600115-005 intake duct

[Andy95W]

But with the exception of the M20E that crashed (and that had been modified/repaired with duct tape), does anyone have credible information of any other failures that caused crashes?

[EricJ]

3 hours ago, Pinecone said:

How is the orginal intake boot in any way fail safe????

I didn't design it, so I can't talk definitively to all of it or anything, but, as already mentioned, the originals are composite construction with an internal mesh.   Some have described that there were wire windings through theirs as well.   Both of those features reduce the likelihood of rips, tears, etc., from opening the bellows and allowing chunks and debris into the intake.   They also improve the integrity of the material to reduce fragmenting in the event of a failure.  They can also provide the additional rigidity so that most tear failures don't result in collapse of the boot.   

Basically, the original units have design history that seems to have born out well with few known catastrophic failures over many decades of deployment.   They weren't unsafe designs, so a deviation from a system with some good history using a different technology might reasonably present an expectation of some pretty thorough testing to vet "equivalent or better" performance.  Imho, anyway.

[MikeOH]

Frankly, I'm a little shocked at this talk of 'how is the original fail-safe'.  Good grief!  These ducts have been in planes without care or maintenance for decades with but ONE demonstrable catastrophic failure!  Yet, there is some expectation that they shouldn't be failing???

Come on!  If an oil hose burst because it hadn't been replaced since 1964 would you be questioning its lack of fail-safe design?

It seems to me that the original design is pretty well thought out and reliable.  I think we should be very careful before accepting a 'modern' design as an airworthy substitute.  Don't let scarcity and high cost color our judgement when it comes to a safe replacement.

[EricJ]

38 minutes ago, MikeOH said:

It seems to me that the original design is pretty well thought out and reliable.  I think we should be very careful before accepting a 'modern' design as an airworthy substitute.  Don't let scarcity and high cost color our judgement when it comes to a safe replacement.

I'm all for a new and modern design with modern materials, but a component like an intake boot that has some nasty consequences to potential failure modes should be able to demonstrate that those failure modes are mitigated in the design.   It should be attainable goal, but it shouldn't be trivial, either.

[PT20J]

The FAA requires that only approved parts can be installed on certificated aircraft.

An OPP done correctly can be an approved part. It is not required that the part be unavailable elsewhere, although it is usually easier and less expensive to procure the part if it is available than it is to make one. But, sometimes it's easier to make one. For instance, the rubber tail pipe hanger was ripped halfway through and I removed it and used it as a template to cut one from an old tire. The OPP had the same dimensions and was made of material similar to the original. My A&P was fine with it; he gave me the tire.

An OPP can only be produced by the owner (or operator) for their own aircraft and the owner must participate materially in the production although the owner can contract out the actual manufacture of the part.

The OPP generally must be installed by an A&P. If the OPP is part of a major repair requiring a 337, then it must be signed off by an IA. Once the part is installed and logbook entry made, it is considered airworthy. A subsequent maintainer should only reject it if it somehow becomes unairworthy (say your new duct has developed a hole). If it is made correctly, the OPP should be virtually indistinguishable from the original.

The difficulty in producing an OPP is that it must conform to approved data to be in accordance with the original type design. Since you are not going to get manufacturing drawings and material specifications from Mooney, the usual way to accomplish this is to copy an original. This is fairly simple with metal parts. At the museum, we had a brake caliper for a Bearcat machined to match the original unavailable cast part. Sheet metal parts are easily replicated. It gets more complicated when trying to replicate something like an intake duct. And, if you make it different than the original, even if it is "better", it might require a one-only STC.

Skip

 

[Pinecone]

11 hours ago, MikeOH said:

Frankly, I'm a little shocked at this talk of 'how is the original fail-safe'.  Good grief!  These ducts have been in planes without care or maintenance for decades with but ONE demonstrable catastrophic failure!  Yet, there is some expectation that they shouldn't be failing???

