• If You Are Having Trouble Logging In with Your Old Username and Password, Please use this Forgot Your Password link to get re-established.
  • Hey! Be sure to login or register!

Super Cub Structural Failure

If you've ever gotten a replacement for a change of address, the date on your license will be that date, not the original issue date.
 
If I recall this thread started with concern of rear spar failures.
A number of years back when I was looking at the Javron cub at his display at Osh I was curious about the box structure aft of the rear spar, In this image look at the top center forward of the aileron and flap juncture.
P5240011.jpg

I now recognize the sheet structure mid span is there to reduce the chance of the spar buckling. This structure is added to greatly resists any fore-aft movement of the spar caps reducing the chance of the spar bending from high loads.
 

Attachments

  • P5240011.jpg
    P5240011.jpg
    134.1 KB · Views: 349
With regards to Chutes in Cirrus’s: are there any Cirrus owners on here, that have done the Cirrus recurrent (or initial) training, that can tell us what Cirrus teaches?

I had a Cirrus owner tell me Cirrus standard Ops was to pull the BRS in the event of an engine failure?

Is there anyone who has gone through the Cirrus Simulator program that can elaborate?
 
I’m not a owner nor have I had citrus training, but my understanding from those that have is you are correct. ANY power issue and they say pull the handle.


Sent from my iPhone using SuperCub.Org
 
Demonstrated Deployment Parameters

400’ (561’ G5*) - Demonstrated loss of altitude from a straight and level CAPS deployment

920’ (1081’ G5*) - Demonstrated loss of altitude from a 1 turn spin

135 KIAS - VPD (SR20 G1/G2) - Maximum demonstrated deployment speed for CAPS

133 KIAS - VPD (SR20 G3/SR22/SR22T) - Maximum demonstrated deployment speed for CAPS 140 KIAS - VPD (SR22/SR22TG5) - Maximum demonstrated deployment speed for CAPS

*Demonstrated parameters for the G5 were calculated from G5 parachute drop tests simulating a 3,600 lbs airplane

Possible CAPS Deployment Situations

CAPS should be activated in the event of a life-threatening emergency where CAPS deployment is determined to be safer than continued flight and landing.

• Loss of Control - A loss of control is when the airplane does not respond as the pilot expects and may result from flight control or system failure, turbulence, disorientation, icing or pilot loss of situational awareness. If a loss of control occurs, CAPS should be activated immediately.

• Engine Failure Not Over a Runway – If a forced landing is required onto any surface other than a runway, CAPS activation is strongly recommended. If a forced landing over rough or mountainous terrain, over water, in fog, at night, or in low IMC conditions is required, CAPS activation is strongly recommended.

• Engine Failure Over a Runway - During engine failures within gliding distance of a runway, the pilot must continually evaluate the situation.

- At 2,000 ft AGL, if the landing is assured the pilot may continue to the runway. If not assured then activate CAPS.

- At 1,000 ft AGL, if the landing is still assured, the pilot may continue, recognizing that the risks associated with landing short, runway overrun or low altitude loss of control likely exceed those of a timely CAPS deployment. If the landing is not assured by at least 400 ft (561 ft G5) AGL the pilot should immediately activate CAPS.

• Pilot Incapacitation - Pilot incapacitation may occur from a wide variety of causes, ranging from a pilot’s medical condition to a bird strike that injures the pilot. If incapacitation occurs and the passengers are not trained to land the aircraft, CAPS activation is strongly recommended.

• Mid-Air Collision - A mid-air collision will likely render the airplane uncontrollable by damaging the control system or primary structure. Unless it is apparent that structural and control system damage has not occurred, CAPS activation is strongly recommended.

• Structural Failure - A structural failure has never occurred in a Cirrus aircraft. However, if a structural failure were to occur, CAPS activation is strongly recommended.


Sent from my iPhone using SuperCub.Org
 
Another approach at this structural failure, A question for Mike Patey if you are in audience.
You stated the investigators cut out all the structure as they determined they need.

How about the other wing?

Could the fabric be opened up to photograph the structure for all to see?
My curiosity is if any additional bracing had been added around the lift strut attachment.
I expect there is no additional structure there. We have many guys and gals here who would love to get a good look at the lightening holes in these spars.
I was going to add more comment but will refrain until we all can get a chance to see both front and rear spars in this plane.
 
Charlie, that's what I was thinking, too. Thanks for bringing it up.

I keep looking at that Javron box structure too, doing my best to imagine the forces.
 
Charlie, that's what I was thinking, too. Thanks for bringing it up.

I keep looking at that Javron box structure too, doing my best to imagine the forces.

In the Pipers, the metal spars get most all their strength in the web. The spar caps offer very little strength in the overall picture. The spars in reality rely on the ribs to prevent twisting which long ago it had been determined to not have much in reserve. Therefore the additional structure is used to box the spar to prevent twisting which is the act of buckling.
 
