Preliminary results of the ERAU PA-28 accident

[Tantalum]

Do you have any evidence to support your claim they used junk aluminum?

Was my thought too.. but this should be a relatively easy thing to test for. IF.. and that's a big IF, it turns out that Piper is using substandard metals that's going to be a major problem for them. At this point, seems to me more likely that this plane was just abused too hard one too many times.. and somehow those cracks were missed. If ERAU pulls wings at 10K hours anyway, then they're aware that their planes get ridden hard and put away wet

Or maybe there will be blame on both parties

I have to think that with the thousands of PA28s out there flying around if there was an issue with the wing structure and materials we'd see more than one failure in 1987 during a pipeline inspection and one failure from a flight school

 

[Checkout_my_Six]

remove the covers on the belly.....and the nuts and lower spar is visible for inspection.

And that is ok ne of the big picture questions here. With a plane subject to 100 hrs inspections, should that have been seen. Here is a ok view of the underside of the Arrow wing. Between the access ports and the wheel well, I am not sure how visable that joint is. This shot shows several aces ports at the leading edge, and the spar lies just forward of the wheele well

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[Clark1961]

It's tougher to test than one might initially think. The material has already been altered by the failure. The material will be further altered by cutting a test coupon. About the only thing one can be certain of is the chemistry. Welcome to metallurgy where disputes are resolved in courtrooms, not in labs.

 

[drotto]

Was my thought too.. but this should be a relatively easy thing to test for. IF.. and that's a big IF, it turns out that Piper is using substandard metals that's going to be a major problem for them. At this point, seems to me more likely that this plane was just abused too hard one too many times.. and somehow those cracks were missed. If ERAU pulls wings at 10K hours anyway, then they're aware that their planes get ridden hard and put away wet

Or maybe there will be blame on both parties

I have to think that with the thousands of PA28s out there flying around if there was an issue with the wing structure and materials we'd see more than one failure in 1987 during a pipeline inspection and one failure from a flight school

I believe the wing spar is steel not aluminum.

I was not aware that this school was already pulling wings at 10,000 hrs. That is a significant argument that they are aware of the beating their planes take. But it also makes it worse if this plane was missed ( despite their self imposed wing pull at a higher TT).

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[Clark1961]

I believe the wing spar is steel not aluminum.

I was not aware that this school was already pulling wings at 10,000 hrs. That is a significant argument that they are aware of the beating their planes take. But it also makes it worse if this plane was missed ( despite their self imposed wing pull at a higher TT).

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PA-28 spars are aluminum.

 

[drotto]

PA-28 spars are aluminum.

Ok, thanks.


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[Clark1961]

Ok, thanks.


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No worries. There can be steel doublers at the fore and aft attachments. Dunno 'bout the main spar attachment. All I've seen is hefty chunks of aluminum.

 

[Velocity173]

Same here. I had the same thought the first time I saw all the cowlings off a plane, and a puny little engine sticking out there on some metal tubes

Yeah... I mean, there must be an incredible amount of stress on that joint and the associate bolts right there. My hats off to the engineers that design and build these things

Anyone ever looked up at that one tiny little bolt that chairlifts swivel on that connect it to the cable? 6 fat guys sitting in a chairlift with all their ski gear on held to the cable by one bolt that appears grossly undersized.. and chairlifts dangle out there in the snow and moisture for decades

Metal is an incredible thing.. until you push it past its limits and it begins to fatigue

I wonder how a Diamond 20 or other composite plane treated the same as the accident bird would have behaved. We've been told composites are unforgiving when they give up the ghost.. but I can't imagine too many things more unforgiving then your wing coming off

How many composites have come apart in flight? Not many. While delam in the spar is possible and inspecting for it next to impossible, the odds of it happening to the point of breaking is slim. They have low stress memory and better stiffness than aluminum.

Do a search on NTSB for Beech or Piper with “in flight breakup” in the remarks. You’ll see several that come back. While yes, most happened during IMC over loads, you won’t see many composites that come back with that search.

 

[mondtster]

remove the covers on the belly.....and the nuts and lower spar is visible for inspection.

The lower spar would not be visible with that cover removed, nor do I believe you'd see the defect that took this airplane out. All you're going to see removing those covers is the belly of the airplane and the threaded end of the close tolerance bolts and the nuts that attach the bottom side of the spar to the carrythrough.

Now, you could pull the rear seats, the wood floorboard, and the carpet covering the spar carrythrough and have a look at things from the inside of the plane. Using that method the cracks may have been detected. One problem with doing this is that Piper glues foam in some of the holes in the spar carrythrough in the newer airplanes (I don't remember ever seeing it on the older ones). If a mechanic was unwilling to remove the foam to do a proper inspection it is going to be hard (but not impossible) to see the area that broke.