Come on!  If an oil hose burst because it hadn't been replaced since 1964 would you be questioning its lack of fail-safe design?

It seems to me that the original design is pretty well thought out and reliable.  I think we should be very careful before accepting a 'modern' design as an airworthy substitute.  Don't let scarcity and high cost color our judgement when it comes to a safe replacement.

The point was, some were saying the OPP design had to be fail safe.  And the original is not really fail safe.  It is a good design based on the performance, but it does not mean it has any specific fail safe design elements.

Yes, the replacement needs to be properly designed for the task, but it does not have to perform better than the original, just as well as.  

[MikeOH]

25 minutes ago, Pinecone said:

The point was, some were saying the OPP design had to be fail safe.  And the original is not really fail safe.  It is a good design based on the performance, but it does not mean it has any specific fail safe design elements.

Yes, the replacement needs to be properly designed for the task, but it does not have to perform better than the original, just as well as.  

Then the point wasn’t clearly made as I don’t disagree with what you just wrote.

The discussion had the flavor of, “since the original isn’t fail safe don’t worry about it”

My opinion is that without reinforcing of some kind an OPP duct is NOT going to be of “better performance” than the OEM part. Testing to establish equivalence is not going to be easy when it comes to showing long-term performance in the field under real-world environmental conditions.

[Pinecone]

I agree that if there is a way to make it more fail safe, that is good.  But equal to original is fine.

 

[EricJ]

3 hours ago, MikeOH said:

The discussion had the flavor of, “since the original isn’t fail safe don’t worry about it”

"Fail safe" apparently means different things to different people.    I'm used to the engineering standards I've been held to over my career, but that may not mean the same thing to someone else.   Simply put, I think it means that common failure modes aren't catastrophic and safety isn't put at risk, or, optimally, even compromised.   Common failure modes for things like intake ducts are tears, wear holes, cuts, etc., due to abrasion, etc., etc.   If it can be anticipated that those can happen without causing a catastrophic failure (which in this case could be anticipated to be ingestion into the intake or restriction of the intake), then that's a "fail safe" design.   On the original intake boots this was primarily covered by the composite design including sturdy mesh fabric, wire., whatever, so that the duct maintained shape even in the event of localized material failure, and helped to prevent chunks from being ingested.

I don't know whether the OPP part has similar features or not, but the suggestion was that it was the sort of thing that should be checked in order to meet the "same or better" criteria for OPP parts, since the original parts clearly had such features.

[MikeOH]

@EricJ

I, too, am used to more formal "fail safe" standards including single and multiple fault tolerance.  IHMO the original is a single fault tolerant design because of its composite construction; exactly as you point out it is tolerant of 'localized' material failure.

That is exactly my concern with a homogeneous materials approach (e.g. made entirely from molded silicone); it may not be even a single fault tolerant design.

[PT20J]

Question still is, if you make an OPP, what are you going to use for approved data to qualify your part?

[Shadrach]

1 hour ago, PT20J said:

Question still is, if you make an OPP, what are you going to use for approved data to qualify your part?

I don’t think this is as complicated as everyone is making it out to be. The part that was posted in this thread was clearly fabricated using the factory part as a guide to match original dimensions. The difference in fabrication material is the issue at hand - fabric reinforced rubber vs silicone. It should not be that difficult to compare the properties of each material to determine if silicone is a lessor, equal or superior substitute. 

[PT20J]

1 hour ago, Shadrach said:

I don’t think this is as complicated as everyone is making it out to be. The part that was posted in this thread was clearly fabricated using the factory part as a guide to match original dimensions. The difference in fabrication material is the issue at hand - fabric reinforced rubber vs silicone. It should not be that difficult to compare the properties of each material to determine if silicone is a lessor, equal or superior substitute. 

Ultimately it's up to the A&P that agrees to install it to ensure conformity with the type design.

[Shadrach]

21 minutes ago, PT20J said:

Ultimately it's up to the A&P that agrees to install it to ensure conformity with the type design.

