Q-talk 87 - Safety Alert - Cracked Carbon Spar
- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 16:24
- Written by Dave Richardson
- Hits: 1394
Cracked Carbon Spar
Extracted from the Q-List e-mail server
Ed Note: The following is a series of e-mails describing the cracked carbon fiber spar Jim Patillo found upon doing his annual. Anyone with the LS1 canard with the carbon fiber spar needs to be aware of this potentially serious problem.
Helllp! I started doing my yearly inspection yesterday and found a strange thing!
My carbon spar was cracked at about BL15 on the passenger's side, (between passengers legs). The crack is on top of the spar about 3" long and is perfectly horizontal to the spar. It does not appear to be a compression failure as it looks like a hairline fracture. I jacked up the fuselage so the right wheel was off the ground and the crack widened. Can anyone explain what might of happened? I have had only a couple of hard landings and have never pulled any serious negative "G"s. This is unnerving because nothing on the fuselage is cracked.
What is the procedure for repairing the spar? Do I repair it with glass or carbon? How much past the crack must I glass for the structure to be sound again.
Jim Patillo N46JP Q200
It sounds as though you have had a primary structural failure. If the carbon spar has indeed cracked, this is an extremely serious occurrence, and you have been extremely lucky not to have experienced a catastrophic in-flight failure. This aircraft should not be flown under any circumstances until the cause of the failure is established and the repair designed, verified, installed and tested.
In the normal course of events, this would be cause for a
mandatory grounding of all aircraft of this type until the cause of the failure has been established. At the very least there should be a mandatory before next flight inspection of every Q-200 in the area you have identified on both sides! Because it is easy to inspect, this should be included in the preflight checks for all carbon sparred Q's. If anybody else finds or has found a similar failure, it is very important to post notification to the list.
There is no engineering authority or manufacturer who has responsibility for this type so I guess that this list is it.
As to why it may have happened, several possibilities exist. These include, but are not limited to, the following:
1. A substandard spar being supplied. In order to establish if this is the case, you need to establish precisely where your spar came from, who built it, what materials were used and to what design spec the spar was built. If this information cannot be established, it does not mean the spar is wrong but rather that it cannot be conclusively proven to be correct.
2. A manufacturing defect was incorporated in the spar. The above applies.
3. The spar was incorrectly installed - it is necessary to revisit the build documentation, to verify if each of the steps specified in the plans was documented/ticked off whatever each of us does to ensure that we did not overlook any step. It is possible that one interpretation of the plans may differ from another and this may have resulted in omission or misalignment of some lay-ups. The point to note here is that if it happened to you, it could happen to others.
Unidirectional composite elements need to be wrapped in order to prevent the buckling/splitting failure, which you have experienced. It is a compressive failure. If those wrapping layers are missing or incorrectly oriented, what you have seen is to be expected. If you are seeing a straight axial crack in the top of the spar, can you confirm that the carbon spar material is exposed to your view?
As to the most likely scenario, as your airframe is relatively young, and by your own admission free from mistreatment, it is most likely that either there is some structure missing or you have a dodgy spar. There are some fairly high hour Q 200's out there and they exhibit different (and much more benign) failure modes.
Do not be surprised by the fact that the fuselage shows no sign of failure - it does not react any significant portion of the bending load in the canard at all.
Before starting to even consider repair, it is essential to unambiguously establish why the failure has occurred, otherwise we finish up fixing things that are not broken whilst not fixing the real problem.
Regards, John Tenhave
The main theory is that I may have had a defective spar from the git go. I found a picture of Ron Lungren at QAC load testing a spar in newsletter 21-24. It was captioned that "every spar is load tested". Take a good look at the device he was using. My spar failed just in front of the clamping device. The spar may have been over stressed in test and the crack was so small it went undetected.
I will cut out the underbelly and into the canard tomorrow and inspect the whole area for cracks, etc.
Here is some background regarding my airplane. I do not have the full width header tank. My tank holds 5.5 gallons and is over the passenger side only, terminating about 3" from the firewall and from right side to just clear of the radios. Therefore, I suppose I lost some rigidity not having the tank completely across the top of the fuselage as per plan. (I originally saw this tank design on Gordon Pratts Q2/C85 and used it because of radio clearances).
My two side gussets are about 2.75" wide x 9". They connect over the spar and run up side walls only, not connecting to the tank. The original plans showed this gusset in a picture with a side note, stating to run from canard to tank but additional note said "omitted from the picture for clarity." I missed that!
Additionally, I did not have the two gussets (2.5" x 7") installed on the firewall at 16" apart. I received the original plans and spar one week after release from QAC and my plans did not show the firewall gussets. Farnam has a later picture showing these gussets. This situation has been discussed in detail by our EAA members and most all do not feel this caused the problem. It certainly did not help the situation but we do not believe it caused it. There is no sign of cracking on my exterior skins.
The spar repair looks straightforward. I think this will be the same as originally joining the two halves. We are still debating exactly how the spar repair takes place. Additional repair will include gusseting from the spar to the tank and forming another gusset on the left side from spar up and over to the tank on the right. Firewall gussets will be installed.
Would you agree the crack looks like a tension failure more than a compression failure? As we all know, unless one is doing outside loops, it would be hard to induce a tension failure. I HAVE NOT BEEN DOING OUTSIDE LOOPS! If it was a compression failure, why isn't the crack more deviated? I have flown my AC at 1285 lbs. several times. I have made at least two hard landings but not enough to go around. Again things are pointing to a possible flawed spar.
