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Structural failure - what, why, and how likely?

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Structural failure - what, why, and how likely?

Old 30th Sep 2018, 14:13
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Structural failure - what, why, and how likely?

Hello! This is my first post after many years of lurking...

Why don't the wings fall off? Or rather, what causes structural failure in small aircraft? I know that the wings don't fall off planes very often, but sometimes the wings do fall off, and I don't know how to deal with this fact!

I've had lessons in a C42 and a CT. I've also been up in a Grob, a Cub, a couple of Cessnas, some bizjets and even a Harvard, so I wouldn't say I was scared of flying. But I don't know what I have to do to prevent catastrophic failure in the aircraft's structure, and I don't know (for example) how much turbulence could shake a fixed-wing microlight apart. A couple of years ago I was passengering in an AX3 and the windscreen folded in around 400ft into our initial climb. The pilot got us back to base, albeit with the door flaps hanging off, but it makes me wonder what else can go wrong on these little flying machines. I'm instinctively nervous about structural failure, and instinctively reassured by the presence of a ballistic parachute, but seems to be the opposite of what experienced pilots think.

I like microlights but what stops them falling apart? Are they less sound than a Cessna? How am I supposed to be confident in the wings not falling off when sometimes the wings do fall off!?

Am I being a massive wet blanket?
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Old 30th Sep 2018, 17:40
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My answer would be - one of either over stressing so pulling g outside of the operating limits, the other options weakening by corrosion or preexisting structural damage (hard landings, or other unreported over stressing) while operating at normal operating limits.

The latter happened with a PA28 in Florida operated by a flight school only this summer - the wing basically fell off, of course it did not end well.
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Old 30th Sep 2018, 17:58
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You repeatedly state you do not know the answers to the questions you pose. Why not do some very basic research or reading to discovery the answers? Why does n't a building fall down, a bridge collapse or a car break apart? For very similar reasons - all available with less than 10 seconds googling.......
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Old 30th Sep 2018, 18:00
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Indeed, @Gasax. I cannot help smelling some unpleasant odour... Troll, perhaps?
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Old 30th Sep 2018, 18:29
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Seems more like a gnome or an elf
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Old 30th Sep 2018, 20:16
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Thanks Ebbie, though that's what's worrying me a little! I think I know the incident you're talking about, which involved a relatively new aircraft at a well-regarded flying school, with an experienced instructor...

I've read up on what I can in training books/the internet, and while these incidents are clearly rare (especially in comparison to pilot error) they happen often enough to be at the back of my mind while in the air. Metal fatigue and over stressing could be invisible during visual checks. Sometimes it's only during forensic examination after a crash that the cause of a structural failure becomes clear. Am I wrong to find that frightening?

I don't mean to come across as daft or obtuse, it just feels like an exercise in faith at the moment!
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Old 30th Sep 2018, 21:46
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Originally Posted by birb View Post
Hello! This is my first post after many years of lurking...

Why don't the wings fall off? Or rather, what causes structural failure in small aircraft? I know that the wings don't fall off planes very often, but sometimes the wings do fall off, and I don't know how to deal with this fact!
In really simplistic terms, a structural failure occurs when the flight envelope is exceeded - normally by at least 50%, as that is the minimum margin most airworthiness standards require between the maximum permitted load in normal flight, and the structural failure point. So that's more than 150% of the g-limits (in a light aeroplane those are usually at +3.8/-1.4g, in a microlight +4/-1.5, in an aerobatic aeroplane at-least +6/-3, in an airliner almost always +2.5 / -1.25, fighters typically sit around the +8g point, display aerobatic aeroplanes in the bracket 8-12g).

There are more complex cases - control deflections above 1/3rd deflection at Vne, tapering up to full deflection at the manoeuvre speed, Va (which is the stall speed multiplied by the square root of the g-limits - in most cases



I've had lessons in a C42 and a CT.
C42 is 4g x 1.5 = 6g to break it
CT is 4g x 1.5 x 1.5 = 9g to break it. That's when new - the extra safety factor is for the composite wings, which are assumed to degrade in service.

