APPENDIX 3. SEAT FAMILY DEFINITION
1. This appendix discusses the primary components that make up a seat assembly and how those components can vary between seat assemblies within a family. In addition, a discussion of appropriate means of substantiation for each element is given.
2. While the discussion below addresses the evaluation of seat components as individual members, the dynamic performance of the entire seat assembly with all variations/modifications incorporated must also be evaluated against the tested seat assemblies. For example, a seat with varia tions from the tested seat in legs, beams, and spreaders might require test even though the change in any one element might not require test.
Primary Load Path Elements Legs Frame Fittings/Track Restraint System Spreaders Upper Beams Legs Fittings/Track Restraint System Spreaders Beams Lower Beams Other Primary Load Path Elements (not pictured):
Seat Back row to row HIC Primary load path for a ft facing seats Attach Fittings
e.g., between spreaders and beams Bottom Cushion Seat Pan
Frame Figure A3-1 64
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED)
3. The following discussions of the various seat components are structured as follows:
• Description of the family concept/principles governing a specific component.
• Discussion and guidelines for component variations within the family that are acceptable, using rational analyses without test. This is generally for changes that do not make that component or other features more critical than those found on the tested seat.
• Discussion and guidelines for variations in a seat family that will require substantiation by test. Generally, component variations that make the seat more critical than the tested seat(s) will require additional tests. There is a degree of redundancy in this portion of the discussion in that variations that require test are explicitly stated rather than simply relying on implications of the preceding section, which documented those things permissible by analysis. While this approach does increase the size of the document, it should reduce confusion when specific circumstances arise.
4. The term “variations” denotes both variations in design from one seat to another and changes/modifications made post test or post certification and those resulting from test failures. It is also important to note that the point of reference for variations is the seat that was tested. Guidance with respect to restraint systems addresses the potential for variations across several seat models in the family with more than one restraint.
5. As noted previously in paragraph 5c of this AC, the guidelines given here are intended to be integrated with the design phase. Application of these guidelines and the test article selection process of paragraph 5 to previously approved seats should be done with caution. Since the object ive of this process is to minimize the amount of testing necessary through commonality among designs, it is expected that the seat family will incorporate compromises to that end which might not otherwise have been included. In the case of certification programs that are already completed, this might not have been the case. Therefore, it is doubtful that the pr inciples given here could be applied in total after the fact. These principles should still be useful, however, in assessing the magnitude of changes and variations to designs that have already been approved.
6. Seat Legs.
a. Family of seat principles. Seat legs are typically the vertical structural members of the seat that provide the load path from the upper seat structure (e.g., upper beams or pan) to the lower seat structure (e.g., lower beams or track fittings). Energy-absorbers may be incorporated into the seat leg design (see paragraph 14 of this appendix). To be eligible to belong in a particular seat family, seat legs must utilize the same design philosophy, section properties, and energy absorber (if used).
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED) NOTE: Seat track fittings that interface with airplane structure are addressed in paragraph 13 of this appendix.
b. Variations and post certification changes acceptable by analysis. Variations to the seat leg geometry are acceptable without additional test(s), provided it can be shown by rational analysis that the strength, stiffness, and seat permanent deformation are equivalent to or less critical than the tested seat(s). For example, an increase in distance between the front and rear fitting would be acceptable, provided it could be shown by rational analysis that:
• The floor fitting loads are equivalent to or less critical than the seat leg of the tested seat (for example, linear interface loads analysis),
• The strength of the portion of the leg that varies to accommodate the increase in distance is equivalent to or less critical than the seat leg of the tested seat,
• The stiffness of the leg is similar to the critical leg in the longitudinal and vertical load conditions.
Holes or other minor variations to the seat leg that are not located in a highly stressed area are acceptable. For example, holes drilled in the leg web to attach under-seat electronics boxes are acceptable, provided the hole is not in a highly stressed area of the leg.
c. Variations and post certification changes requiring additional tests. Additional tests would be selected, based on the role that the variation plays in the seat performance.
For example, a material change to a portion of the seat leg may require an additional 16g forward structural test but not require additional HIC or lumbar tests. An additional test must be performed for:
• Any seat with a seat leg geometry that is determined to be more critical with regard to strength, stiffness, or seat permanent deformation than the critical leg of a similar tested seat.
• Any seat with a seat leg energy absorber that has a variation in the load path or a variation that affects the load rating or stroke/deformation of the energy absorber, compared to the seat(s) included in the baseline testing.
Static or dynamic component tests may be acceptable to substantiate variations to seat legs. Component test methods should be c oordinated with the appropriate regulatory agency in advance of the certification program.
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED) Test/Analysis Basic Design/Geometry Test Fittings/Attachments Test Material Test/Analysis Energy Absorber Seat Legs Note: Basic design/geometry includes small, local changes to the legs.
