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PostPosted: Tue Apr 17, 2018 8:46 pm 
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The following is an NTSB preliminary report regarding an in flight break up of a PA-28R-201. As an A&P I find these kind of findings both interesting and disturbing. I look forward to any thoughts that any of you may have in regards to this.

https://app.ntsb.gov/pdfgenerator/Repor ... m&IType=FA

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PostPosted: Wed Apr 18, 2018 1:29 am 
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Wow that's rather shocking.
I'm very surprised at where the fatigue cracks occurred considering the design has been around for ages. It is only 10 years old, considerably younger than the majority of pa28s.
It is surprising because normally if a design issue causes stress fractures it would be discovered long before this point due to similar incidents occurring.
I wonder if there was a recent minor design change around the wing root and spar attachment in that version compared to older pa28s?
If that's not the reason then maybe a manufacturing defect in the quality of aluminium used or a defect in the bracket and bolt hole leading to increased flexing and stress?
If it is neither of those then I'm stumped as in general this is not the sort of failure you would expect in such an established and proven design.
One thing of note is the hours are high for a young airframe, it is averaging a couple of hours every day for its whole life. Unless my maths is out. Probably not unlike other flying schools though.

I wonder if there will be a directive to check all pa28s for fatigue? The ones I fly are significantly older than that, worried now....

Chris


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PostPosted: Wed Apr 18, 2018 5:35 am 
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There has been Piper's Service Bulletin No. 886 out for years, dating back June 8th, 1988. It orders inspections for fatigue cracks precisely in this area. It determines compliance requirements in rather complex way, depending on the kind of usage the aircraft has been subjected to. It is, somewhat interestingly, limited in applicability to given serial number ranges.

Curiously, it is not listed in Piper's website for technical publications...

Edit: It appears that the inspections detailed in SB No. 886 are incorporated to the maintenance manual, and therefore applicable to the accident aircraft as well. However, airplane's usage history is usually assumed to be "normal", so that initial inspection is only required at 30600 hours time-in-service.

-Esa


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PostPosted: Fri Apr 20, 2018 10:03 am 
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I thought those pipers were built like tanks, that nothing short of a collision would ground them and then something like this happens...CAPS for me please and RIP to the pilots.

Edit. Do you guys have any ideas about how to control this thing without a wing to at least a survivable crash landing ? Riding the rudder, unloading the remaining one with the elevator, any aileron, opening a door if available ?


Last edited by Caldemeyn on Fri Apr 20, 2018 10:20 am, edited 1 time in total.

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PostPosted: Fri Apr 20, 2018 10:18 am 
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I doubt that CAPS would have saved them at that low altitude. If there is an AD to inspect for these fatigue cracks, then that means that they are at risk by design.

In a training environment, these aircraft are subject to an extreme number of takeoff and landing cycles with respect to their airframe total time. On top of that it has probably seen a high share of hard landings and maneuvers including commercial steep turns and stalls. Lots of of non steady unaccelerated flight.

The is also located in a region known to be one of the worst in the US for airframe corrosion.

combine ask these factors and you have an elevated chance of a defect becoming a problem. There is also the possibility that this particular unit had an undetected flaw.

What is strange to me is the phase of flight when this happened. They were at a low airspeed in a climb shortly after takeoff. They couldn't have done much to load up the wing without stalling. It must have been barely holding on at that point in time.

Very sad for the families and terrible to contemplate.

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PostPosted: Fri Apr 20, 2018 10:35 am 
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This topic made me to look up and recall some documents from the days I was involved with airworthiness management of some PA-28 series airplanes. I also looked up a bit beyond of I had to back then. This unfortunate case is indeed rather shocking, as Chris stated, but not entirely out-of-the-blue, given this specific failure mode has been identified in the type in question.

I intended not to write a post of it, but then I thought that why not - it is a piece of history of this classic aircraft anyways! :| (And of course, the NTSB findings in the opening post are preliminary, and this brief history write is not intended to speculate the final probable causes of that crash beyond of what has been officially reported, as required by forum guidelines as well.)

On March 30, 1987, a PA-28-181 registered N8191V, crashed after its left wing separated in cruise flight at low altitude near Marlin, Texas. The aircraft was owned and operated by a pipeline patrol company, and performing that very job during the accident flight. Aircraft had approximately 7488 hours time-in-service.

The evidence soon pointed out that fatigue cracking of the wing spar cap originating from the wing attachment bolt holes was the cause of the wing separation. Some other areas of cracks were noticed as well when examining the wreckage, most shockingly to me, including a 10-inch-long chordwise crack that had been stop-drilled (and thereby detected) in the upper wing skin. Another similar aircraft flown by the same operator with closely similar TIS was examined, showing similar damage.

