Preliminary results of the ERAU PA-28 accident

Discussion in 'Flight Following' started by jbrinker, Apr 17, 2018.

  1. hindsight2020

    hindsight2020 En-Route

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    Sure, but you misunderstand my motivation. IOW, I have zero quarrels with my wings or how they're attached, it's the FAA buffoonery and ensuing peanut gallery panic I'm trying to skirt here. I used to believe having a really proliferate airplane was a good thing for my ownership economy. Now I'm potentially finding out this thinking was flawed. I realize that thanks to the monkeys around me, we can never have nice things.

    To the point, Lances are virtually absent from the training environment, and PA-23s are old enough and low enough in training fleet volume these days they aren't likely to spring up a wing spar AD at this point on account of a derelict puppy mill dream-seller. I suppose the Seneca I is the only one I'd be walking into a trap on that list, since it's fairly proliferate in the training environment. so I'll concede you that one and scratch it off the list.
     
    Last edited: Apr 19, 2018
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  2. RudyP

    RudyP Line Up and Wait

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    Exactly. If you look at the spar on these planes, you can picture a discussion going sort of like this: “ok so we built a sizable spar out of carbon fiber, what do you think?” “You know, composites fail in weird ways and are hard to really inspect...” “alright, what about if we made it 4x beefier?” “Seriously? Ok yeah, that should do it!”
     
  3. 3393RP

    3393RP Pattern Altitude

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    It appears several posters have assumed that systemic and ongoing mishandling during training and/or maintenance oversight must have been causal factors in this accident. I think it more likely the accident airplane experienced a single event that initiated the fatigue damage, and the overstress failure occurred before a maintenance check could discover it.

    One would expect certain things to have already occurred if the ERAU Arrow fleet was abused with "impunity" (from another post) and it was "common".

    ERAU airplanes would have had to remain in service without any specific spar connection inspections after repeated significantly abnormal landings or turbulence encounters for a spar failure to occur.

    It is almost certain ERAU instructors are required to report flight encounters or landings which could have damaged the airplane. The CFI should be aware enough to discern the difference between a hard landing and one which buckles the firewall or damages the MLG. Flight in moderate to severe turbulence would not go unreported.

    This also would not have been the first ERAU Piper Arrow to shed a wing in flight. Similarly, post sale inspections by subsequent owners would have disclosed some instances of similar spar damage (which would probably have resulted in FAA involvement) or some number of these former ERAU Arrows would have had a spar failure.

    However, Piper didn't build them with the caveat they had to be treated gently. They had to assume the Arrow would become a staple of the trainer fleet, and that the average aircraft would experience a certain percentage of their lives operated by students in a wide variety of weather conditions (like summertime Florida convective turbulence) and varying degrees of landing skill.

    I think it unlikely that the incident is indicative of willful indifference by ERAU.

    What a sad incident. There was nothing the student and examiner could have done to save themselves. May they rest peacefully.
     
    Last edited: Apr 19, 2018
  4. 3393RP

    3393RP Pattern Altitude

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    The report didn't say that at all. It said the right wing center section lower spar cap had cracks at the same location (the two bolt holes on the end of the center section lower spar cap stub), but they were just 0.047 in depth.

    The left wing lower center section spar cap had cracked through 80% of the material.

    Note that only the left center section lower spar cap had fatigue damage. When it failed, the remaining components of the spar connection joint (upper cap and doublers) failed from overstress.
     
    Last edited: Apr 19, 2018
  5. 3393RP

    3393RP Pattern Altitude

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    From the NTSB preliminary report:

     
  6. murphey

    murphey Final Approach PoA Supporter

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    Not long after I bought the cherokee, I had the infamous SB 1006 done, which involved removing the fuel tanks. Made it very easy to check the bolts and such. About 10 yrs later I had a fire on the ground (thanks to the shop that screwed up the brake system and no longer exists). This time the wings were removed, a few skins replaced on the wings, and I assume/hope the shop (Beegles) examined the spar, bolts and everything around that area.

    Can a dye penetrant test be done without taking the wings off?
     
  7. 3393RP

    3393RP Pattern Altitude

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    I'm going to say no. The upper and lower caps of the wing main spar have doubler plates at the joint with the center section, and several rows of bolts connecting the doubler to the wing and center section spar caps.

    The bolt head, nuts, and washers conceal the perimeter area of the bolt holes, precluding their inspection.
     
  8. murphey

    murphey Final Approach PoA Supporter

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    oh krap, removing the back seat without damaging the entire side panels....there goes the new interior....
     
    Last edited: Apr 19, 2018
  9. murphey

    murphey Final Approach PoA Supporter

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    I always wiggle the wings on the cherokee. Even more fun when it's Young Eagle day, and I explain I want to make sure the wings are still glued on.
     
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  10. Clark1961

    Clark1961 Touchdown! Greaser!

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    Well somebody is wrong...
     
  11. Pilawt

    Pilawt Final Approach

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    My point was that there's nothing to stop the buffoonery and panic from extending from the PA-28R to its progeny, including PA-32R and PA-34.
     