I've also been up in a Grob, a Cub, a couple of Cessnas, some bizjets and even a Harvard, so I wouldn't say I was scared of flying. But I don't know what I have to do to prevent catastrophic failure in the aircraft's structure,
Don't exceed the operating limits.


and I don't know (for example) how much turbulence could shake a fixed-wing microlight apart.
Probably an infinite amount - the low inertia means that the whole aeroplane moves, not breaks. Airliners can be threatened by turbulence - microlights are only threatened by the risk of loss of control, not structural failure.

A couple of years ago I was passengering in an AX3 and the windscreen folded in around 400ft into our initial climb. The pilot got us back to base, albeit with the door flaps hanging off, but it makes me wonder what else can go wrong on these little flying machines.
Canopies and doors are secondary (aerodynamically necessary) as opposed to primary (structurally necessary) structure, so may be tested to lower standards. Bloody annoying, but almost certainly you were not in any danger at any point.


I'm instinctively nervous about structural failure, and instinctively reassured by the presence of a ballistic parachute, but seems to be the opposite of what experienced pilots think.
In flight structural failure on small aeroplanes, is possible, but fantastically rare. The BRS is in many cases, particularly on lower speed aeroplanes like microlights, nothing but a comfort blanket as the vast majority of life-threatening risks occur too close to the ground to use them anyhow.

On an aeroplane like the Cirrus, it makes more sense as it has a lot more energy - even at the stall, so will be much harder to safely land in a field.

On gliders personal parachutes are common - because they operate in a way (lots of aeroplanes chasing the same thermal...) that make mid-air collision much more likely than any other civil aeroplane class. It is far to say that basically no aeroplanes are built to take a mid-air collision - avoiding those is a piloting issue.

I like microlights but what stops them falling apart? Are they less sound than a Cessna? How am I supposed to be confident in the wings not falling off when sometimes the wings do fall off!?
Microlight structural requirements are simplified compared to light aeroplanes, but that simplification is compensated by requiring slightly higher g-limits. Basically, there's no real difference in structural strength.

Am I being a massive wet blanket?
No, but in the politest possible way, you're being ignorant. That's probably not your fault - as this is poorly taught, particularly to PPLs - and if you're not even at that level, you've just not been exposed to this.

If you don't mind spending money, I can recommend some books - but as I may have written some of them (and they are quite expensive) PM or email me for references rather than my causing a daft public p*****ng contest on the subject as happened last time I mentioned them on PPRuNe.

G

Last edited by Genghis the Engineer; 1st Oct 2018 at 03:26. Reason: Typo
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Old 30th Sep 2018, 22:14
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So aside from exceeding the flight envelope and corrosion, it seems like a little bit of luck as well? I've done a little reading on the incident mentioned and it seems like the aircraft wasn't stressed past the flight envelope, wasn't particularly neglected (article mentions just passing annual inspection 2 weeks prior), yet still suffered this failure. So just bad luck that despite all the measures in place, this failure still occurred?

Article referenced:
NTSB: Wing of plane in deadly ERAU crash showed fractures, metal fatigue - News - Daytona Beach News-Journal Online - Daytona Beach, FL
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Old 30th Sep 2018, 22:16
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An unprompted wing falling off is very unlikely to be the prime cause of one's death.

Loss of control in a perfectly serviceable airplane is more likely to be the cause and one can minimize that - although I suggest to nowhere near the risk of a wing falling off - minimizing it by training and practice is the way.
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Old 30th Sep 2018, 23:25
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I think the discussion is more towards HOW these things happen and how it can be spotted/fixed/avoided rather than the probability of such things happening.... otherwise we can just pick up statistics on the chances of us dying from an accident on the motorway on the way to the school compared to the chances of us dying from a rare event of a wing falling off.
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Old 1st Oct 2018, 00:29
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As Genghis has well explained, there are many safety margins built in to GA airplane structure. Slower than Va, you'd have to massively abuse an airworthy airplane to break it. Indeed, the "break" will generally be a "bend" so far, that once landed, the aircraft is obviously unairworthy. I am aware of three aircraft in which this has been the case - massively abusive flying, the aircraft was landed safely, but scrap afterword.

