Structure modification no OME loads

[ashco]

Hello,

I am working for a small company installing equipment on a FAR 23 aircraft fuselage. The installation includes antenna and external pod affecting the fuselage skin stringers and frames. Since we have no access to the OME stress reports, internal loads, external loads, I wonder what design and analysis approach I can use to ensure compliance and FAR 23 STC certification

 

[rb1957]

welcome to my life !

First thing ... talk to your local certification organisation, and/or look around for someone to help with certification. Where are you ?

For practicalities, it depends on the scope of your mod. Maybe you'll have to re-do the airplane loads, maybe just local anlaysis. If your modding the wings, there's a tonne of work to do; if the fuselage, much more straightforward. If a radome, more work (birdstrike ?). If working within the cabin (ie not at the extreme nose or tail) then easier (you are effectively moving payload from the cabin to the fuselage).

But the basic idea in working without OEM data is to make conservative assumptions (like inertial loads of 10g).

Working with FAR23 ... use the old version (amendment 64?) ... the new version (amendment 2000?) is "crap" !

Read up on the ACs (advisory Circular) ... it outlines an acceptable means of compliance.

Good Luck !

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

Yep, as rb says, you need to develop your own loads, and get them approved for your mod.

 

[verymadmac]

Note that Loads can be as simple as demonstrating that its still going to break somewhere else first. Max possible skin shear loads can be determined by IDing & counting rivets in the skin panels, etc etc. Do be aware of methodology limitations, such as flange crippling calculations should be used comparatively (with care) rather than to determine an absolute. SRM's are also a god place to look to get a handle on local stresses.

 

[WKTaylor]

Pressurized or unpressurized?

Upper [crown], lower [belly] or to-the-side locations?

High speed or low speed?

Antenna size and mass + cabling...?

Any new frame or bulkhead cutouts/holes/enlargements required for mounting and/or wire-harness pass-thru?

Straight-forward load-path or complex load path?

Interfere with any interior placements/equipment/interior close-out?

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[ashco]

Hi Buddies,

Thank you very much for sharing ideas from your experience! I forgot to mention that the aircraft is pressurized.
I am now in preparation of a plan to be presented to our local certification authority (like FAA). The issue of cutting small penetrations for antennae and hardpoints for external equipment attachments involving skin and stringers seems not too complicated as SRM solution could be implemented using higher than skin gage doublers to restore limit and ultimate load capabilities. The issue I am facing is with the hardpoints supported by intercostals connected to adjacent frames. Unknowing the frame stress level and strength capability, I am adding loads impacting the frames locally. Any idea how to strengthen the frames at the attachment? I am planning to add a frame, back to back, with the existing ones with capability to carry the additional loads, ignoring the existing ones. The issue is how far to go with the new frame.
Please help with any idea.

 

[Ng2020]

Hi Ashco, to clear up and ambiguity about the structural mod youre proposing, could you provide a sketch of the hardpoint, intercostal, frames, and back-to-back frame segments? Are the intercostals new or existing?

On another note - are the externally-mounted items large enough that they could be adversely affected in the event of a cabin decompression?

 

[rb1957]

ok, you're doing what we call a certification plan. Not sure why you're not telling us where you are ?
At it's core a cert plan describes which standards you're meeting, which amendment level (the latest amendment ? the level at type certification of the plane, something inbetween ... what we call "Changed Product Rule"), and how you'll show compliance (analysis only, analysis and ground test, analysis and flight test).

These days we add a lot of description of the mod. The questions you pose (the extent of structural reinforcement) depend on the magnitude of the loads. If some little antenna (even large VHF blades) I find that applying an abuse load requires sufficient structure to "hand wave" the real loads ... typical a simple loads analysis to show the abuse load is clearly the critical loading. Yes, learned colleagues can poke holes in that ... remember, it's a simple sentence not a complete description of all possible cases, remember the correct story depends on the loads of the particular mod. A simple antenna would usually have a C-channel beaming the loads to the frames, local doubler on the frame, typically good enough. Larger loads require more.

When you talk to your local airworthiness people they'll remind of things you haven't included. But, they can also remind you of things late in the project which can be a significant bummer.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

Hi ashco,
Welcome to the STC world.

If I try to look ahead to the end-goal of your modification, I think you need to find out who will be signing off the findings of compliance for the requirements. If your local airworthiness authority will do it themselves, then they should be directing you on acceptable ways to carry out the analysis and/or testing. More likely, you need a DER to be signing off the project's certification plan, so that's the person setting the standard for your work. If you don't have a DER involved in your project then your project has a low probability of success.

