rudder deflections at relatively high speeds or in gust load conditions. It’s unlikely the aircraft’s service as a floatplane had any significance to the failure, unless extra exposure to water made corrosion pitting more prevalent.

    If it’s a fatigue issue (which it appears to be), it won’t even necessarily be the highest loads that cause the failure over time, it could be just normal flying causing a crack to form from a material flaw, a scratch, or corrosion pit (a 70-80 year old carbon steel part? No chance of that having corrosion, is there?).

    These aircraft were never built assuming 70-80 years of continuous service. Fatigue was not a real design consideration at the time. If the NTSB did some calculations that showed the in-flight stresses of a pristine part would be close to or above the endurance limit of the material, having the parts replaced would be a really good course of action. You may have a rudder post that could fail at any time with little warning.

  5. Perhaps that big beacon/strobe fixture is the problem. I’ve been around J-3s my entire life and restored a couple with my dad as well as owning one for over 20 years and worked on others along with Super Cubs. Never seen a problem like this, nor even corrosion there.

    • Yes, I also would believe that the beacon caused the problem (possible vibration in flight).
      Were beacons installed on the other failures??

    • NOT an opinion, but only an uneducated WAG (wild-ass guess): aside from the mass of the beacon, could a leaky seal at the lens enable moisture to pool in the beacon and leak down to cause corrosion at the point of failure? Especially if it’s salt water?

  6. Just one opinion but, I’m going with the 150 plus horsepower as the cause. The 1940’s engineers with their slide-rulers found the rudder strength was sufficient for 115 horsepower. Many of the engines overhauled in the past decade are balanced, ported, tuned carburetor, ignition and exhaust improvements. I’ve seen a couple dynamometer reports with over 165 HP on a basic O-320 and over 190 HP on O-360s. Combine that with more efficient propellers and just maybe the ol’ Piper Rag-Wings are hitting their 1940’s structural design limits?

  7. It appears that the failure is just above the top hinge. An improper weld can weaken the tubing in the weld area. The area also needs to be very well protected by zinc chromate and paint or epoxy primer and paint externally and should have had linseed oil introduced internally at time of manufacture. The effect of the beacon is a big unknown. Was the aircraft certificated with the beacon ? If it is a 337 or STA(C) was there a vibration analysis. Alex R and Black Bart are right on. When further incidents result in a crash, then FAA will act.
    From an A & P /Canadian AME

  8. February 9, 2022 – Our evaluation showed that the peak stresses on the nominal rudder post
    approach the endurance limit of AISI 1025 steel.7 All five of the rudders the NTSB
    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

    NTSB-Structural Failure of Piper Rudder Posts
    Notice Number: NOTC2252
    NTSB recently issued an Aviation Investigation Report AIR-22-02 highlighting an urgent safety issue involving Piper part number 40622 rudder posts made of American Iron and Steel Institute (AISI) 1025 carbon steel, which our investigations have found to fracture due to fatigue.

    Select this link to view Aviation Investigation Report AIR-22-02:
    In this AIR, we called on the Federal Aviation Administration to issue an airworthiness directive that describes the safety risk associated with the continued use of this part and require 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.
    What You Should Know
    We developed this report and recommendation as a result of investigations into two accidents both occurring in Anchorage, Alaska–ANC20LA059 on June 8, 2020, and ANC21LA064, on July 23, 2021–involving airplanes, designed and built by Piper Aircraft Inc., that sustained substantial damage when their rudders structurally failed in flight. In both cases, the airplanes were being operated as Title 14 Code of Federal Regulations Part 91 flights. The NTSB also examined three additional similarly fractured rudders.
    What You Can Do
    We encourage 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 Inc. 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.

    Recent publications are available in their entirety on the NTSB website. Other information about available publications also may be obtained from the website or by contacting—
    National Transportation Safety Board
    Records Management Division, CIO-40
    490 L’Enfant Plaza, SW
    Washington, DC 20594
    (800) 877-6799 or (202) 314-6551