Arrow PA-28 Experienced In-Flight Break-Up

[Radar]

Sleeve Wing,

My thoughts exactly, when I came across this thread a couple of days ago. It has been "time for the Mods" for some time in relation to this ' gentleman', IMHO.

[DX Wombat]

I shall be saving this thread for my own future reference.
There is one thing which hasn't been mentioned so far regarding break-up in flight and that is unreported, and therefore unchecked, bumps to the aircraft. (I am NOT saying that this is implicated in this case.) This report on G-DELS makes chilling reading. The pilot knew he had hit something, did a very cursory check then set off again with fatal results. All the more reason for having those seemingly minor bumps checked out by an engineer.
Sternone, as others have said, your attitude to people who really DO know more than you is worrying. None of us is perfect, but if I make a mistake, and I have made many, I acknowledge the fact and try to build on what I have been told is the correct version of matters. As one of my former FIs said (of another student) "I foresee a starring role in an AAIB report for him." You really DO need to think, listen and read carefully. I don't want to hear that you have become another statistic.

[sternone]

Ok

Anyways, i will try to be more gentle and not go into replys and atacks on my comments. Maybe it's a rolling stone and i have no problem at all taking the responsibility on me.

I will see what future will give and will see how other users of this forum will do, in my opinion i still think users should be allowed to give their consern on a certain insident but as i understood it seems to be better to keep personal comments for me own, just like in real life ?

I hope some of you accept this, we'll just see how it goes. If you take this as an appoligy of some of my comments to you if i have hurt you, please do so!

[VFE]

I fear Sternone is yet another one of those PPRuNe personalities whose main intention is to stimulate user debate centred largely around themselves or their assumed online persona rather than anything of substance and relevance to the raison d'etre of this oft abused website.

Do you feel ignored in your real life Sternone? That is probably because you have nothing much to bring to the table. Hence my advice that you perhaps ought to listen instead of rabbiting for once - then maybe when the weight of your knowledge equals the weight of the chip on your shoulder you too might bring something to a (ridiculous) debate. However, as you rightly state, it is indeed your choice, and I'm not really fussed moreover I just feel for those at your chosen training establishment who appear to be tasked with a job beyond the call of duty, namely that of winding your neck in sufficiently enough for you to learn something about flying.

But until then you'll be pleased to know that this here cherished thread on PPRuNe is all yours so please have a good sunday on me whilst I sleep content in the knowledge that I'm not tasked with the apparently thankless job of sitting next to you on your flying lessons.

VFE.

[IFMU]

I am very interested to learn more about the failure mode of the last 2 PA28R's. In the UK report on G-BKCB, they mentioned no evidence of fatigue, corrosion, or anything else wrong. Are the UK investigators blessed with scanning electron microscopes, etc for looking at this stuff? Is the FAA? I just hope the right technical people do the analysis so we can come away smarter.

-- IFMU

[SNS3Guppy]

There is one thing which hasn't been mentioned so far regarding break-up in flight and that is unreported, and therefore unchecked, bumps to the aircraft. (I am NOT saying that this is implicated in this case.) This report on G-DELS makes chilling reading. The pilot knew he had hit something, did a very cursory check then set off again with fatal results. All the more reason for having those seemingly minor bumps checked out by an engineer.

This is an excellent point, especially with respect to rental aircraft. We really have no way of knowing what the last student, the last instructor, the last renter did to the airplane. Did he overstress it? Did he run the engine too lean, cause detonation, and set the engine up to fail on my flight? Did he "shock cool" something, overstress something, tweak or twist something for which the airplane will mete out vengence on me?

Metal remembers. Fatigue is cumulative (which is why it's called fatigue). Cracks grown, damaged areas are weakened...and mechanical things do not repair themselves. Problems don't go away. They just hide for a while.

I believe I mentioned previously the C-130 that shed it's wings. I was in the airplane about five years prior to it losing the wings, when a pilot decided to show is anger at being kept out in the field months longer than he desired. He made a combat landing (short field, roughly speaking) so hard that it pulle the map case free from the cockpit wall, and it hurt. I mean hurt. My neck and back. He was the pilot flying, and I had no idea he was going to do what he did. I have a personal suspicion that he intended to break the airplane in order to go home.

I personally removed the wings on that airplane as part of a team doing inspections on two small cracks discovered. The cracks weren't discovered then, but while out of the country on an assignment. When we entered the wing structre to do ultrasonic testing, the two small cracks we'd discovered visually and identified with dye penetrant inspection were found to be inisible, but extnding the full circumference of the wing. If you don't know the early model C-130 wing, it doesn't have anything holding it together but thick skin. Internally there are hat-section stringers and some shear web spars that don't really carry a load so much as for the fore and aft walls to the integral fuel cells...crack the skin on the wing, the wing is coming off. The crack went all the way around both wings. Only a very small bit of it was actually visible.