Actually it is the compression ribs which control the twist. The ribs only hold the fabric shape.

I've often wondered why the compression struts did not have some vertical triangulation. The only compression member with anti-twist between spars is the N-Strut.

I did a test on wood ribs and found that each had a measurable amount of resistance to spar twist. I'll have to go back to my notes, but I think it was around 12-15 pound-feet twist before failure at the rear spar point. Add those ribs up and there is something there.
 
Actually it is the compression ribs which control the twist. The ribs only hold the fabric shape.
Hate to be the critic but you should go over the math some more. The struts are way to few and far between to offer much of any resisting to twist / buckling of the spars.

The testing RV did is pretty darn close to the numbers that Piper's style of riveted ribs as well.
If you refer back to the image that Steve P posted of a CC failed rear spar you will note the total buckling is between ribs. If the added support of what was designed in for the 2000# gross weight had been added that buckling might well have been much different if not even failed.
 
Hate to be the critic but you should go over the math some more. The struts are way to few and far between to offer much of any resisting to twist / buckling of the spars.

The testing RV did is pretty darn close to the numbers that Piper's style of riveted ribs as well.
If you refer back to the image that Steve P posted of a CC failed rear spar you will note the total buckling is between ribs. If the added support of what was designed in for the 2000# gross weight had been added that buckling might well have been much different if not even failed.
This picture?
attachment.php


The buckling was prevented at the compression rib. The buckling which did occur was caused by an outside impact force.
 
This picture?
attachment.php


The buckling was prevented at the compression rib. The buckling which did occur was caused by an outside impact force.

Air loads or impact, it is still a single point failure. Looks to me it is very clear nothing prevented or reduced any buckling. The failure was not spread out as should be if the engineering were closer to what is should be.
 
Air loads or impact, it is still a single point failure. Looks to me it is very clear nothing prevented or reduced any buckling. The failure was not spread out as should be if the engineering were closer to what is should be.
Charlie, You know that the engineering is done for the flight loads, not crash damaging loads. You are mixing apples and oranges.
 
Charlie, You know that the engineering is done for the flight loads, not crash damaging loads. You are mixing apples and oranges.
No, no I am not. A simple ground loop should not destroy a major component of an aircraft. There have been in flight failures of Cub wings dating back many decades.
A fuselage should not collapse around it's occupants with mild mishaps. There are to many that have done so.
I have spent my life working on survivability of structures. The plane I am building is just one example showing the differences in structure to achieve the goal. It is not for everyone but I believe in walking away, even if one might be limping.
Single point failures should have been designed out of these structures long ago. Some companies have addressed a number of the issues, but the severity of that spar failure from what is not even a crash makes it clear that some have not.
The Cub fuselages have needed quite a few added braces which most people do, kudos to them since they care and understand. But no one has addressed failure of landing gear mount points, Why? Safety cables, Why? Fix the problem.
The main reason I am building a look alike is I have no desire to even consider the lack of survivability in my original J4.
Someone might want to continue with the restoration of my original plane but it is not me.
On my current build some people feel my wings are a copy of the RV series of wings, they are not. Wings should not buckle in flight especially when the fix does not even add weight. End
 
There have been in flight failures of Cub wings dating back many decades.

okay, now you have me worried. What is the failure mode of that wing? Does it happen to J3s as well? I had not heard of an in-flight Cub wing failure, except after failure of a lift strut.

I have cold- straightened spars. The amount of force that takes is stunning!
 
There have been in flight failures of Cub wings dating back many decades.
.

I have cold- straightened spars. The amount of force that takes is stunning!

That because you didn’t heat it first and reheat treat it before straitening it. Bringing it to it’s annealed (soft) condition temporarily. (I am not a fan of doing this either..) just replace spar