 

[Checkout_my_Six]

ok...the nuts are visible from the outside....the spar is visible from the inside.

 

[drotto]

How many composites have come apart in flight? Not many. While delam in the spar is possible and inspecting for it next to impossible, the odds of it happening to the point of breaking is slim. They have low stress memory and better stiffness than aluminum.

Do a search on NTSB for Beech or Piper with “in flight breakup.” While yes, most happened during IMC over loads, you won’t see many composites that come back with that search.

This is also why the initial composit planes had an airframe lifespan. If I remember the Cirrus was 6000 hrs. As the planes proved themselves and their were no composit failures seen it was raised to 12,000 hours. They now say that number may be extended as the planes get older.

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[Clark1961]

The lower spar would not be visible with that cover removed, nor do I believe you'd see the defect that took this airplane out. All you're going to see removing those covers is the belly of the airplane and the threaded end of the close tolerance bolts and the nuts that attach the bottom side of the spar to the carrythrough.

Now, you could pull the rear seats, the wood floorboard, and the carpet covering the spar carrythrough and have a look at things from the inside of the plane. Using that method the cracks may have been detected. One problem with doing this is that Piper glues foam in some of the holes in the spar carrythrough in the newer airplanes (I don't remember ever seeing it on the older ones). If a mechanic was unwilling to remove the foam to do a proper inspection it is going to be hard (but not impossible) to see the area that broke.

I'd change that from 'could pull the rear seats, the plywood cover' to 'after the rear seat are pulled and the plywood cover removed' because that must be included in the annual inspection of a PA-28. The carpet is removed with the plywood cover so it's not a separate step.

 

[Tantalum]

I was not aware that this school was already pulling wings at 10,000 hrs

I can't take ownership of that comment, someone else noted it:

ERAU has an internal policy of pulling wings at 10K

^but I would imagine for ERAU heavy wear and tear on a plane wouldn't be their first time at that rodeo.. so I am include to believe the poster.. and, as you indicate, it would suggest (if true) that they're well aware of the abuse their planes take.. they know they have big visibility and in flight breakup would be very bad for business..

 

[midwestpa24]

I have talked to mechanics about metal vs. composit. Many of them like the metal planes because they say you can usually tell visually when metal is fatiguing. It warns you that is is going to have a problem. You can see wrinkles, cracks, changes in color, etc.

Composites on the other hand do not show issues to the naked eye till failure. Apparently, you can do things like ultrasound the composite to see hidden damage, but that is expensive and time consuming. So there is damage there, but you can not see it. So when it does fail there appears to be no warning.

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That has been one of the issues as airliners become more and more composite. Ramp rash on metal airframes was easy to see, diagnose and repair, but composites can hide impact damage well, and require specialized repairs.

 

[Tantalum]

While yes, most happened during IMC over loads, you won’t see many composites that come back with that search

I agree. I know metal is the classic "go to" for strength.. but having done a fair amount of competitive sailing on composite boats it blows my mind the types of insane loads composites can take.. some of these yachts have virtually paper thin hulls and heavy lead bulb keels held on with a shockingly small amount of composite material. Yes... those boats do tend to break up from time to time, lose masts, keels, etc., but when that happens they're usually in a 60 knot gale with their sails up hammering down 20 foot seas.. and even then hull damage is often the result of hitting something, like a whale, etc. Not to mention these boats bake in the open sun all day, in a salt environment, with usually some water salt water sitting in their bilge (leakage from rudder posts, prop seals, etc.)

 

[ColoPilot]

Anyone ever looked up at that one tiny little bolt that chairlifts swivel on that connect it to the cable? 6 fat guys sitting in a chairlift with all their ski gear on held to the cable by one bolt that appears grossly undersized.. and chairlifts dangle out there in the snow and moisture for decades

Going off topic (but hey, it's POA) the high speed quad/6 pack lifts are impressive, the chair is taken off the cable and put back on every time it arrives at the top and bottom stations. How many times a day is that multiplied by the number of chairs? You think a chair or two wouldn't quite make it back on each season.

 

[SkyDog58]

Was my thought too.. but this should be a relatively easy thing to test for. IF.. and that's a big IF, it turns out that Piper is using substandard metals that's going to be a major problem for them. At this point, seems to me more likely that this plane was just abused too hard one too many times.. and somehow those cracks were missed. If ERAU pulls wings at 10K hours anyway, then they're aware that their planes get ridden hard and put away wet

Or maybe there will be blame on both parties

I have to think that with the thousands of PA28s out there flying around if there was an issue with the wing structure and materials we'd see more than one failure in 1987 during a pipeline inspection and one failure from a flight school

Agreed.

