14 CFR Part 25.619 – 625: Various Factors Including Fitting Factor
In this post, we will explore the next series of regulations, 14 CFR Part 25.619 thru 625. These cover special factors, casting factor, bearing factor and fitting factor. But mainly we will focus on the “fitting factor”.
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For each fitting (a part or terminal used to join one structural member to another), the following apply:
(a) For each fitting whose strength is not proven by limit and ultimate load tests in which actual stress conditions are simulated in the fitting and surrounding structures, a fitting factor of at least 1.15 must be applied to each part of -
(1) The fitting;
(2) The means of attachment; and
(3) The bearing on the joined members.
(b) No fitting factor need be used -
(1) For joints made under approved practices and based on comprehensive test data (such as continuous joints in metal plating, welded joints, and scarf joints in wood); or
(2) With respect to any bearing surface for which a larger special factor is used.
(c) For each integral fitting, the part must be treated as a fitting up to the point at which the section properties become typical of the member.
(d) For each seat, berth, safety belt, and harness, the fitting factor specified in § 25.785(f)(3) applies.
In the previous posts, we looked at:
- 14 CFR Subpart C Section 25-301: Loads
- 14 CFR Subpart C Section 25-303: Factor of Safety
- 14 CFR Subpart C Section 25-305: Strength and Deformation
- 14 CFR Subpart C Section 25-307: Proof of Structure
- 14 CFR Subpart C Section 25-365: Pressurized Compartment Loads
- 14 CFR Subpart C Section 25-561: General Emergency Landing Ultimate Loads
- 14 CFR Subpart C Section 25-562: Emergency Landing Dynamic Loads
- 14 CFR Subpart C Section 25-601: Hazardous Unreliable Design Features
- 14 CFR Subpart C Section 25-603: Materials
- 14 CFR Subpart C Section 25-605: Fabrication Methods
- 14 CFR Subpart C Section 25-607: Fasteners
- 14 CFR Subpart C Section 25-609: Structure Protection
- 14 CFR Subpart C Section 25-613: Strength of Materials
(a) Fitting Factor: Applicability
14 CFR Subpart C Section 25-303 provides guidance on the safety factors related to limit and ultimate loads. In addition, this regulation requires a factor called the "fitting factor".
The pesky question many engineers face is:
"What is a fitting factor and when do we need to apply this fitting factor?"
The answer varies by company, sometimes the fitting factor can be an obscure thing to pin down. But, if you were to sum the reason up in one word, it would be "UNCERTAINTY".
Reading the text in part (a) of this regulation we can interpret it as follows:
Considering a structural load path, if the members along this load path are not actually tested to the actual loading and stress conditions, instead certified by analysis, then a fitting factor must be applied to account for the variability in analysis/material/manufacturing conditions of each part of the installation, viz.,
- The fitting itself (section checks, crippling checks etc. as applicable)
- The fasteners (means of attachment - fastener tension and shear interaction checks)
- Joint bearing (fastener hole bearing checks)
Fitting Factor: Components
For limit and ultimate cases, a fitting factor of 1.15 is applicable (only in cases where wear and tear is not an issue, or frequent removal and installation is not required). Panel pin joints also require the 1.15 fitting factor.
The figure below is a schematic of a sandwich panel floor fitting installation (upstream of the fitting), and its components. Each of these components will require the 1.15 fitting factor:
- The fitting itself
- Panel inserts
The same is applicable to components downstream of the fitting, up to the floor structure (not shown in the figure above):
- The tie down bolts
- Fitting base checks
- Fitting tie down bolt hole bearing and pull out checks
- Any washers (if applicable)
- Seat track fittings (if actual test data for the exact seat track fitting is available), joint fitting factor of 1.15 is required
- Seat track lip
(b) Fitting Factor: Exceptions
So, in what situations can the fitting factor be omitted? Again, this can vary from company to company.
- When an actual production assembly is tested and this assembly is used only once, no fitting factor is necessary
- If a joint is manufactured based on a standard process already tested and certified and allowable values are known from comprehensive tests
- If a larger special factor is used for a cast part in the fitting installation, then that part does not need any other fitting factor
- In general, a fitting factor is not required for the sandwich panel checks in cabin interiors
(c) Fitting Factor: Integral Fittings
If one portion of a structural member is machined or formed or cast as part of that larger member and used to transfer load, then it is considered an integral 'fitting' or attachment structure.
- Integrally formed attachment rings
- Radome attachment structures
- Lugs on larger machined or formed or extruded parts, for example engine pylon or nacelle hinge or latch beam lugs
- Hydraulic tubing end fittings etc.
The fitting factor of 1.15 is applicable to these features and beyond for relevant strength checks. It must be applied until the section properties of the larger portion of the structure become consistent or typical and do not change much.
(d) Fitting Factor: Wear and Tear
Part (d) of this regulation refers to a different regulation that requires a higher 1.33 fitting factor for components subjected to frequent removal and installation or frequent operation, thus leading to wear and tear.
Examples of such components may be:
- Restraint devices such as quarter turn bolts
- Door latches
- All seat belt components
- Jump seat or flight attendant seat attachments
- Depending on the DER or ODA requirements, you may also need this factor for seat track fittings and bolts
Now let us briefly touch on the other factors.
Casting Factor: Additional factor to account for the Casting process
In some cases, critical components may be made using the casting process, and 14 CFR Part 25-621 provides guidance on it.
But why is this done? Generally it is done to reduce manufacturing costs for high volume ship sets, and cutting down machining and material costs.
For a detailed explanation of the impacts and safety factors, refer to this FAA AC (Advisory Circular): AC 25.621-1, Casting Factors
Bearing Factor: Additional factor to account for loose fitting bearing joints
14 CFR Part 25-623 provides guidance on pin or fastener joints that may have a significant gap by design. In such case, the parts will most likely pound each other's bearing surfaces due to vibration. This has a detrimental impact on the bearing strength margin. Therefore, a safety factor is used to account for this behavior.
But, if a larger special factor is used for some reason (for example a larger casting factor in case of cast parts), no special bearing factor or fitting factor is required.
Fitting Factor: Conclusions
To be safe, in most instances, a DER or an ODA Unit Member or the customer may require the 1.15 fitting factor for all parts of the load path.
The general form of use for the fitting factor is as follows:
[math] MS = Allowable Load / (FF * Applied Load) - 1 = Allowable Load / (1.15 * Applied Load) - 1 [/math]
[math] MS = Allowable Stress / (FF * Applied Stress) - 1 = Allowable Stress / (1.15 * Applied Stress) - 1 [/math]
But there may be situations where you get a negative MS with an unnecessary fitting factor requirement, for example:
- The MRB or production stress engineer does not have enough wiggle room in the MS value in case of a repair or a defect tag
- May be there is a change in applied loading
- Or a may be there is a change in part design
In such cases, the 1.15 fitting factor may be negotiated out upon the approval of the signatory authority (DER) or signatory body (ODA Unit).
That's it for this post... Hope you enjoyed this post.
How did you use the fitting factor in your own experience? Comment below the post....