I had occasion to be involved in the "repair" of a Cessna 206, in which a careless installation on the wing resulted in a drill hole being made into the critical area of the wing spar (right in the wing strut attach area). After analysis, I determined that the wing remained airworthy without a repair being accomplished. I hired a structural Engineer to make a formal determination. The result was that with the drill hole damage, the wing had a predicted fatigue life exceeding 1.17 million flight hours - many orders of magnitude beyond what it could ever fly.

This, of course, presupposes that the airplane is maintained in an airworthy condition. The failure of the aforementioned PA-28 wing was in large part the result of the wing being unairworthy. This would have been very difficult to detect though (it's not an easy area to inspect). This was not the first instance of such a failure in a PA-28 wing, and in the late '80's, I was involved in AD required inspections for the same airworthiness concern. I expect that the newly developed inspection technique will become a means by which the airworthiness of a PA-28 series airplane can be assured.

In the mean time, it's your job as the pilot to conduct the prescribed preflight inspection, and thereafter, to operate the aircraft within its limitations, and with appropriate technique. As said, BRS will be of little use through a lot of the flight envelope in which a training aircraft normally operates, and also introduces other expenses, and reductions in safety. Just fly airworthy airplanes as they were designed to be flown, and otherwise, don't worry about it.
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Old 1st Oct 2018, 00:59
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Originally Posted by Ebbie 2003 View Post
An unprompted wing falling off is very unlikely to be the prime cause of one's death.

Loss of control in a perfectly serviceable airplane is more likely to be the cause and one can minimize that - although I suggest to nowhere near the risk of a wing falling off - minimizing it by training and practice is the way.
Yes: the most dangerous and most likely to fail component in any aircraft is the nut that holds the yoke / stick....... i.e. us!
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Old 1st Oct 2018, 14:29
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One thing which deserves mention is the dreaded thundercloud.
Stay well away from anything which you even suspect might be convective, if you fly under one it can suck you up to unconcious altitudes and then spit you out in pieces, it has happened many times.
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Old 1st Oct 2018, 17:44
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Met two guys who had wings fail and survive..one was Bwana Drum who was spotting Mau Mau in a tripacer. Took off before sun rise and would search for heat disturbances in the fog layer, then call in Lancasters to pattern bomb the area. Aircraft had been over stressed on a previous flight and he described to me how he played with the controls after one wing had fallen off. Rebuilt in east grinstead and had the GM plus police medal for gallantry.
The second was the Lak test pilot during a low level beat up at VNE when the wings fluttered and folded. Used Russian composites on a western designed glider but did't know that the materials fatigued at a different rate.
lost a very good friend who was ex frog carrier pilot who had been warned about aerobatics in his kit motor glider..thought he knew better.
Similarly watched a three axis hang glider doing aerobatics extremely badly at Coupe Icare..next day he pulled the wings off fortunately it had a ballistic chute.
Flown two aircraft which had been over stressed and not reported..one I found myself inverted over the new forest with my brief case on the ceiling..
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Old 1st Oct 2018, 18:07
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blind pew.......... Interesting you mention "Bwana Drum". I lived next to his parents in Nairobi for a while. In David's own book no mention is made of the aircraft having been stressed in a previous flight. The wing was sliced off by a support wire of VHF mast; they had been dropping a message canister at low level. David recovered from the accident and went on to fly for East African Airways: finally becoming head of security for EAA. Those Lancasters were Lincolns !! Remember lying in my bed at Nyeri Primary and hearing the bombing on the Aberdares.

