Aircraft Cabin Interiors Structures Certification
From a structures standpoint, aircraft cabin interiors structures certification with the FAA can be achieved in a few different ways.
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In this post we will discuss the three main methods and how they differ from each other.
- Certification by Stress Analysis
- Certification by Full Scale Static Test
- Certification by Comparative Analysis
1. Certification by Stress Analysis:
This is one of the most common methods for VIP completions, business jets, some major commercial jet OEMs and other aircraft cabin integrators. In this method, a detailed stress analysis specifications and guidelines document is generally provided by the customer or integrator for the stress engineers to strictly follow. This document may be known as the “Structural Design Criteria (SDC)”, or it may have other names such as “Stress Analysis Control Document (SACD)” etc.
This document will generally list:
- Load Factors and Load Cases
- 14 CFR Part 25 compliance requirements
- FEM modeling requirements
- Analysis requirements
- Margin calculations
- Panel materials and other specifications
- Aircraft structure attachment stiffness
- Load and constraint application guidelines
- FEM validation requirements
- Any special modeling requirements on specific type of structures and aircraft attachments
- Margin of safety calculation requirements
- Inter and intra structure interaction requirements
- Report documentation requirements
- FEM validation requirements
Adhering to this document is required in order to satisfy the approving authorities and the customer's internal procedures for compliance. Full scale structural tests are not required in this method. However, tested allowable loads are quite common. It is common for the customer or integrator to provide standard installations and allowable load test results on various approved components. Industry standard methods are used for analysis purposes, these are known as classical hand calculations.
Aircraft structure interface loads are extracted using the FEM models. Then classical hand calculations are used for writing safety margins for all the critical components along the load path to the aircraft structure based on historically accepted reference materials or books such as Bruhn, Roarks, Niu, MMPDS etc.
Various load and material or special uncertainty factors are accounted for in the margin calculations. Finally everything is documented in the form of an approved report format and then submitted as part of the entire certification package.
2. Certification by Full Scale Static Test
A majority of the activities are similar in terms of building a finite element model, calculating aircraft attachment interface loads etc. The following are the differences:
- In addition to the interface loads report a static test plan is required and must be approved before testing
- Critical load cases are selected and tested on the full scale structures for the full load held for 3 seconds in each load case
- In addition to the full scale unit tests, detail tests for the 1.33 fitting factor may also be conducted on components such as dual bolt restraint devices, quarter turns or seat track fittings
- If 1.33 fitting factor tests are not feasible then analysis is performed based on techniques discussed in the section above
- The test plan and results are documented in approved report formats and then submitted as part of the entire certification package
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3. Certification by Comparative Analysis
There will be cases in which the structural modifications to an existing unit or a new unit similar to a certified unit are not major enough to require testing.
In such cases the previously certified unit's report contents, tested component allowable loads, materials, full scale test results etc. can be used to present a logical argument.
The substantiation for the changes can be done by comparison and by demonstrating that the new unit, or changes to an existing unit, do not alter the overall load path significantly. It is also necessary to demonstrate that they do not result in the overall weight increase compared to the certified weight.
Again, all analysis is documented in approved report formats and then submitted as part of the entire certification package.