  12. Tantalum

    Tantalum En-Route

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    I always thought I was the only one who did this
     
  13. GRG55

    GRG55 Final Approach

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    +1

    But in my case I am also worried an engine may fall off with the wing...so double jeopardy. :p
     
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  14. Bill Jennings

    Bill Jennings Touchdown! Greaser! PoA Supporter

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    I wiggle the wings on the Mooney during preflight, but I doubt it would have helped those guys.
     
  15. Bell206

    Bell206 Line Up and Wait

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    I believe the NTSB report stated visible corrosion. I think your point on stress-corrosion may be closer. Since there is no fire damage to the spar the lab guys will be able count and follow the failure marks right to the crack origin. And determine the number of cycles it took to failure. You can see those faint "beach" marks on the spar and they tend to move away from the bolt holes. It can be a pit .001" deep or a minute stress riser from installing those bolts that could start the failure process. Not to mention the failure is through both bolt holes (steel bolt/aluminum spar). Will be interesting how the spar load cycles vs flight hours will work out.
     
  16. Tantalum

    Tantalum En-Route

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    I give it a firm wiggle. Actually learned that from my brother in law who flew sailplanes.. since they routinely took the wings off it was a good way for them to check if anything rattled or seemed loose, etc. I imagine though that on the accident plane it probably won't have done much. NOW, had they peeked under the plane, I wonder if maybe there would have been some odd or concerning looking evidence under the plane. I can also imagine the look of otter shock on the pilot if you wiggled a wing and it buckled and came off. Doesn't the Mooney have a solid one piece spar? I wish all planes did

    Didn't @denverpilot say a buddy of his scrapped a flight once because the wings weren't even when they did the preflight? I think he said it turned out the thing had a hard landing and the plane never flew again

    I remember the first time I flew in a 402 and we hit some moderate bumps, it was very odd to see the propeller and spinner moving independently of the wing and nacelle (cowl?) in the bumps. Also weird to see the same thing on a 757, you have this (comparatively) massive engine out there just dancing away on the wings. Metal (when not fatigued!) is incredible.


    *I must say though.. in recent years as I spend more and more time flying Cirrus I am appreciating more the strength of composites. I might even dare say I feel safer on a composite plane now!:stirpot:
     
  17. Velocity173

    Velocity173 Final Approach PoA Supporter

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    Yeah but there’s more to spar strength than just geometry. What material / quality is the spar made of? How thick is the material? How large is the spar itself?

    I know you’re a PA28 fan because you own one but the fact is, there are 22 in-flight breakups listed on NTSB JUST for the PA28 alone. Not a single in-flight breakup is listed for not only the AA5 but the AA1 series as well. Even the ones that crashed from over G showed signs of wrinkled wing skin but no spar failure. Sorry, but I’ll take the strength of Grumman over a Piper any day of the week.
     
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  18. KSCessnaDriver

    KSCessnaDriver Pattern Altitude

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    Late to the party, but it doesn't surprise me too much. Those PA28Rs were used for power off 180's a ton, and probably got dropped in a ton as well. The commercial course only had maybe 10 hours in the Arrow, but it was all commercial maneuvers and whatnot. I never flew them (as while I went to ERAU, I saw the value in 61 training). Wouldn't surprise me to see this be ERAU trying to be cheap, as it seems like everything they do is about lining their pockets with cash.
     
  19. Tantalum

    Tantalum En-Route

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    I have a soft spot for Pipers but I also prefer composites (like the Cirrus)... so I won't take sides. But, I've never had a composite component let me down in life.. (aviation or otherwise) but I've had many steel or metal parts fail or disappoint me. For anyone doubting the strength of composites check this out. That small little foil, which already has a sizable bend built into it, withstood 4,600 kg of force.. or a little over 10,000 lbs!! I'll happily fly on a Grumman wing any day.. same with any composite plane

    *note: those guys walking around so close during that test are buffoons.. if that piece had let go and released 10,000 lbs of force in an instant there would have been shrapnel flying every which way
     
  20. GRG55

    GRG55 Final Approach

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    That would be irrational.

    Composites and metal behave differently, including how they fail. One isn't necessarily better or superior to the other.

    All airplane design is a compromise between competing objectives. Strength, weight, corrosion resistance, fatigue life, ability to inspect/repair and so forth. There's lots of glassed in metal fittings in a composite airframe. ;)
     
  21. GRG55

    GRG55 Final Approach

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    My read of the NTSB preliminary report suggests it has ruled out stress corrosion cracking, and put it down to fatigue. On closer inspection as the investigation progresses they may change that view, but what has been published so far seems pretty clear there's no evidence of a chemical environment (such as chlorides) to promote SCC.

    That there's a stress concentration at the bolt holes should not surprise. The question is the selection of materials and configuration of the attachment fundamentally inadequate, did this airplane exceed the airframe cycle limits, or is there something unique about the way this wing/spar was fabricated and bolted together that is different from all the others (e.g. an important step in the manufacturing that was overlooked on this one assembly).
     
  22. Checkout_my_Six

    Checkout_my_Six Final Approach

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    I'm sure they didn't recognize fretting as corrosion....but, it is.
     