NTSB issued a safety recommendation to "Issue an airworthiness directive to require an immediate inspection of the main wing spars and upper wing skin at the wing root of Piper PA-28 airplane with over a specified number of service hours for evidence of cracking. Particular attention should be placed on inspecting the bottom surface of the lower spar cap adjacent to the outboard forward attachment bolt hole at the wing root attachment [...] (Class I, Urgent Action)"

This resulted in an Airworthiness Directive 87-08-08, which required "wing removal and inspection on many PA-28 and PA-32 series airplanes with more than 5000 hours total time in service". By what has been written by Piper, this AD resulted in "over five-hundred" inspections with "only two negative findings" reported. Piper also conducted their own wing fatigue and fracture "analyses" as they call them. The AD was suspended on September 28, 1987.

I don't know what was discussed in the cabinets, but it appears that at least the Piper was somewhat uncomfortable with the situation at hand. They published a couple of Service Bulletins over the years, namely No. 886 and No. 978A. Similar instructions were incorporated into the maintenance manuals of many, if not all, subsequent aircraft models affected by the issue (AMM Chapter 57, and for some models Chapter 5, Time Limits / Maintenance Checks). In no cases, to my knowledge, these have been incorporated into the Chapter 4, or Airworthiness Limitations.

These instructions provide for removal of the wings for spar cap inspections and some other stuff. Importantly, they divide the flying fleet into "usage classes" by their history. I don't want to reproduce details of maintenance instructions for obvious reasons (Google 'em up if interested), but basically for "normal" usage, the initial inspection requirement could be as long as 30600 hours TIS, and then repetitively each 3000 hours. For airplanes in "severe" usage, the initial inspection is extremely low in comparison, at 1800 hours, and each 800 hours thereafter. The definition for "severe" usage included the following.

EXCERPT FROM OUTDATED MANUAL - DO NOT USE AS REFERENCE FOR ANYTHING! ..defining "severe usage" wrote:
Aircraft which have engaged in severe usage, involving contour or terrain following operations, (such as power/pipeline patrol, fish/game spotting, aerial application, aerial-advertising, police patrol, livestock management or other activities) where a significant part of the total flight time has been spent below one-thousand (1000) feet AGL altitude.
The fatigue was clearly noted as the key issue in the design. However, the determination and compliance with the inspection requirements have been left to the operators, and in my experience, one seldom identifies reasons to make the "usage class" worse than "normal", nor is one often compelled enough to force it "unknown". It may be worth noting that a gross 'g-overload' in Piper Cherokee series breaks the wing around mid-span, not from the area where it has been separated in the fatigue-related cases, the failure modes of structural overload and fatigue are different.

I certainly follow this investigation, and wish they make a proper report of it. Any in-flight failure by fatigue, IMO, is a failure of our maintenance system thoroughly, and I always feel bad of them because they should be caught in time.



The sources include Piper maintenance documentation as quoted and NTSB safety recommendation A-87-42 among the others.

-Esa


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PostPosted: Fri Apr 20, 2018 10:44 am 
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Well, from what i know, brs is rated to about minimum 500 feet safe pull, they were about 900 msl with DAB elevation of 33 feet, of course one has to take into account possible initial shock that would lead to wasting altitude...maybe they would have made it.

If this wing may have hung by a thread, i wonder if this could have been catched during a walkaround ?

Who maintained this plane i wonder ? It was a property of embry-riddle aeronautical university


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PostPosted: Fri Apr 20, 2018 10:44 am 
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Oracle427 wrote:
I doubt that CAPS would have saved them at that low altitude. If there is an AD to inspect for these fatigue cracks, then that means that they are at risk by design.

In a training environment, these aircraft are subject to an extreme number of takeoff and landing cycles with respect to their airframe total time. On top of that it has probably seen a high share of hard landings and maneuvers including commercial steep turns and stalls. Lots of of non steady un-accelerated flight.

The is also located in a region known to be one of the worst in the US for air frame corrosion.

combine ask these factors and you have an elevated chance of a defect becoming a problem. There is also the possibility that this particular unit had an undetected flaw.

What is strange to me is the phase of flight when this happened. They were at a low airspeed in a climb shortly after takeoff. They couldn't have done much to load up the wing without stalling. It must have been barely holding on at that point in time.

Very sad for the families and terrible to contemplate.