The good news is that it was caught in the annual inspection. I'm still here to talk about it and hopefully this information may someday save someone else's bacon. If this can happen to me, it can happen to others.
I will keep everyone informed through the process.
Regards, Jim Patillo N46JP Q200
The crack is about 11.5" from centerline on the passenger side. Everyone agrees this is a strange area for this crack to appear and the best guess is factory damage. Again I would say, if this can happen to me, I could happen to you. If you have a carbon spar, please check it over carefully on a regular basis.
Thanks to John Tenhave, my plan now is to ultrasound or x-ray the spar before the repair begins to see if there is other damage. The point here is to know for as sure as possible what caused the problem before the repair begins, so as to not duplicate it again I feel confident this repair can be made satisfactorily.
The weight of the AC on the ground is about 4 G, as far as I can tell. In normal level in-flight condition, loading is about 1G. The thinking here is once repaired and fully grossed, I should be able to apply 7 or 8 G's just by pushing up and down on the fuselage. Should I consider other over gross loading tests? I will not do drop tests as I believe there are other ways to accomplish the objective without destroying the paint job.
When the repair starts, I will probably use E-glass with double 45-degree orientation Bi-directional UNI for the first 4 wraps, then install 30 ply of straight UNI for top caps and 25 straight UNI for bottom caps. Once complete, I'll back out of the hole I cut with micro and blue foam and reapply wing UNI covering as per schedule.
I had a long talk with Duane and Scott Swing, formerly of QAC fame, and found that Scott was indeed the "head spar checker outer". He relayed the spar test process.
There was a fixture used which was a tube the size of the large end of the spar and mounted on a frame 5-6' off the ground. The spar was slipped into the tube small end first and then pounded into place. The "tube" held the spar in grip at between 12-18" from its end. Since the tube was square, it did create a shear point as the spar exited the tube according to Scott.
The test was to load 700 lbs. or 2G at the end of the spar. If the spar snapped, creaked or the load touched the floor (from a certain loading height) when fully loaded, it was rejected. Scott, in fact, told me that he used failed spars and reworked them for his Q200 without any problems.
He surmised that my problem was just as I had expected. "The spar did not touch the ground but might have been damaged in the test", since they did throw several away.
This may explain the appearance of a tension crack. As the plane was taxied and landed repeatedly and after a few gross weight landings, the area failed, either in tension or compression. SO, IF I OWNED AN LS1 CANARD WITH A CARBON SPAR, I WOULD MAKE A SPAR CHECK MANDATORY ON REGULAR INTERVALS! It might just make the difference in having a good day or not.
There was no orientation for installation (spar is symmetrical) so the damaged spar may have been installed in such a way as to have the weak point on the top side of the spar as installed in the airplane.
He and Duane agreed the repair was straight forward and suggested I do the same lay-up schedule according to original plans but use 5" carbon graphite unidirectional tape over the 3 ply bi-directional wrap. He further stated the plane has approximately 4 G's on it when setting on the ramp. His opinion was that taxing around would put more load on the structure than I will ever get airborne. He pointed out that the flex of pushing down on top of the fuselage induces more load that you ever see looking out the window in flight. I am also considering static loading inside the cockpit to about 3 G's @ gross weight. Any comments? Has anyone confirmed ground G-loading by calcs? Are there any "numbers guys" out there that can verify this loading?
Scott also said he had found a Q kit and was planning to build another one. He said that he had considered putting the Q kit back into production. Further stating because of what he had learned from designing and selling the Velocity, he felt he could improve on the design and build time significantly. I hope he moves forward with his idea. That would be good news indeed!
Regards, Jim Patillo N46JP Q200
This message was sent to Scott Swing at Velocity Aircraft. He was in charge of spar testing during the introduction of the Q200.
It was good to talk with you and your dad the other day. Since you supervised this test (Ron Lungren probably did it!) at QAC, it's great to get information from the horse's mouth even though it was light years ago.
The pix you are looking at shows the crack which viewed from the right wheel pant goes from 270 degrees (gas tank side) to 20 degrees (firewall side). Note the location of this crack in the airplane. Also note, I made an accidental cut on the bottom of the spar with a Dremel Tool when opening up the area. (Damn - How do I repair that?)
Having looked at these pics, do you think the spar could have been over stressed during the load test at QAC? Take a close look just left of the crack. It looks like clamp or tool marks are imbedded in the spar at about 10.5" from center-line. Can you make any suggestions to help me in the repairs?
Should I use glass or carbon and should the lay up be the same schedule as we used when joining the two halves? Do you think I can make this repair without going through the side of the fuselage? I have about 6" on the right side of the crack and 9" on the left side of the crack.
Can you tell me anything about the thickness of the spar? Any help would be greatly appreciated.
Regards, Jim Patillo N46JP Q200
For your reference, there were two types of carbon spars made for the Q200. One built by Larry Howell from Dallas, Texas and another spiral wound spar built in California somewhere. My information according to Scott Swing and Larry Howell is that the spar failures were with spiral wound units and not the mandrel wrapped auto clave versions that Larry made. The spiral wrap is black and the autoclave version is yellow or orange colored, I think. Larry put on an outer color for sanding purposes.
Jim Patillo N46JP Q200
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