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NTSB search for breakup accidents.

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p.mulwitz(at)worldnet.att
Guest

Posted: Sat May 12, 2007 6:06 am Post subject: NTSB search for breakup accidents.

I just completed a search of the NTSB accident database.  I was 

looking for fatal accidents involving 601XLs.  I also included RVs so 

I could compare results for the two different families of kit 

planes.  I am not sure how to interpret the results, but I am happy 

to report the data I found.



For the period from January 1, 2005 I found 135 fatal accidents in 

the database where home built airplanes were involved.  23 involved 

either CH601 or RV and 3 involved CH601s.  (one 601HDS was involved 

in a mid air collision, and one 601XL that was made by AMD had a 

breakup but was not included in the 135 accident count)



There were three in flight structure failures for XLs (N10028, 

N158MD, N105RH) and only one for RVs (N43EM).  There was an 

explanation that a service bulletin for the RV-3 involving wing 

weakness was not complied with for the RV-3 failure, but no 

explanation for any of the XL failures.



The RV accidents seemed to be a random assortment of 

misadventures.  One particularly notable problem with RVs was 4 

accidents related to formation flying.  ALL OF THE FATAL ACCIDENTS 

WITH XL'S INVOLVED IN FLIGHT STRUCTURE FAILURES.



I don't feel qualified to draw any firm conclusions from this little 

study.  However, I feel some anecdotal comments are in order.  The 

total number of fatal accidents for XLs compared to RVs seems to 

reflect the much larger number of RVs in the air.  That leads to the 

conclusion that flying an XL is no more life threatening than flying 

an RV.  However, the failure mode in the XL fatal accidents is 

alarming.  Compared to RV fliers, XL fliers seem to be at a much 

higher risk of experiencing structural failure.



I am afraid I don't have any action to recommend for XL owners.  It 

would seem that ballistic parachutes might help us survive a 

structure failure, but the accidents seemed to happen at low 

altitudes where the ballistic 'chutes might not be much use.



It would be nice if a design study could reveal the actual cause of 

the structure failures.  That could lead to a design change which 

might reduce the chances of future fatalities.  So far, there isn't 

even a clue about why the XL tends to break up in flight.  There are 

many speculations, but not a single piece of evidence to support them.



Paul

XL fuselage

naumuk(at)alltel.net
Guest

Posted: Sat May 12, 2007 8:59 am Post subject: NTSB search for breakup accidents.

Paul-

    As long as everyone is speculating and since you've already found the 

sources, how many HD/HDS outboard wings have failed? A lister reported 0 for 

the 701, but that configuration isn't even close to the 601.

     The reason I ask is because there are 3 radical wing design differences 

between the XL and HD/HDS-

    1. The shorter outboard wings and shorter distance to the attach points 

of the HD/HDS result in a shorter moment of arm and consequently less stress 

on the attach points and pull on the rivets spanwise.

    2. HD/HDS wing spars have no fore or aft tip angle I'm aware of.

    3. No flaps on the HD/HDS.

    To answer the first thought that comes to mind, yes, I'm trying to 

convince myself that this problem is restricted to the XL. The HD/HDS was 

notorious early on for nose gear problems, which I've done my best to 

compensate for with builder/Zenith upgrades. I doubt it, but if it turns out 

my outboards need attention, the time to address the problem is when they're 

30" rather than 2000' off the ground.



Bill Naumuk

HDS Fuse/Corvair

Townville, Pa

Ron Lendon

Joined: 10 Jan 2006
Posts: 685
Location: Clinton Twp., MI

Posted: Sun May 13, 2007 1:48 pm Post subject: Re: NTSB search for breakup accidents.

I think the real telling piece would be to some how find out which of the XL's were scratch built.  The spar design looks very solid but material selection and preparation play a major part in the integrity of the finished product.

_________________
Ron Lendon
WW Corvair with Roy's Garage 5th bearing
CH 601 XLB
N601LT - Flying
http://www.mykitlog.com/rlendon
Corvair Engine Prints:
https://sites.google.com/site/corvairenginedata/

steveadams

Joined: 10 Jan 2006
Posts: 191

Posted: Mon May 14, 2007 9:41 am Post subject: Re: NTSB search for breakup accidents.

While I appreciate your effort to search for failures, if you look back a bit further you will find no more 601 failures and 8 RV structural failures involving VMC-IMC, hard pull-up after low passes over airports and fields, aerobatics, improper construction, and 1 case that implied the pilot had overstressed the aircraft on repeated occasions in the past, and it finally failed in relatively benign conditions. While we need to ask questions, saying that the 601 "tends to break up in flight" is quite a stretch of the imagination.