If they are using a substandard alloy or one that doesn’t meet the specifications in the design that the FAA approved and certified, they will be facing pretty dire consequences. I’d venture to guess that would mean having their production certificate pulled and every plane built with that aluminum sent to the junk yard with revoked airworthiness certificates. The FAA civil penalties plus lawsuit verdicts could cripple them to the point of no return. And there is also possible criminal charges for those in the know.

All of that are well known possible consequences and is why it is practically unfathomable that they would have used such materials.

 

[Tantalum]

the chair is taken off the cable and put back on every time it arrives at the top and bottom stations. How many times a day is that multiplied by the number of chairs? You think a chair or two wouldn't quite make it back on each season.

Indeed, it really is incredible watching them detach and reattach... and it is such a simple yet beautifully elegant mechanics that do the attaching

 

[SoCal RV Flyer]

I was surprised at how short the spar connection to the carry thru is. Just looking at the lever arm.

Yeah, my RV-9A spar sticks out about the same from the wing, maybe less. I was a little surprised at this when I first received the kit. But mine looks a lot beefier than the PA-28's, with very healthy rectangular aluminum bar stock forming the caps of the spar's I-beam. Stubbier spars just need sturdier spar carry-throughs.

It may not have been an issue in this failure, but de-burring all imperfections/edges to eliminate stress risers is taken very seriously in the Experimental world. A very labor-intensive and unglamorous task, but of utmost importance.

 

[SoCal RV Flyer]

This is also why the initial composit planes had an airframe lifespan. If I remember the Cirrus was 6000 hrs. As the planes proved themselves and their were no composit failures seen it was raised to 12,000 hours. They now say that number may be extended as the planes get older.

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My Diamond DA-20 trainer also had a temperature limitation on the airframe. There was a heat sensitive sticker in the cockpit, and when it turned a certain color, the flight was a no-go. The temp was quite high, IIRC, but interesting that it's a constraint. Also, many composites can only be painted white, to keep the absorbed heat down.

I don't think I'd every buy a composite experimental...too hard to ascertain build quality. With aluminum, it's much easier to evaluate.

 

[overdrive148]

Indeed, it really is incredible watching them detach and reattach... and it is such a simple yet beautifully elegant mechanics that do the attaching

 

[paflyer]

Jane! Stop this crazy thing!!

 

[SixPapaCharlie]

I feel better about the oddball Grumman spar.
I will admit at least 2 times I have flown after this incident, I have grabbed a wing during preflight and shook it a little.

 

[GRG55]

If you look at the picture of the spar, it looks like it failed right where the bolts attach, in the middle of the bolt holes. At a minimum I think you would be missing those two bolts.

The bolts would fail in shear. The bolts didn't fail. The structure around one side of the bolt failed. Why would you expect them to be missing?

 

[Tantalum]

Well, not sure about Georgian skilift tech. My friend actually skiied there and rode that lift a few winters ago

 

[hindsight2020]

I feel better about the oddball Grumman spar.
I will admit at least 2 times I have flown after this incident, I have grabbed a wing during preflight and shook it a little.

View attachment 62063

Lol, you do realize the extruded I beam construction is stronger in bending moment than a hollow cylinder right? Torsional rigidity goes to the tube, but this quality is usually attained in conventional wings by the addition of a much smaller secondary spar, where the control surfaces and drag devices are usually attached to. Which is basically a sunk cost in all spam cans since they all have flaps and ailerons regardless.

Grumman put a tube in lieu of a properly constructed I beam as a cost cutting measure. The geometry of your spar is in fact not stronger than the geometry of failed wing in question. But let's not let facts get in the way of the placebo we all need to rationalize our need to break ground, myself included. The more you know....

 

[GRG55]

Black corrosion on aluminum seems a bit out of place without heat and moisture. OTOH, I live in a desert. Do you guys closer to a coast see black corrosion on aluminum?

The black is fine oxide the forms at high localized temperature from fretting. The fretting is from the initial crack propagation being subjected to repeated cyclical stresses over some period of time.

 

[RudyP]

This is also why the initial composit planes had an airframe lifespan. If I remember the Cirrus was 6000 hrs. As the planes proved themselves and their were no composit failures seen it was raised to 12,000 hours. They now say that number may be extended as the planes get older.

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They also tend to overbuild them to throw in a lot of extra margin. I can't really speak for other composite designs since I have not seen them but the carbon fiber spars on SR22s are REALLY beefy. As in, massive overkill beefy and they carry through the whole wing. They make what we are seeing here in this example look like a matchstick in comparison. I think you'd have a really tough time snapping the wing on a Cirrus or Columbia.