The issue of accumulated stress has always been a prime issue for aircraft used in the training environment. Botched maneuvers, even simple basic maneuvering flight coupled with hard landings all take a toll on aircraft.
The problem is that a 100 hour inspection done to FAA requirement does little to deal with accumulated stress. Many AD's as we used to say, are written in blood, reflecting post accident "fixes" when discovered.
The truth is hard to face when one starts delving into this issue. Unless one is willing to start magnifluxing every joint on an aircraft, latent stress can remain basically unnoticed until it actually fails. When one adds to this the factor that this type of stress is individually suffered by specific airplanes and not generally through an entire type, the true scope of the problem begins to rear it's ugly head.
You can literally have two exact planes of the same type where one has been flown correctly and never over g'd or stressed on landing while the same type next to it on the ramp due to hard operation contains a latent failure just waiting to occur during the next gusty flight.
It has been my experience over time that the ONLY security one has when picking a plane to buy or rent is to obtain a complete history on that specific airplane before flying it. (Read the logs and research where and how operated and by whom).
I hate to say it, but unless all this has been done, although the odds are well in your favor when flying any aircraft, there is always the chance that something can go wrong.
There is no 100% security involved with flying an airplane. The absolute best one can hope for is that YOU have been the sole operator of a specific plane since new.
Dudley Henriques


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PostPosted: Fri Apr 20, 2018 11:09 am 
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Indeed, by what Dudley wrote, and I did a few minutes earlier, if I summarize a little what I find uncomfortable is this:

A given PA-28 is considered, by its manufacturer, worth of 30600 total time-in-service worth of fatigue life if flown "normally" before one even bothers to inspect it in a laborious way. If flown in a "severe" way, it is worth of only 1800 hours, or about 6 % of its "normal" fatigue confidence. If I combine that with the final definition of "severe usage", by the manufacturer, being flying operations "where a significant part of the total flight time has been spent below one-thousand (1000) feet AGL altitude", I am not entirely positive of the way this type was engineered. Put bluntly.

Fatigue is a scary enemy of any airplane made of aluminum structure. It must be respected, and appropriate maintenance/inspection requirements must be introduced by the manufacturer, and asked to be enforced by the competent authority.

Working bottom-up, if I had to rationalize an increase of fatigue life worth of 1600 % if simply flying primarily above 1000 ft AGL, I'd find the engineering question overwhelming. But then, I never was into metallurgy. Being somewhat into maintenance systems...well, not comfortable at all.

-Esa


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PostPosted: Fri Apr 20, 2018 2:22 pm 
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Condolences to the families involved. Metal fatigue truly is one of those things that you can try like hell to minimize, but there is always that chance, as with anything in life. I hope the lessons from this tragedy will help others, and reinforce the importance of strictly sticking to inspection schedules.

I agree that perhaps in this instance a CAPS system might have increased the chances of survival, but overall I've never been quite sold on the whole CAPS system. I think it is a mixed bag. If I have a CAPS system, I would also want self sealing fuel tanks, because a CAPS system with wet wings and brittle composite just tells me that the chute is going to drop you into a post impact fire. This might be fine on a clear VFR day where you can get an idea of what you are going to land on...and thus plan your escape, but at night or IFR, it might as well be Russian Roulette given the chance of landing on things that break airplanes ie trees, rocks, buildings...etc.

TJ

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PostPosted: Fri Apr 20, 2018 3:56 pm 
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I do doubt that CAPS would have helped here. Even at the slow climb out speed and 900msl, as soon as the wing came off it was a rock, and I doubt that pilot reaction time would have saved it. That being said, I have never heard of an aircraft that has descended to the ground ever being lowered into a post crash fire, and just about all GA aircraft do not have self sealing tanks, I most because I am sure there are some STCs out there that add them. The metal fatigue is the thing that really got me, and I suspect that E-R will probably have to increase the frequency and scope of their inspections.
As Dudley said, ADs and safety features are paid with blood and lives, and are often referred to as "tombstone technology". Ours is a hazardous field of work and while mitigating aspects can be worked in, there is no way to make flying 100% safe, all we can do is learn and apply and advance. I used to tell my ground service team when I was a ramp lead and trainer, "Flying begins and ends being dangerous the moment we hook that tow bar to the tug in the hangar till we chock and detach it in the hangar after it comes back."

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PostPosted: Fri Apr 20, 2018 4:33 pm 
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I meant with 'self sealing tanks'...might be the wrong term, I mean tanks that are more than just a wet wing. For instance Diamond Aircraft uses separate aluminum fuel tanks inside the wing structure. This comes with weight penalty, however I believe it is worth it's weight in gold considering there have been very very few post crash fires involving Diamond Aircraft. Having plain wet wings made out of composites with a chute that drops 1700fpm (~20mph) in to trees, rocks, structures is a recipe for a post crash fire. I thought I heard somewhere that new Cirrus aircraft are equipped with Aluminum tanks, tho this might have been a rumor....if so, that would be great news.

But like with all things in aviation, we pick our poison and take our risks.

TJ

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PostPosted: Fri Apr 20, 2018 5:29 pm 
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Here i have a short vid where an aerobatic loses a wing quite close to the ground and brs saves the day :) https://m.youtube.com/watch?v=4a8cntPdRtk


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