We immediately removed the wings, stripped them, replaced one, repaired the other, and began instituting inspections involving stripping wing sections and performing dye penetrant and ultrasonic inspections every week or so. Even with that elevated inspection schedule, the wings still came off some years later.

Past sins can be paid today, but you may the one paying even though you didn't commit the crime. Yet another good reason to carefully respect the limitations of the airplane and not stray too close to the edges.

In another thread on this forum just lately a discussion has been in play regarding ultimate load bearing capability on the airplane...it's published for these numbers XXX, but it can really stand a hundred fifty percent of these numbers...so we're okay, right? Wrong. The airplane has been stressed and bent and twisted...so we're not gauranteed anything Metal remembers. All those other stresses to which that airplane has been subjected? They're still there, waiting for more. Like a big spinning prize wheel, but big question is when will it stop, and who will be the lucky winner when it does?

Some years ago drop studies were done to explore the reasons that perfectly intact cherokee airframes have been found with dead occupants. The airframe is in good shape from outward appearances, but the occupants are all dead of crushing injuries. Why? Drop tests from different heights and different angles with high speed photography showed that the airframe deformed substantially on impact, crushing the occupants and killing them, then springing back to look relatively intact and normal.

Is this a weakness in the design? No, not really. The airframe of an airplane is a very flimsy, weak structure; it's designed with minimum strength, and it's chief citeria is that it's aerodynamic, and it's light. Unlike fabric airplanes of old, the Cherokee is not built around a reinforced stress-frame steel cage. It's just bits of 6061 and 2024 aluminum riveted together. Add speed and gravity...don't expect it to perform miracles.

It's a strong, well designed airplane. The gear system design, particularly with automatic ram-air sensed gear safety and other such features, is ridiculous. However, it is what it is. The pitch is particularly sensitive, being a flying tail, and the potential to overcontrol the airplane does exist. What this calls for isn't fear, but common sense. Many production copies are out there flying safely...jump in one and you can too. Push the airplane beyond it's limits, and that privilege is no longer yours...just as you've been taught since day one a student.

The biggest weakness in any airplane is the pilot.

[IFMU]

Originally Posted by G-EMMA

IFMU, The AAIB report for G-BKCB looks robust enough, they didn't find fatigue, corrosion, or anything pre-existing as in my opinion it wasn't there to find. The finite element model they created is convincing,...

Originally Posted by SNS3Guppy

Metal remembers. Fatigue is cumulative (which is why it's called fatigue). Cracks grown, damaged areas are weakened...and mechanical things do not repair themselves. Problems don't go away. They just hide for a while.

I spent a part of my aerospace career in test. We would chug on metal parts until they busted, then send them out to the metlab. The metab would put the parts under a microscope, look at grain stucture, origins, etc. Under the microscope you could see what parts were ripped apart by fatigue, and then as the part got weaker static overload would take over and finish the job up quickly, assuming the initial fatigue damage was not detected with non-destructive test. We would take all the test data and figure out s-n curves, to help in predicting the life of parts. Miner's law, cumulative damage and all that.

What struck me about the G-BKCB report was the one liner:

Also, no evidence was found in the failed area of the wing of corrosion, fatigue cracking, repairs, material defects, or any other feature, which could have degraded the strength of the structure.

Maybe the investigator turned the pieces over to the metlab, they did the detailed analysis, electron microscope study, etc of the failed parts and the terse one liner came out of the results of that report. Or maybe not. Why would they do a detailed analysis of the metal, and not show it, then do a detailed FEM analysis, and present it in gory detail?

I have seen a lot of FE analysis. It's a great tool. But it is not always right. I've seen it both ways, where somebody comes along and does a FE analysis of a 30 year old part with a million fleet hours on it and says it only has a 5 hour life. I've also seen parts that were shown to have huge margin in analysis end up with a limited life and a redesign. Even if the FE model for G-BKCB was spot-on, it seems all that it did was to validate the static strength of the PA28R spar splice area. I bet that part was designed, static tested, and shown to have margin over the standard category g-loads a long time ago.

-- IFMU

[Contacttower]

Don't want to burst your bubble or anything, but here's a brief, partial list of a few Spitfires that did...

The comment was meant to be half in jest...hence the . To be fair though the Spitfire tended to be subjected to much greater in-flight loads than your average GA hack. Part of my point as well was the assertion by Sternone that the Bonanza was strong- it is, but as someone reminded us earlier in the thread the early ones did have a very well know reputation for loss of control in flight and subsquent break up.