Fittings include attachments to cross-beams, fittings and spreaders.
Component tests may be appropriate instead of a full scale test.
Different Design Philosophy = Different Family Figure A3-2
7. Lateral Beams (Cross Tubes).
a. Family of seat principles. Lateral beams (cross tubes) are typically the structural members that provide the load path from the fore-aft linkages (for example, spreaders) and bottom cushion support to the vertical structure (for example, legs). Lateral beams at similar locations within the seat assemblies must have the same design philosophy.
(1) Lateral beams may also have stiffeners in the form of tubes-within-tubes that vary within the family. Two types of stiffeners are considered here. The first is a local doubler added to reinforce areas with high stress concentrations. A local doubler is defined as one whose length is of the same order of magnitude as the maximum crosssectional dimension of the beam. The second is a longer stiffener (for example, nested tubes) used to increase beam stiffness and strength over a substantial part of the beam length. Lateral beams with long stiffeners should be treated as a different family, requiring a new, different test program since the dominant cross-section for the beam is different than other beams in the seat family.
(2) Lateral beams can include local inserts within the family (for example, doublers) that typically provide local strengthening of the beam. Inserts at similar locations within the seat assemblies must have the same material and manufacturing process and must have similar attachment methods. An insert configuration used in the primary load path (for example, at the leg or spreader attachments) at all similar primary load path locations within the seat for all seats does not need additional substantiation beyond the baseline testing. For example, an insert included at any rear beam leg 67
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED) attachment should be included at all rear beam leg attachments for all seats in the family.
Variations in geometry (length and thickness) are discussed below.
(3) Nested tubes within a seat family must have the same material and manufacturing process and must have similar attachment methods. Variations in length are discussed below.
b. Variations and post certification changes acceptable by analysis.
(1) The following variations in local inserts are acceptable without additional test(s), provided it can be shown by rational analysis that the strength, stiffness, and seat permanent deformation are equivalent to or less critical than the tested seat(s).
• Insert thickness
• Insert length
• Insert location
• The elimination of a local insert in some locations of the seat assembly, if the analysis clearly demonstrates the adequacy of the attachment without the insert.
(2) Variations in the lateral beam length to accommodate differences in seat width are acceptable without additional test/analysis, provided the seat is included in the interface load analysis used in the test article selection process of paragraph 5 of this AC.
(3) Variations in nested tubes length and location are acceptable without additional test(s), provided it can be shown by rational analysis that the strength, stiffness, and seat permanent deformation are equivalent to or less critical than the tested seat(s).
c. Variations and post certification changes requiring additional tests. The structural performance of any seat with a lateral beam or nested tube possessing a variation in material, geometry (except for length), or manufacturing process from the seat(s) evaluated by test. An additional test(s) must be performed for any seat that:
(1) If lateral beam doublers are used, does not have lateral beam doublers at all similar primary load path locations within the seat,
(2) Has lateral beam doublers that have a variation in material, geometry (except for length or thickness), or manufacturing process from the tested seat(s).
(3) Has lateral beam doublers or nested tubes that have a variation in length that is determined to be more critical with regard to strength, stiffness, or seat permanent deformation than the tested seat(s).
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED)
(4) Has lateral beam doublers or nested tubes that have a variation in attachment method that is determined to be more critical with regard to strength than the tested seat(s).
8. Seat Spreaders.
a. Family of seat principles. A seat spreader is typically a fore-aft linkage between the lateral beams. Seat spreaders often provide the structural load path for other features of the seat (for example, a seat belt attachment or seat back attachment). Spreaders at similar locations within the seat assemblies should have the same design.
b. Variations and post certification changes acceptable by analysis. Variations to parts of the spreader that are not in the primary load path (for example, between the seat belt/seat back attachments and the top of the armrest) are acceptable without additional tests or analysis. An example is the area of the spreader that extends beyond the seat belt or seat back attachment that incorporates an armrest attachment. The armrest attachment may vary, provided:
(1) The variation does not extend into the seat back/seat belt load path.
(2) The variation does not affect any area of potential ATD head contact from an occupant in the seat behind.
(3) It can be shown by rational analysis that the retention of the armrest is not significantly affected.
Variations to parts of the spreader that are in the primary load path (between the seat back attachments and the lateral beams or legs) are acceptable, provided it can be shown by rational analysis that the strength (compression/bending) is equivalent to or less critical than the tested seat.
c. Variations and post certification changes requiring additional tests. An additional 16g longitudinal structural test is required beyond the baseline testing for any seat with variations to parts of the spreader that are in the primary load path (between the seat belt attachments and lateral beams or legs).