I'm basing part of my answer on your member profile, which says you are in Illinois, USA. If that's not correct, then you may have to play by different rules (but in most parts of the world the rules are quite similar).

 

[ashco]

Hi all,
Thank you for your response. Now I can be more specific. I am no longer in the US. Sorry cannot provide more details about it.
The project is a mod of King Air 350. We are still in negotiations with the certification authority on which amendment to be included. We are trying to do to the original one (9 g crash landing required vs 18 g recent amendment). This is a preliminary evaluation to prepare a certification plan to present to our authority for review so I am not yet sure which approach to use. One option (the costly one) is to prepare full external and internal loads and to modify existing structure accordingly. The second is to propose a structure conservative mod that adds modifications assuming all added loads carried by newly added parts. Since my background consists of working only with OME’s where information is available, I am not sure what approach to follow in my tiny organization.
As the installation area of the pylon is in fuselage/wing common area, this further complicates the matter. The section is under the cabin floor, cramped with systems (control and electric cables, pipes etc.), design planning not to remove system. It requires designing parts mate of several pieces and assembling them in place.
I am adding some pictures hoping to answer some of your questions.

 

[rb1957]

King Air is part 25, no? 18g crash landing ?? That may be as a dynamic load, as opposed to the static 9g

Indeed, checking 25.561 (current) tells me this, that current crash case is 9g fwd.

Where are you seeing 18g ?

Having looked at your pic, why crash landing loads ? 25.561 applies to items of mass within the cabin.
Your pylon is going to be affected by "wheels up" landing and ditching.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

RB,
The King Airs span across Part 23 Normal and Commuter.

ashco,
Why didn't you post your diagram?

Not knowing if it's a typo or you honestly believe in your acronyms: The OEM is the Original Equipment Manufacturer and it refers to the Beechcraft and/or Textron in the case of the King Air aircraft. You should stop using the "OME" acronym as it will interfere with the results of Google searches and confuse readers not as familiar with the aviation jargon.

Since you've ignored my DER suggestion and say you aren't in the USA, you sort of come across as finding yourself in a lawless no-man's land... I exaggerate but I do so for a reason. It sounds like you are doing your project in the most difficult manner possible. It is probably a decision made above your pay grade, so I don't aim my concern at you personally. If your superiors were advised by some expert to take this course of action, then in my humble opinion they were given bad advice. They should be seeking an individual who has (a) already completed a civilian STC certification process themselves (b) done so for external modifications (c) supervised the required flight tests and (d) have some delegated/designated authority with your civil aviation authorities to act on your behalf. Since 99% of STC projects in North America and Europe are done this way, I am concerned that your project management will balloon extraordinarily without such a person on your staff.

Case in point, you just mentioned that your team are considering the use of the original or latest amendment of 23.561 for this project. That's a clear sign that they do not understand the Changed Product Rule and are at risk to saddle their project with a substantial number of unnecessary costs and complications.

 

[rb1957]

please show where you need 18g crash loads, which standard imposes this ?

if you are referring to part 23, then be careful using the latest amendment ... it is IMHO awful. Most use the previous version, amendment 64 I think.

Why do you think your mod is so significant that you need to use the latest requirements ? This is called (in NA at least) Changed Product Rule. Typically mods are done to the certification standards of the original type certificate (or the type certificate for the relevant model). Be careful in dealing with authorities, they'll usually take as much as they can. If you offer the latest requirements, they'll probably say "ok". If you offer the type certificate standards, they'll probably say "justify why". Once you offer the latest standards, it's hard to get that off the table. Changed Product Rule includes a decision tree so you can usually have a weight increase < 10% approved to the type requirements, but a weight increase >10% will usually needs the latest ... always subject to how the authority views your abilities, your work, your confidence in "getting it done right".

Your dealings with your authority should be cordial and constructive (you're both aiming to make your planes safe). Dealings can become factious (argumentative) and then it's just a long day for everyone.

Where are you situated ? Europe ? Russia ?? China ?? India ?? South America ?? Africa ?? Mexico ??
You're planning on doing CFD, to understand the flow around the pod, and it's impact on the airframe in general ?
You're planning on verifying this with flight test ?

another day in paradise, or is paradise one day closer ?

 

[crackman]

Dear ashco

From your diagram, seems like you are modifing tha wing carry thru with an external pod? If yes, this is a big deal. I have worked the 350 on numerous projects as a DER, this is no trivial mod. As the others suggested, you need to find some expert help. In addition to external loads, you will need to develop good internal static and fatigue loads.

Good luck with your project.