  23. SoCal RV Flyer

    SoCal RV Flyer Pattern Altitude

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    As long as you don't have a soft spot in your composite, you're good!
     
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  24. Clark1961

    Clark1961 Touchdown! Greaser!

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    Agree that many folks haven’t been exposed to some of the more technical concepts that are part of metallurgy. In particular some of the failures accelerated by a corrosive environment appear to be normal fatigue failure until they get they fancy cameras out. Cracks early in life and without deformation are clues. Not conclusive at all but they are clues.
     
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  25. Mtns2Skies

    Mtns2Skies Super Moderator Management Council Member PoA Supporter

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    While it's certainly true composites can be very strong, that test doesn't take into account the strain rate at which force is applied. Most materials can withstand a truly massive load when it's applied slowly, but when all of that force comes in a sharp whack, they're much more brittle and prone to yielding.

    Composites are not great at absorbing shock loads, vs metals which generally are.
     
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  26. Bell206

    Bell206 Line Up and Wait

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    I think we're getting a few terms crossed. Corrosion fatigue and stress corrosion are not the same thing. Corr fatigue requires the wet environment or as you suggested with interaction with chlorides. Stress corr is more similar to cyclic fatigue and usually can only be seen at the molecular level. The NTSB report said they sent those items out for further study. I read it as the spar did not corrode apart. The difference between stress corr crack and cyclic fatigue crack is stress crack starts at a micro-pit which usually starts from dissimilar metals in bore holes.
     
    Last edited: Apr 19, 2018
  27. A1Topgun

    A1Topgun Pre-takeoff checklist

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    2007 Arrow "cross country training" accident in Texas where the wing came off. I would never buy an aircraft which was a "trainer". This is not normal usage when there is a hot shot instructor in the mix.

    Also, the graphics depict a "T" Tail, it was a straight tail.
     
  28. X3 Skier

    X3 Skier En-Route PoA Supporter

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    Believe it or not, in the early days of composite parts in the USAF, the standard field NDI to check for delaminations was tapping with a nickel and listening for funky sounds.

    Cheers
     
  29. Bell206

    Bell206 Line Up and Wait

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    And still practiced today on helicopter honeycomb structures down to using a coin if you can find the correct tapping hammer.
     
  30. GRG55

    GRG55 Final Approach

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    I know they are not same thing. My point was there is, at least at this point in the investigation, no evidence of stress corrosion cracking, or that the environment needed for that ever existed in this instance.

    However, the text from the preliminary NTSB report is specific to visual evidence of corrosion, so I'll accept SCC as a contributing factor can't yet be ruled out either.

    "...Preliminary examination of the left wing main spar revealed that more than 80% of the lower spar cap and portions of the forward and aft spar web doublers exhibited fracture features consistent with metal fatigue...The fatigue features originated at or near the outboard forward wing spar attachment bolt hole..."

    "...None of the surfaces exhibited visible evidence of corrosion or other preexisting damage..."


     
  31. GRG55

    GRG55 Final Approach

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    I don't think that method as a test has changed much.
     
  32. PaulS

    PaulS Final Approach

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  33. Omalley1537

    Omalley1537 Line Up and Wait

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    To add some levity to an otherwise horrible incident...
    In your description i’m picturing the first pilot wiggling the wingtip, only to have the plane go “ker-thump” onto the ramp, and the other pilot looking at the first and saying “what’d you do?” like Chris Farley at the gas station in Tommy Boy...
     
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  34. paflyer

    paflyer Final Approach

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  35. Stan Cooper

    Stan Cooper Line Up and Wait

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    This.

    While it's true that the strength-to-weight ratio of aerospace composites is superior to aluminum and most steel alloys, the Achilles Heel of composites is that they have low (really low) elongation, generally around 1 or 2 percent. Metals typically have elongation in the 10 to 20 percent range. Elongation is a measure of brittleness; low elongation is brittle, high elongation is ductile. Materials with low elongation when stressed beyond their yield strength, particularly when the stress is very high and locally concentrated like an impact, fail spectacularly (shards and splinters everywhere), while materials with high elongation will typically deform by bending and/or denting rather than breaking.

    The debate over materials choice for different applications isn't simple.
     
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  36. SoCal RV Flyer

    SoCal RV Flyer Pattern Altitude

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    Yes.

    Exhibit A, Robert Kubica at the 2007 Canadian GP. Data show a peak momentary impact of 75g, but he lived to tell the tale.

    accident-kubica-2007-canada.jpg
     
  37. paflyer

    paflyer Final Approach

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    Um, not my experience w/HB wings.
     
  38. denverpilot

    denverpilot Taxi to Parking

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    That’s an amazing photo, could have easily been a photo of his death.

    Energy dissipation is saving his life there. All the parts getting launched is keeping his body from being destroyed and the coroner writing “blunt force trauma” as his cause of death.
     
  39. KSCessnaDriver

    KSCessnaDriver Pattern Altitude

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    Exactly, the straight wing PA-28's come out of the sky like the space shuttle.
     
  40. SoCal RV Flyer

    SoCal RV Flyer Pattern Altitude

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    Indeed...he was a lucky man to survive that.