Yep. So some approvals/denials will be more complicated than others.

[Joshua Blackh4t]

3 hours ago, Shadrach said:

I don’t think this is as complicated as everyone is making it out to be. The part that was posted in this thread was clearly fabricated using the factory part as a guide to match original dimensions. The difference in fabrication material is the issue at hand - fabric reinforced rubber vs silicone. It should not be that difficult to compare the properties of each material to determine if silicone is a lessor, equal or superior substitute. 

The problem is, is the silicone better in ALL ways.

Sure, it might be more heat resistant, stronger, more flexible and many many more. But it only has to fail in one way for it to be not suitable.

When I compare it to plastic intakes on cars, I would need a new part to have similar fail characteristics. So no matter how it cracks, is damaged, or downright cut in half, it still can't block the intake.

I'd make a suggestion that if its of one material, maybe each 2nd ring should be thicker so it will stay structurally stable even when the thinner ring cracks.

Then we can all argue about OPPs and what happened with downblocks. Although, if its 3d printed, the file could be shared (at a price) so anyone can make one

[EricJ]

5 hours ago, MikeOH said:

@EricJ

I, too, am used to more formal "fail safe" standards including single and multiple fault tolerance.  IHMO the original is a single fault tolerant design because of its composite construction; exactly as you point out it is tolerant of 'localized' material failure.

That is exactly my concern with a homogeneous materials approach (e.g. made entirely from molded silicone); it may not be even a single fault tolerant design.

That's my concern as well.    In this application it's not an unreasonable assumption to limit considered failures to localized issues due to deterioration or abrasion or whatever kind of damage, since it's supposed to get inspected periodically to catch the larger failure cases.   The recent catastrophic failure case doesn't fall under that umbrella since it was dragged out way past the anticipated design failure modes, sadly due to part unavailability.    So somebody coming up with a suitable OPP part is admirable and desirable, but it does need to be able to do the job.

[MikeOH]

1 hour ago, EricJ said:

That's my concern as well.    In this application it's not an unreasonable assumption to limit considered failures to localized issues due to deterioration or abrasion or whatever kind of damage, since it's supposed to get inspected periodically to catch the larger failure cases.   The recent catastrophic failure case doesn't fall under that umbrella since it was dragged out way past the anticipated design failure modes, sadly due to part unavailability.    So somebody coming up with a suitable OPP part is admirable and desirable, but it does need to be able to do the job.

That's it in a nutshell.  Will a new design part survive as long as the old one before deteriorating to a dangerous condition.  As you say, they should be periodically inspected but we know the old ones aren't...yet they survive surprisingly well over many decades...it's going to be tough to prove a new design will be as robust to neglect!  Functional longevity is going to be difficult to prove equivalent to the OEM design.

[PT20J]

40 minutes ago, MikeOH said:

That's it in a nutshell.  Will a new design part survive as long as the old one before deteriorating to a dangerous condition.  As you say, they should be periodically inspected but we know the old ones aren't...yet they survive surprisingly well over many decades...it's going to be tough to prove a new design will be as robust to neglect!  Functional longevity is going to be difficult to prove equivalent to the OEM design.

Well, if I were the A&P, asked to approve the installation of the part, I would ask the owner for approved data. The owner could present the original part, which I believe was reinforced neoprene, and the new silicone part. They are different. By what basis would I know that the new part was equal to or better than the original? It would be the owner’s responsibility to prove that since the owner produced the part. How would you do it?

[MikeOH]

1 minute ago, PT20J said:

Well, if I were the A&P, asked to approve the installation of the part, I would ask the owner for approved data. The owner could present the original part, which I believe was reinforced neoprene, and the new silicone part. They are different. By what basis would I know that the new part was equal to or better than the original? It would be the owner’s responsibility to prove that since the owner produced the part. How would you do it?