Sent from my iPhone using SuperCub.Org mobile app
 

Attachments

  • IMG_9952.JPG
    IMG_9952.JPG
    170.7 KB · Views: 176
  • IMG_9953.JPG
    IMG_9953.JPG
    87.4 KB · Views: 186
  • IMG_9954.JPG
    IMG_9954.JPG
    91.4 KB · Views: 190
  • IMG_9955.JPG
    IMG_9955.JPG
    84.5 KB · Views: 188
  • IMG_9956.JPG
    IMG_9956.JPG
    86.4 KB · Views: 169
  • IMG_9957.JPG
    IMG_9957.JPG
    82.1 KB · Views: 174
  • IMG_9958.JPG
    IMG_9958.JPG
    92.3 KB · Views: 160
  • IMG_9959.JPG
    IMG_9959.JPG
    86.7 KB · Views: 174
  • IMG_9960.JPG
    IMG_9960.JPG
    98.2 KB · Views: 159
  • IMG_9961.JPG
    IMG_9961.JPG
    87.4 KB · Views: 144
  • IMG_9962.JPG
    IMG_9962.JPG
    81.3 KB · Views: 139
  • IMG_9963.JPG
    IMG_9963.JPG
    88.3 KB · Views: 147
  • IMG_9964.JPG
    IMG_9964.JPG
    84.7 KB · Views: 143
  • IMG_9965.JPG
    IMG_9965.JPG
    90.1 KB · Views: 147
  • IMG_9966.JPG
    IMG_9966.JPG
    94 KB · Views: 130
  • IMG_9967.JPG
    IMG_9967.JPG
    93.6 KB · Views: 133
  • IMG_9968.JPG
    IMG_9968.JPG
    93 KB · Views: 133
Last edited:
No, no I am not. A simple ground loop should not destroy a major component of an aircraft. There have been in flight failures of Cub wings dating back many decades.
A fuselage should not collapse around it's occupants with mild mishaps. There are to many that have done so.
I have spent my life working on survivability of structures. The plane I am building is just one example showing the differences in structure to achieve the goal. It is not for everyone but I believe in walking away, even if one might be limping.
Single point failures should have been designed out of these structures long ago. Some companies have addressed a number of the issues, but the severity of that spar failure from what is not even a crash makes it clear that some have not.
The Cub fuselages have needed quite a few added braces which most people do, kudos to them since they care and understand. But no one has addressed failure of landing gear mount points, Why? Safety cables, Why? Fix the problem.
The main reason I am building a look alike is I have no desire to even consider the lack of survivability in my original J4.
Someone might want to continue with the restoration of my original plane but it is not me.
On my current build some people feel my wings are a copy of the RV series of wings, they are not. Wings should not buckle in flight especially when the fix does not even add weight. End

Thats the dog chasing his tail theory. Stronger heavier is more survivable but in fact it will crash at a faster speed. A lighter craft designed for the job + 10% added for stupidity will crash at a lower speed. Speed kills.

Glenn
 
Piper spars can only be legally straightened cold. There is a service bulletin, memo or instruction with the criteria. They don't cost that much, buy a new one and get on with life.
 
Both of those are true. I replaced the spars, then tried my hand at the service bulletin straightening. I was successful. It was a lot of work - you do not do it without some pretty robust tools.

Again, I want to know what the failure mode is - if it fails in flight, it would be fatal. It only took a couple of lift strut fatalities to cause an AD of some fame. I think there were four lift strut failures, not counting the ones caught at inspection. You would think that if the rear spar had a history of buckling under aileron load, there would be an AD against it. Does it have such a history?
 
No, no I am not. A simple ground loop should not destroy a major component of an aircraft. There have been in flight failures of Cub wings dating back many decades.
A fuselage should not collapse around it's occupants with mild mishaps. There are to many that have done so.
I have spent my life working on survivability of structures. The plane I am building is just one example showing the differences in structure to achieve the goal. It is not for everyone but I believe in walking away, even if one might be limping.
Single point failures should have been designed out of these structures long ago. Some companies have addressed a number of the issues, but the severity of that spar failure from what is not even a crash makes it clear that some have not.
The Cub fuselages have needed quite a few added braces which most people do, kudos to them since they care and understand. But no one has addressed failure of landing gear mount points, Why? Safety cables, Why? Fix the problem.
The main reason I am building a look alike is I have no desire to even consider the lack of survivability in my original J4.
Someone might want to continue with the restoration of my original plane but it is not me.
On my current build some people feel my wings are a copy of the RV series of wings, they are not. Wings should not buckle in flight especially when the fix does not even add weight. End
Charlie, I agree with you that should not destroy, should not collapse, should have been designed, should not buckle in flight ought to be designed into an airplane. However show me in CAR 3 and CAR 4 where it says should not. There are specific parameters spelled out, should not is not among them. FAR 23 was an attempt to correct the deficiencies of those earlier regulations. That is just one reason why the costs of certifying a new design have risen as it has.

I wasn't there, but that wing in Steve Pierce's picture was not the result of a simple ground loop. Landing gears do have side load requirements, there are no requirements that a wing not be permitted to buckle when it strikes the ground.

You are to be commended for your attention to detail and safety in the building of your plane. You will also have to admit no manufacturer would design and build a plane such as yours, expecting to turn a profit. The manufacturers follow the regulations to a minimum, and sometimes exceeding the regulations, but to the extent you are doing.... not so. If the CAA now the FAA required it, that would be a different story.
 
I have never heard of an in flight failure of a Piper wing exceept in the book "Wager With the Wind" on a PA14.
 
Yes, and in that case consider the source. if I’m not mistaken, all the strut issues I’m aware of, were a result of corrosion or other factors which weakened the structures. If you want a scary picture, look at a photo of the Taylorcraft “strut failure” that brought about the latest AD. That airplane was not even close to airworthy. If I recall, the original Piper strut AD was precipitated by similar conditions.

MTV
 
Back
Top