An additional row-to-row HIC test may be required, if variations to the spreader in any seat are within the ATD head contact area from an occupant in the seat behind or if it changes the seat back performance with regard to HIC.
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED)
Changes to Spreaders Bars Figure A3-3
9. Bottom Cushion.
a. Family of seat principles. The bottom cushion is the component that the occupant sits directly upon. The primary considerations for this component regarding variations/changes are the effect upon lumbar load and the positioning of the occupant in the seat place. Occupant position is assessed using the Seat Reference Point (SRP), as defined in AS8049 Revision A or later. Variations in SRP dimensions discussed in this document are in the component X, Y, and Z directions (the XYZ resultant change is not considered). The bottom cushion assembly (for example, the foam sandwich) must have the same material (that is, the same density, material, and manufacturing process), must be either molded or fabricated within a family, and must be similar in contour and thickness.
b. Variations and post certification changes acceptable by analysis. Contour variations are acceptable without additional 16g and 14g structural tests, provided the SRP does not vary by more than 0.75 inch in any direction (that is, fore, aft, inboard, outboard, up or down) from the SRP of the tested seat. This 0.75 inch variation recognizes the inherent 0.25 inch tolerance in the SRP measurement in addition to an allowable design change of 0.5 inch. Experience has shown that geometry variations in an area 3 inches forward, 2 inches rearward, and 2 inches sideward of each buttock reference point have the most influence on the SRP. Areas of the cushion outside this zone have little influence on ATD performance.
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED) Variations in seat cover fabric are acceptable without additional analysis, provided the variations do not significantly affect the SRP location.
c. Variations and post certification changes requiring additional tests.
(1) Any variation in the cushion contour in an area 3 inches forward, 2 inches rearward and 2 inches sideward of the buttock reference point of the previously tested cushion that results in a vertical change to the SRP of greater than ½ inch would require a 14g vertical lumbar load test.
(2) Variation in bottom cushion material (excluding fabr ic and common fireblocking material) would require a 14g vertical lumbar load test and a 16g longitudinal head path test, if one was included in the baseline testing.
(3) Variation in cushion contour that moves the SRP location more than 0.75 inches up would require a 16g longitudinal structural test.
(4) Variation in cushion contour that moves the SRP location more than 0.75 inches in any direction would require a 16g longitudinal head path test, if one was included in the baseline testing.
10. Bottom Cushion Support.
a. Family of seat principles. The bottom cushion support (for example, seat pan or diaphragm) is the structure immediately below the bottom cushion supporting the occupant weight. The primary considerations for this component regarding variations are the effect on structural performance, lumbar load performance in a 14g vertical test, and the positioning of the occupant in the seat place. The bottom cushion supports at all seat place locations must have the same materials, manufacturing processes, and construction method, and they must be similar in geometry and method of attachment, except as noted in paragraph b below.
b. Variations and post certification changes acceptable by analysis.
(1) Variations to the seat bottom cushion support geometry and method of attachment are acceptable without additional test(s), provided it can be shown by rational analysis, based on test data, that:
(a) The variations have no significant influence in increasing lumbar compression load (including deflection such that contact occurs with any item beneath),
(b) The strength is equivalent to or less critical than the tested seat.
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED)
(2) The following variations are acceptable without additional tests:
(a) Variations in the bottom cushion support geometry to accommodate small difference in the seat place width (3 inches or less), provided other aspects of the geometry and the method of attachment do not vary.
(b) Variations in the bottom cushion support geometry having an influence on SRP location, provided the SRP does not vary by more than 0.5 inch in any direction (fore, inboard, outboard, or up) from the SRP of the tested seat. In general, if all other features of a seat remain constant, head excursion with respect to the seat is shorter when the SRP moves aft. Similarly, structural loads due to overturning moments decrease as SRP is lowered. These general trends can be examined to eliminate duplication of some tests.
c. Variations and post certification changes requiring additional tests. Tests are required for any seat with a variation in seat bottom cushion support material or construction method from the tested seat.
(1) Tests are required for any seat with a variation in seat bottom cushion support that has significant influence on lumbar load (including deflection such that contact occurs with any item beneath) or that is determined to be more critical with regard to strength than the tested seat.
(2) If a variation in the seat bottom cushion support varies the SRP more than
0.5 inches in any direction from the tested seat, the following tests or analysis must be performed:
• A 14g lumbar load test
• 16g longitudinal structural test, if the SRP moves upward.
• 16g longitudinal head path analysis (if one is included in the baseline testing).
This analysis would graphically modify the head path collected in previous tests to account for the change in SRP.
• A row-to-row HIC analysis, if the SRP moves up or forward more than 0.5 inches. This analysis may result in an additional HIC test or a modification of the installation limitations for the seat family. If the SRP moves down or aft, graphical analysis of the data collected in previous testing should be used to determine, if the head might strike a different object.