 

[GRG55]

I am new to this airplane stuff, but am I looking at this correctly? When interpret the pic, it looks like the wing only attaches at two small points? That can’t be correct.

It is. The web of the spar takes the bending loads distributed along the wing. That's why it is widest at the root and tapers towards the tip. The flanges of the spar are in tension or compression. The bolts are in shear under load. Steel bolts in shear are hellaciously strong. It's not the bolts that failed in this instance; it's the material around them that developed cracks and eventually failed.

 

[SixPapaCharlie]

Lol, you do realize the extruded I beam construction is stronger in bending moment than a hollow cylinder right? Torsional rigidity goes to the tube, but this quality is usually attained in conventional wings by the addition of a much smaller secondary spar, where the control surfaces and drag devices are usually attached to. Which is basically a sunk cost in all spam cans since they all have flaps and ailerons regardless.

Grumman put a tube in lieu of a properly constructed I beam as a cost cutting measure. The geometry of your spar is in fact not stronger than the geometry of failed wing in question. But let's not let facts get in the way of the placebo we all need to rationalize our need to break ground, myself included. The more you know....

Want to buy a Grumman?

 

[Pilawt]

Grumman put a tube in lieu of a properly constructed I beam as a cost cutting measure.

Not quite. That wing design began with the Bede BD-1 of 1964. The intent was to have the hollow spar do double duty as the fuel tank, as it did on the American Aviation and Grumman-American AA-1 series, developed from the BD-1. Bede also originally intended the BD-1 wings to be readily removable for road transport, but that feature was (fortunately) abandoned for the production versions.

 

[Fearless Tower]

As far as bounced landings in a PA28, I can’t imagine...those things don’t want to touch the pavement...they float for damned near forever.

Fixed gear PA28s will float forever. Arrows with the power at idle? Not so much.

 

[hindsight2020]

Want to buy a Grumman?

Hell naw. The way things are going I might dump this arrow for a 180 apache, Seneca Uno or hail damaged Lance (only kind I can touch lol) if they slap some stupid wing pulling AD as a result of this Easter egg hunt. I'm going for straight power loading on this one, cosmetics and avionics be damned. If it wasn't for the move and this stupid house purchase that's about to wreck my play money kitty, I'd done it already. The wife thinks I'm gonna skip out on the closing date to go haggle some lances she caught me cheating on her online with the other night lol.

 

[Pilawt]

Fixed gear PA28s will float forever. Arrows with the power at idle? Not so much.

True that. Hear the whistle as a gear-down Arrow flies overhead? That's the sound of drag!

 

[Fearless Tower]

How many composites have come apart in flight? Not many.

AA 587 comes to mind.

 

[Pilawt]

The way things are going I might dump this arrow for a 180 apache, Seneca Uno or hail damaged Lance (only kind I can touch lol) if they slap some stupid wing pulling AD as a result of this Easter egg hunt.

Don't the Lance and Seneca have a similar wing attach structure to the Arrow?

 

[dell30rb]

The bolts would fail in shear. The bolts didn't fail. The structure around one side of the bolt failed. Why would you expect them to be missing?

Because if the structure around the bolts failed they would drop out of the airplane.

 

[GRG55]

They also tend to overbuild them to throw in a lot of extra margin. I can't really speak for other composite designs since I have not seen them but the carbon fiber spars on SR22s are REALLY beefy. As in, massive overkill beefy and they carry through the whole wing. They make what we are seeing here in this example look like a matchstick in comparison. I think you'd have a really tough time snapping the wing on a Cirrus or Columbia.

There is still the view there's a higher level of uncertainty working with composites, so stuff is overdesigned.

My business owns both steel and composite structures (pressure vessels). The Federal regulators require the carbon fiber composites to be re-tested and re-certified every 5 years, half the interval for steel. And we have to scrap the composite structures at 15 years no matter what. The steel units have been used for multiple decades if they are maintained properly.

Composites have little ductility. When they fail they snap or shatter. Because of that our composite pressure vessels have to be completely without flaws on the exterior surface. Any damage, even so much as a scratch, is cause for immediately scrapping it as there is no reliable repair method with carbon fiber, and the flaw is a potential stress concentrator.

 

[hindsight2020]

Fixed gear PA28s will float forever. Arrows with the power at idle? Not so much.

Yeah I miss my warriors wings. This arrow floats like a greased brick lol. I don't even bother practicing engine outs anymore. Wherever im aiming at, I know I won't make it, so I just worry about steering through whatever the hell is in front of me and flaring when I get to landing height lol. Insurance got the wheel at that point anyways.

 

[GRG55]

Because if the structure around the bolts failed they would drop out of the airplane.

The bolts join two pieces together. Only the material on one side of the bolts failed.