Last edited by Planemike; 1st Oct 2018 at 18:21.
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Old 1st Oct 2018, 19:10
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Just thinking about my limited experience on this: We had a single Comanche in for some check or other.and I noticed an oddity in the leading edge of the stabilator. Had a closer look and it was a dent that had been filled with polyester putty. Bells rang in my memory and I ran a check on the balance. Which was out. The bells were the event that the CAA used to make airtests conducted by CPL's. A PPL owner of a Comanche 250 conducted his own C of A test with some friends to make the weight. He reached the Vne while still in a shallow dive, rather than pulling it up before to hit the number in level flight. The stabilator fluttered at the higher speed due to the imbalence and it removed the whole unit. The a/craft nosed over and the wings failed in negative G.
We had a Zlin 526 in with a private owner. When Neil Williams had a structural failure practicing for the aerobatics championship in a similar a/c and had a wing fold up. We got a phone call the same day to ground the Zlin. The main spars were tested with NDT methods, along with the rear attachments, All OK.
Several French flying clubs used CAP22 aerobatic a/c. After a couple broke up in flight, they found that the G meters had recorded frequent overstressing and it was common on others they looked at. Difficult to see what they thought they were thinking of, if they could see they were overdoing it.
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Old 1st Oct 2018, 19:31
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Originally Posted by Planemike View Post
blind pew.......... Interesting you mention "Bwana Drum". I lived next to his parents in Nairobi for a while. In David's own book no mention is made of the aircraft having been stressed in a previous flight. The wing was sliced off by a support wire of VHF mast; they had been dropping a message canister at low level. David recovered from the accident and went on to fly for East African Airways: finally becoming head of security for EAA. Those Lancasters were Lincolns !! Remember lying in my bed at Nyeri Primary and hearing the bombing on the Aberdares.
Small world. I never met him but knew his wife (Hilary) when I was a very young student and she was a very mature administrator in Nairobi. Incredible tales of his time in the MauMau.
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Old 1st Oct 2018, 20:04
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plane mike

I went on a camping safari with him whilst I was flying the death cruiser ..dc10..we left Wilson field on the dc3 then went back with an engine problem. waited an hour or so whilst they thought they had fixed it but it didn't sound great on the pre take off engine run up..got airborne but couldn't climb up high enough to get out of the cumulous tops..crew decided to return but he persuaded them to continue and land in the massi mara where the aircraft stayed a day until they flew in some carb spares.
Around the camp fire he told me the story after I asked him what the GM stood for..seemed shocked that as a Brit I didn't know but I thought he was swedish as in the accident he had mangled his face. talked about bader who was a regular visitor in east grinsted. told me that it was a spiral dive where it had been overstressed the night before and he hadn't done an outside check and how they found him still straped in the seat 100 yards from the wreckage. he had an eight freight in his garden ..my father was on the footplate and I was into steam locos. Took his biography out of the library when I got back to UK.
Would give you Lincoln's but still remember the story he told.
Had another friend survive a wing coming loose but that was on a luscombe that had been restored in southern England but the guys had forgotten to lock the clevis pin in the rear turn buckle which adjusted the angle of attack. he tried a high speed pass low over the runway hopeing that some one could see the fault..when he put Aileron in it twisted the wing like a servo tab and the aircraft rolled in the opposite direction..rudder had helped but not in the pass..survived but later died in a Christian eagle.
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Old 1st Oct 2018, 20:20
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blind pew................... Only reporting what is written in his biography "" Bwana Drum "" , certainly a good read.
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Old 2nd Oct 2018, 06:26
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Ahh...
One of the other bits I asked Dave about was another set of initials..police medal for gallantry in a deadly shoot out with drug traffickers in Belgium.
What isn't mentioned in the book is that he was the presidents body guard and was instrumental in getting the hunting ban in Kenya which lost him lots of friends and created enemies.
We walked in single file through the Bush with him leading us carrying a rifle; the dangerous bit was crossing a river as the hippos were the deadliest animals. At night the servants collected the camay soap, which was kept in aluminium soap dishes, dug a hole and buried it otherwise the cheetahs would enter the tents and eat it!
My Swiss skipper filled me in on details that I didn't dare ask, some of which are in the book especially dyeing his skin with potassium permangenate and wearing a curly wig and mingling with the Mau Mau.
Read that when the country deteriorated he went to Jersey where he passed away a couple of years ago.
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