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED) NOTE: The SRP location may be assumed to change directly as a result of any modification to the structural geometry in the seat bottom cushion support. That is, the SRP moves by the same amount that the bottom cushion support moves. Therefore, no SRP measurements are required in determining the “new” SRP.
11. Seatbelts and Anchors.
a. Family of seat principles. The seat belts (occupant restraints) provide the load path from the occupant to the seat structure. The seat belt typically consists of a latching mechanism, a belt anchor (which connects the belt to the seat) and webbing (which links the latch mechanism with the belt anchors). The latching mechanism must have the same materials, manufacturing processes, construction method, and means of webbing retention; it must also be similar in geometry. The belt anchors must have the same materials, manufacturing processes, and construction method; they must also be similar in geometry. The webbing must have the same material, manufacturing process, construction method, and geometry. The stit ching used to attach restraint system hardware to the webbing must be identical to the tested seats. In general the same principles apply to upper torso restraints and lapbelts.
(1) To date, there are no standards for seat belts that are sufficient to reduce or eliminate full-scale testing when they are substituted on a seat family. At this time, one or more full-scale dynamic tests would be required to substantiate a seat belt replacement.
Certain changes may be substantiated using comparative testing on rigid seats, provided the loads are not increased and head path data are not necessary.
(2) The quality and workmanship of the restraint system must be consistent with TSO/JTSO C22 or TSO/JTSO C114 or equivalent.
(3) The seat belt anchor provides the load path between the belt anchor (part of the belt assembly) and the seat structure (for example, spreader). The seat belt anchor at similar locations within the seat assemblies must utilize the same materials and manufacturing processes, must exhibit similar geometry, and must employ similar methods of attachment.
(4) Once a belt system is qualified for a specific seat family, it can replace other qualified belt systems on that same seat family. To qualify a new belt on an existing family, one 16g structural test seat with highest loaded leg (pitch and roll) must be performed. This structural substantiation is sufficient to allow use of the new belt on the seat family. The ATD head path must be compared for the seat with the new belt system and with the old belt system. This may be done on either the structural test noted above or an additional 16g forward head path test, depending on what data is available for comparison with the old belt system.
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED)
(a) If the head excursion along the entire path for the new belt system is equal to or less than the old belt system, no additional substantiation is required.
(b) If the head excursion along the entire path for the new belt system is greater than the old belt system, the installation limitations may need to be modified to account for this difference.
(5) If multiple belts are a part of an existing seat family and a seat component is changed in the family that will require additional testing, it is not necessary to retest with every seat/belt combination. Floor reaction loads for the 16g structural tests for each belt may be used in selecting a single belt for use on testing future changes to the seat family.
This would cover all belts previously qualified using the same webbing material (for example, nylon or polyester webbing). The belt used for this follow-on testing would be the one associated with the highest floor reactions.
NOTE: If any test using the new seat component generates significantly higher floor reaction loads (load increases on the order of 10 percent or more) compared to the test without the new seat component, the belts that were not tested must be addressed to ensure they have sufficient strength.
A plan outlining additional test and/or analysis of the non-tested belts must be reviewed with the appropriate regulatory agency.
b. Variations and post certification changes acceptable by analysis.
(1) Variations to the seat belt anchor or latching mechanism are acceptable without additional tests, provided it can be shown by rational analysis that:
• The variation does not affect the means of webbing retention, and
• The strength and stiffness are equivalent to or less critical than the tested seat.
(2) Variations to webbing color, belt anchor finish, part labeling, connector/buckle “handedness,” latch handle disengagement angle, and adjustable sidewebbing length are acceptable without additional analysis.
(3) Variations of the fixed length of the restraint system are acceptable as follows:
• The adjuster mechanism moves closer to the centerline of a 50th percentile ATD from the previously tested position (Unless the original position of the adjuster was at the extreme side of the occupant (i.e., at the anchorage point)).
APPENDIX 3. SEAT FAMILY DEFINITION (CONTINUED)
• The adjuster mechanism moves to within ±1.5 inches of the centerline of a 50th percentile ATD.
c. Variations and post certification changes requiring additional tests. An additional test(s) must be conducted for any of the following variations, if it is determined to be more critical with regard to the component’s performance in the dynamic test compared to the tested seat:
• Changes in anchor geometry and method of attachment would require substantiation by test. Some changes to the seat belt anchor may be acceptable without test (e.g., changing a bolt to one with higher strength).
• Latching mechanism material.
• Manufacturing process.
• Construction method.
• Stitch Pattern.
An additional test(s) must be performed for any seat with a seat belt anchor that has a variation in material, manufacturing process, or construction method from the tested seat(s), unless substantiated by analysis (above).