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Mon May 14, 2007 9:58 am Post subject: Re: NTSB search for breakup accidents.

I did a close up inspection of an RV this weekend. The RV has ribs spaced 9 inches apart for starters and no sweep.



 	  p.mulwitz(at)worldnet.att wrote: 	
		  I just completed a search of the NTSB accident database.  I was 

looking for fatal accidents involving 601XLs.  I also included RVs so 

I could compare results for the two different families of kit 

planes.  I am not sure how to interpret the results, but I am happy 

to report the data I found.



For the period from January 1, 2005 I found 135 fatal accidents in 

the database where home built airplanes were involved.  23 involved 

either CH601 or RV and 3 involved CH601s.  (one 601HDS was involved 

in a mid air collision, and one 601XL that was made by AMD had a 

breakup but was not included in the 135 accident count)



There were three in flight structure failures for XLs (N10028, 

N158MD, N105RH) and only one for RVs (N43EM).  There was an 

explanation that a service bulletin for the RV-3 involving wing 

weakness was not complied with for the RV-3 failure, but no 

explanation for any of the XL failures.



The RV accidents seemed to be a random assortment of 

misadventures.  One particularly notable problem with RVs was 4 

accidents related to formation flying.  ALL OF THE FATAL ACCIDENTS 

WITH XL'S INVOLVED IN FLIGHT STRUCTURE FAILURES.



I don't feel qualified to draw any firm conclusions from this little 

study.  However, I feel some anecdotal comments are in order.  The 

total number of fatal accidents for XLs compared to RVs seems to 

reflect the much larger number of RVs in the air.  That leads to the 

conclusion that flying an XL is no more life threatening than flying 

an RV.  However, the failure mode in the XL fatal accidents is 

alarming.  Compared to RV fliers, XL fliers seem to be at a much 

higher risk of experiencing structural failure.



I am afraid I don't have any action to recommend for XL owners.  It 

would seem that ballistic parachutes might help us survive a 

structure failure, but the accidents seemed to happen at low 

altitudes where the ballistic 'chutes might not be much use.



It would be nice if a design study could reveal the actual cause of 

the structure failures.  That could lead to a design change which 

might reduce the chances of future fatalities.  So far, there isn't 

even a clue about why the XL tends to break up in flight.  There are 

many speculations, but not a single piece of evidence to support them.



Paul

XL fuselage

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Mon May 14, 2007 10:00 am Post subject: Re: NTSB search for breakup accidents.

I was going to mention this as well. The HD wings have a much shorter moment. Where the range where the xl seems to be failing is right where the HD has a bolt connection.



 	  naumuk(at)alltel.net wrote: 	
		  Paul-

    As long as everyone is speculating and since you've already found the 

sources, how many HD/HDS outboard wings have failed? A lister reported 0 for 

the 701, but that configuration isn't even close to the 601.

     The reason I ask is because there are 3 radical wing design differences 

between the XL and HD/HDS-

    1. The shorter outboard wings and shorter distance to the attach points 

of the HD/HDS result in a shorter moment of arm and consequently less stress 

on the attach points and pull on the rivets spanwise.

    2. HD/HDS wing spars have no fore or aft tip angle I'm aware of.

    3. No flaps on the HD/HDS.

    To answer the first thought that comes to mind, yes, I'm trying to 

convince myself that this problem is restricted to the XL. The HD/HDS was 

notorious early on for nose gear problems, which I've done my best to 

compensate for with builder/Zenith upgrades. I doubt it, but if it turns out 

my outboards need attention, the time to address the problem is when they're 

30" rather than 2000' off the ground.



Bill Naumuk

HDS Fuse/Corvair

Townville, Pa

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Mon May 14, 2007 10:03 am Post subject: Re: NTSB search for breakup accidents.

 	  steveadams wrote: 	
		  While I appreciate your effort to search for failures, if you look back a bit further you will find no more 601 failures and 8 RV structural failures involving VMC-IMC, hard pull-up after low passes over airports and fields, aerobatics, improper construction, and 1 case that implied the pilot had overstressed the aircraft on repeated occasions in the past, and it finally failed in relatively benign conditions. While we need to ask questions, saying that the 601 "tends to break up in flight" is quite a stretch of the imagination. 	