[sternone]

Part of my point as well was the assertion by Sternone that the Bonanza was strong- it is, but as someone reminded us earlier in the thread the early ones did have a very well know reputation for loss of control in flight and subsquent break up

Sure, the 33 model was just a downgraded model of the V-tail, but without the V-tail .. at that time people valued the V-tail higher than any other tail.. might worth noticing that there are no in flight break ups of the 33 model...

[hobbit1983]

Sternone - sorry you're wrong, see here, page 39 -

www.caa.co.uk/docs/33/CAP701.pdf

This document lists a Beech Bonanza 33 which broke up in midair over France, following loss of control in IMC.

[IO540]

Wasn't the V-tail Bonanza back end modified at some stage, which put a stop to the breakups?

There is a big technical advantage in a V-tail: instead of three huge things dragging in the airflow, you have just two. No wonder the new SE jets are V-tails (Cirrus and Eclipse, IIRC). It's got to be worth 10-20kt, which at the relevant cruise TAS is worth an awful lot of fuel flow.

I bet you will notice when the yaw damper servo packs up though

[Tony Hirst]

Some interesting thoughts on a couple of Beech T-34s suffering in flight break ups too.

http://www.avweb.com/news/pelican/182086-1.html

You many need to register.

[hobbit1983]

IO540; link to a piece on the history of the V-tail.

http://www.tsgc.utexas.edu/archive/g...ics/vtail.html

Tony; interesting article, thanks.

[scooter boy]

"Wasn't the V-tail Bonanza back end modified at some stage, which put a stop to the breakups?"

Yup - twas a simple fix, they just stopped selling them to doctors!
i:e If you had an MD you needed the conventional tailed model!
Death rate plummeted!
SB

[IO540]

Thank you for the V-tail URL, Hobbit. Fascinating.

Quote from it:

Out of the over 10,000 aircraft made, less than 250 were involved in fatal in-flight accidents.

That is one helluva lot of deaths.

[wsmempson]

2.5% OF V-Tailed bonanza's involved in fatal accidents? Wow. Is that right? 250 out of 10,000 would suggest that it is....

[SkyHawk-N]

That's why, in the States, Bonanzas have the nickname "Doctor Killers", or "Fork Tailed Doctor Killers".

[Tony Hirst]

Great link Hobbit.

I can't help but notice this quote in relation to my thoughts on Post #8

The in-flight break-up rates of most single engine airplanes with retractable landing gear were significantly higher than for other categories of general aviation aircraft.

[IO540]

The in-flight break-up rates of most single engine airplanes with retractable landing gear were significantly higher than for other categories of general aviation aircraft
Sure, but because they are more slippery (and thus a pilot not paying attention will end up exceeding Vne sooner), not because they have retractable gear.

The availability of reasonably slippery fixed gear GA planes capable of ~ 150kt+ is a recent thing. It was done as a marketing stunt in the USA, where the insurance companies are throwing their weight around more than here. In reality it was just a stunt and hasn't worked because the premium on say a Cirrus SR22 is most definitely not lower than on say a TB20/21. But if you have a relatively inexperienced pilot then the "simplicity" of fixed gear is a rewarding yarn for the salesman to spin.

Fixed gear is itself a compromise. To make it low drag you have to have tight wheel cowlings, which compromises grass field / general dirt runway capability. Even the much looser PA28 cowlings are routinely dumped to avoid these problems - look at any flying school fleet.

[IFMU]

My dad used to fly a Debonair back in the early 70's. He told me there was a rash of wing failures due to people flying into convective weather. The accident pilots hit an updraft of 1000's of feet per minute, and tried to hold altitude with pitch. Like any glider pilot could tell you, the airspeed builds if you try to hold alitude in lift. Then you hit the downdraft on the other side, and suddenly want to go down at 1000's of feet per minute. Haul back on the yoke, at high airspeed, pull the wings off. That was the speculation of the day as to what was happening to the doctors. Not all a fault of the v-tail. My dad flew myself and my two siblings through some bad convection while skirting a thunderstorm. He asked for, and received a block of altitude rather than a single altitude. The recommendation of the day was to hold the attitude, and airspeed in bad convection and let the altitude go up and down if it was safe to do so. I barely remember it, I was 4 at the time. It made a big impression on my 13 year old brother, who vowed to never get into an airplane with my dad again. It also scared the hell out of my sister, but she was into horses and not airplanes.

The T-34's that shed their wings have the same structure as the bonanza/debonair. Fatigue rears its ugly head. Usually fatigue is the domain of the helicopter, due to the many load cycles its parts see. But run the loads up, it takes less cycles.

-- IFMU