Uh, I think we are in violent agreement!  Your point is exactly MY point...just stated a different way.

I want some evidence as the proposed silicone part is DIFFERENT, substantially, from the OEM.

[EricJ]

15 minutes ago, PT20J said:

Well, if I were the A&P, asked to approve the installation of the part, I would ask the owner for approved data. The owner could present the original part, which I believe was reinforced neoprene, and the new silicone part. They are different. By what basis would I know that the new part was equal to or better than the original? It would be the owner’s responsibility to prove that since the owner produced the part. How would you do it?

That's more or less the basis for the concerns.   How "identical" something has to be to qualify as a reverse engineered duplicate, which is a recognized method of producing an approved design, is arguable.    I recall there were people pushing to have the metallurgy exactly duplicated in the downlock blocks, but there was input that they were made of essentially factory scrap potmetal, so that was probably unnecessary overkill.  How much is enough or too much is a little subjective, so there's no rigid guidance.   It does seem more than reasonable, especially for a part where failure of an insufficiently engineered design can create a safety issue, to ask questions when a new design substantially deviates from the original part. 

You're right that the A&P is supposed to be the gatekeeper for detemining whether a part is appropriate for installation, but that can be based on the documentation provided by the owner in the production of the part.

So, yes, a professional butterfly collector can produce a part for his own airplane and it can be approved for installation by any A&P.   Given what we know about the variance in technical evaluation skills across the broader A&P population, it does kinda stretch the imagination of why we ponder these things so much, but it is definitely worthwhile to try to keep the standards reasonable. 

[Allen_D]

On 11/20/2023 at 10:58 PM, MooneyJohn said:

This is a prototype I have been working on. Starting on a mold design to make a silicone version.

Would you be willing to share your drawings of this part? Or is someone willing to send me an old unserviceable part?

I would like to analyze the deformation/stress/strain of this part using different materials.

[00-Negative]

2 minutes ago, Allen_D said:

Would you be willing to share your drawings of this part? Or is someone willing to send me an old unserviceable part?

I would like to analyze the deformation/stress/strain of this part using different materials.

I have my old duct I can send you. PM me your name/address and I'll try to get it in the mail tomorrow. 

- David Vizinat

[Allen_D]

On 1/22/2024 at 11:37 AM, 00-Negative said:

I have my old duct I can send you. PM me your name/address and I'll try to get it in the mail tomorrow. 

- David Vizinat

David,

I appreciate your contribution of the old duct for my project.

I've been conducting thorough analyses using Finite Element Analysis (FEA) to assess the performance of my version of the induction coupler under various loads. The primary goal is to ensure it won't collapse and adversely affect the engine. Considering the potential for backfire in its location, I've decided on using a flame-retardant material with high tear strength.

In examining worst-case scenarios, such as full RPM at 0 knots, I introduced a safety factor by increasing the maximum RPM by 20%. The latest test involved running the coupler at 3,240 RPM, utilizing a Shore 60A urethane. The results indicate minimal deflection, with approximately 2mm on the bottom surfaces and 1mm on the top.

Notably, I conducted a case at an exaggerated 2x the RPM, resulting in only 6mm of deflection. Responding proactively, I reinforced the design to ensure robustness under extreme conditions.

Moving forward, my next step involves creating the mold and experimenting with various urethanes to refine the manufacturing process. I plan to document these practices meticulously to verify and build upon my findings. If successful, I'll certainly send you a usable product.

I'll keep you informed of my progress.

- Allen

[MikeOH]

@Allen_D

How are you addressing the apparent difference that your design does NOT incorporate any secondary reinforcing material (metal mesh)?

[N201MKTurbo]

6 minutes ago, MikeOH said:

@Allen_D

How are you addressing the apparent difference that your design does NOT incorporate any secondary reinforcing material (metal mesh)?

The ones I have repaired didn't have any metal mesh. The tape they were laid up with appeared to be either cotton of fiberglass reinforced butyl rubber.