The 601XL was introdued in 2001 I believe. That's probably the main reason you're not seeing anymore failures further back, there's been more time for fatigue to accumulate.

steveadams

Joined: 10 Jan 2006
Posts: 191

Posted: Mon May 14, 2007 10:26 am Post subject: Re: NTSB search for breakup accidents.

 	  ashontz wrote: 	
		   	  steveadams wrote: 	
		  While I appreciate your effort to search for failures, if you look back a bit further you will find no more 601 failures and 8 RV structural failures involving VMC-IMC, hard pull-up after low passes over airports and fields, aerobatics, improper construction, and 1 case that implied the pilot had overstressed the aircraft on repeated occasions in the past, and it finally failed in relatively benign conditions. While we need to ask questions, saying that the 601 "tends to break up in flight" is quite a stretch of the imagination. 	




The 601XL was introdued in 2001 I believe. That's probably the main reason you're not seeing anymore failures further back, there's been more time for fatigue to accumulate. 	




So what about the RV's tendancy to fail? If they have 9" spaced ribs and no forward sweep, why do they fail with such regularity?

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Mon May 14, 2007 10:43 am Post subject: Re: NTSB search for breakup accidents.

Do they. The guy that started this thread just stated that no structural failures for the RV were noted.



If they do break up regularly, then I'd have to think it had something to do with a narrower spar. The wing isn't as thick. Maybe that's the reason for more ribs. If that's the case then there's a direct correlation between the number of ribs and the strength of the wing, no? The wing is only as string as the vertical component of the spar. If the spar can't twist to accommodate bending of the wing, then the wing can't bend without the spar cap breaking. The thicker the spar, the more vertical the spar and the more the spar is braced (ribs, to keep the spar from twisting as well as keeping the skin from buckling) the stronger the wing. The thing I really don't like about the inclined spar is that's be pre-positioned to flop forward under stress. It's not carrying the stress parallel to the spar, it's carrying that stress at an angle, translating the force into a shear of the rivets that are trying to keep the spar in a NEAR upright position. If it was upright, there would be very minimal shear on those rivets, particularly the top rivets which would then be holding just tensional forces to keep the top skin on. With the inclined spar you now have shear and tension on those rivets. It's like a folding up a corrogated box. The box is pretty strong when it's square, take it a little out of square and you start to feel the strength diminishing rapidly. Not you need something laterally to hold the box from moving sideways to contine to hold it's strength in a less than 90 degree position. Not only that, but now what once was a vertical side (spar) now has a bending moment in it due to the fact that it's being asked to carrying a perdendicular force at an angle, instead of straight through the material.

 	  steveadams wrote: 	
		   	  ashontz wrote: 	
		   	  steveadams wrote: 	
		  While I appreciate your effort to search for failures, if you look back a bit further you will find no more 601 failures and 8 RV structural failures involving VMC-IMC, hard pull-up after low passes over airports and fields, aerobatics, improper construction, and 1 case that implied the pilot had overstressed the aircraft on repeated occasions in the past, and it finally failed in relatively benign conditions. While we need to ask questions, saying that the 601 "tends to break up in flight" is quite a stretch of the imagination. 	




The 601XL was introdued in 2001 I believe. That's probably the main reason you're not seeing anymore failures further back, there's been more time for fatigue to accumulate. 	




So what about the RV's tendancy to fail? If they have 9" spaced ribs and no forward sweep, why do they fail with such regularity?

steveadams

Joined: 10 Jan 2006
Posts: 191

Posted: Mon May 14, 2007 11:01 am Post subject: Re: NTSB search for breakup accidents.

I found 8 RV structural failures with a quick search. My point is not that RV's have a design problem, but that saying there is a tendancy for 601 wings to fail is not accurate, It is also innacurate to say that RV's do not structurally fail. You cannot base a hypothesis off an innacurate assumption, nor can you assume that because a 601 has fewer ribs or carries loads differently than another design that that is a flaw in the design.

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Mon May 14, 2007 11:10 am Post subject: Re: NTSB search for breakup accidents.

Ok, it's not a flaw then, its' just that the 601XL DOESN'T carry the loads perpendicular through the spar and a component of the load is thereby  redistributed through the skin rivets through shear to the top and bottom skins. It's not a design flaw, it's just different, and not as strong as if the spar was truly vertical.



Also, the RV is a 200 mph airplane and people tend to do more aerobatics in them. If the XL was as fast and more people used it the XL as an aerobatic plane, the number of structural failuires in the XL would probably be higher than the RV.



Mark Twain said there are 3 kinds of lies: Lies, Damned Lies, and statistics. Statistics can be misinterpretted in all kinds of ways.



Doesn't matter if we are talking about airplane wings or cardboard boxes. If I have a square box and a Salvador Dali square box that looks more like a trapezoid from the side both made out of the same materials and fasteners, the Salvador Dali box is going to fold first.



 	  steveadams wrote: 	
		  I found 8 RV structural failures with a quick search. My point is not that RV's have a design problem, but that saying there is a tendancy for 601 wings to fail is not accurate, It is also innacurate to say that RV's do not structurally fail. You cannot base a hypothesis off an innacurate assumption, nor can you assume that because a 601 has fewer ribs or carries loads differently than another design that that is a flaw in the design.

planecrazydld(at)yahoo.co
Guest

Posted: Mon May 14, 2007 1:52 pm Post subject: NTSB search for breakup accidents.

I know I said I would not comment any more but this is sort of a different comment: The XL wing is plenty strong - otherwise it could not accumulate the thousands of hours put on it since it started flying.



  It is possible that it is a little more sensitive to all the possible conditions, less than perfect workmanship, flight abuse, etc. I am virtually certain that CH will definitely document the relative change in twist during the currently planned series of limit then ultimate tests (I have not heard from him whether they monitored that during the original testing). In the big world all static airframe tests are monitored to collect all that type of data as it can be an indicator of an opportunistic sensitivity in an otherwise robust design.

   

  Again, CH is one of the people I have always admired - both for the stoutness of his aircraft as well as his willingness to stand up and deliver "ugly" planes that do what they were  designed to do better than the competition. Whatever he learns from the new round of testing will probably be disseminated as "un-spin-modified" as could be hoped for.



ashontz <ashontz(at)nbme.org> wrote:

  [quote]--> Zenith-List message posted by: "ashontz" 



Ok, it's not a flaw then, its' just that the 601XL DOESN'T carry the loads perpendicular through the spar and a component of the load is thereby redistributed through the skin rivets through shear to the top and bottom skins. It's not a design flaw, it's just different, and not as strong as if the spar was truly vertical.



Also, the RV is a 200 mph airplane and people tend to do more aerobatics in them. If the XL was as far and more people used it the XL as an aerobatic plane, the number of structural failuires in the XL would probably be higher than the  [quote][b]

JohnDRead(at)aol.com
Guest

Posted: Mon May 14, 2007 4:23 pm Post subject: NTSB search for breakup accidents.

The bending strength of a wing is in the spar not the ribs, they only serve  to help shape the air over the airfoil. If you read Grassroots Manufacturing by  Chris Heintz you will see that he likes thick wings. Yes more ribs will affect  oil canning but not improve the span wise stiffness. 

  

 The stiffness of a spar is defined by the formula. 

                     Stiffness  = bd^3

  

 the "d"  or depth being very important.

  

 John Read



See what's free at AOL.com. 

  [quote][b]

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Mon May 14, 2007 5:30 pm Post subject: Re: NTSB search for breakup accidents.

I beg to differ, the ribs not only describe the airfoil, they also help keep the spars aligned for and aft through the direct attach points, but more so through keeping the skin profile 'correct' through the various stations.



The ribs give the skin stiffness as well, the same way guides on a fishing pole help distrubute the fishing line force more evenly across the length of the rod. The more guides, the better the performance of the rod and the more the stiffness of the rod is utilized. Think about it, it is in fact the case. THe more ribs, the less moment goes into any one gap between stations, the more that same .025 skin can withstand, both in compression as well as tension, as per the fishing pole example.



There's a reason they call it a cantilever wing. Just as in a cantilever bridge, each one of those rib stations redistributes the loads acting on it. A 200 foot cantilever crane would be weak a hell with struts every 50 feet. but change that to every 10 feet and it's pretty damn strong.



The ribs keep the bottom skin from shortcutting up to meet the top skin at high positive Gs as well as providing attach points to hold the top skin on and restricting the length of any one 'crunch point' on the top to what the skin can handle at a given load.



More ribs do add more strength spanwise.



[quote="JohnDRead(at)aol.com"]The bending strength of a wing is in the spar not the ribs, they only serve  to help shape the air over the airfoil. If you read Grassroots Manufacturing by  Chris Heintz you will see that he likes thick wings. Yes more ribs will affect  oil canning but not improve the span wise stiffness. 

  

 The stiffness of a spar is defined by the formula. 

            ?        Stiffness  = bd^3

  

 the "d"  or depth being very important.

  

 John Read



See what's free at AOL.com. 

   	  Quote: 	
		  [b]

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