compliance can be shown by rational, comparative analysis using data from previous tests.
NOTE: There may be several other aspects of the standardized test procedure that need to be considered when determining the test program required to demonstrate compliance or interpret the test results. The extent of the test program will depend on the most critical case determination and its applicability to other configurations. Further discussion on this aspect of testing is provided in paragraph 5.
3. TEST CONDITIONS
a. General. A minimum of two dynamic te sts is required to assess the performance of an airplane seat, restraints, and related interior system. The seat, the restraint, and the nearby interior are all considered to act together to provide protection to the occupant during a crash. For side-faci ng seats, there may be additional criteria necessary to determine that the seats provide the level of safety intended by § 25.562.
(See paragraph 9d for additional considerations regarding side-facing seats.)
(1) Test 1 (Figure 3-1), as a single row seat test, determines the performance of the system in a test condition where the predominant impact force component is along the spinal column of the occupant in combination with a forward impact force component.
This test evaluates the structural adequacy of the seat, critical pelvic/lumbar column forces, and permanent deformation of the structure under combined downward and forward impact loading. The test may al so yield data on ATD head displacement, velocity, and acceleration time histories.
(2) Test 2 (Figure 3-1), as a single row seat test, determines the performance of a system in a test condition where the predominant impact force component is along the longitudinal axis of the airplane in combination with a lateral impact force component.
This test evaluates the structural adequacy of the seat, and permanent deformation of the structure. It also evaluates be havior and loads of the pelvic restraint and, if applicable, of the upper torso restraint. The test may al so yield data on ATD head displacement, velocity, and acceleration time histories as well as the seat leg loads imposed on the seat tracks or attachment fittings.
This test requires simulating deformation between the seat and airplane floor by deforming the test fixture, as respectively prescribed in Figures 6 and 7 for single occupant and multiple occupant seats, prior to applying the dynamic impact conditions.
The purpose is to demonstrate that the seat/restraint system will remain attached to the airframe and perform properly, even though the airplane and/or seat are deformed by the forces associated with a crash.
(3) For seats placed in repetitive rows, an additional test condition using two seats in tandem placed at representative fore and aft distance between the seats (seat pitch) directly evaluates injury of the head and femur. This test is similar to Test 2 with 4
or without the floor deformation. (Floor deformation is required, if the test also demonstrates structural performance.) These injury criteria are dependent on seat pitch, seat positions occupied, and the effect of hard structures within the path of head excursions in the ±10 degrees yaw attitude range of the Test 2 conditions. See Appendix 4 for a standardized procedure for evaluating row-to-row head impact.
NOTE: It may be possible to evaluate the HIC using alternative tests.
Specific methodologies will require acceptance for certification. (See paragraph 12d.)
Test 1 Test 2
Illustration shows a 100 forward facing seat
Inertial load shown by arrow 300 Yaw Right or Left Min V1 m/s (ft./s) 10.67 (35) 13.41 (44)
Max t, s 0.08 0.09
Min G 14 16 Deform Floor Degrees Roll 0 10
01/10/2006 AC 25.562-1B Degrees pitch 0 10 Tim e 0 t1 Test Pulse simulating 0 Aircraft Floor Deceleration -Time History Deceleration G t1=Rise time The ideal pulse is a symmetrical isosceles triangle V1= Impact velocity Figure 3-1. Seat/Restraint System Dynamic Tests 6
b. Consideration of Test Criteria. The tests should be planned to achieve the “most critical” conditions for the criteria to be evaluated during each test.
Philosophically, the objective of the requirement is to test the critical structural configuration (that is, the seat with the critically stressed components in the primary load path). The primary structural load path and other components that influence occupant injury criteria (for example, HIC or shoulder restraint retention) are evaluated to generate the baseline certification tests. As much as practical, all pass/fail criteria should be assessed during tests that are conducted to show seat structural compliance. Additional structural tests should not be generated to evaluate parts of the seat that are not in the primary load path or do not influence occupant injury criteria. (For example, a dynamic test would not be conducted to specifically evaluate the most critical load on a baggage bar if that is different from the most critical test for the seat structure.)
(1) For multiple place seats, a rational structural analysis is used to determine the number and seat location for the ATDs and the direction for seat yaw in Test 2 to provide the most critical seat structural test. (A detailed procedure is provided in paragraph 5.)
The floor deformation procedure should be selected to increase the load on the highest loaded seat leg and to load the floor track or fitting in the most severe manner. However, a special procedure has been provided, as discussed in paragraph 9c(2), to account for seats that have more than two pairs of legs.
(2) If multiple-row testing is used to gather data to assess head and femur injury protection in passenger seats, the seat pitch must be selected so that the head would be most likely to contact hard structure in the forward seat row. The effect of the 10 degree yaw in Test 2 and the seat back breakover should be considered. (A detailed procedure is provided in Appendix 4.) Results from previous tests or rational analysis may be used to estimate the head strike path of similar seats in similar installations. The front row may be unoccupied.
(3) The separate requirements of §§ 25.562(c)(1) and (c)(3) can result in tests to address both directions of yaw. Section 25.562(b)(2) explicitly requires testing in the direction that would increase the likelihood that the occupant could come out of the restraint (typically over the trailing shoulder). Section 25.562(c)(1) requires assessment of the maximum upper torso restraint loads, which requires testing in the critical structural direction. However, experience has shown that most upper torso restraint geometries have good retention characteristics and can be justified on the basis of similarity to previously tested installations. Thus, if there are test data showing retention (belt over the trailing shoulder) that are applicable to the current installation, only the yaw direction for the critical structural test need be conducted for that installation.
Occupant retention should be assessed during this test and the applicability of previous testing verified. In most cases this should be sufficient, and tests in both yaw directions would not be required.
(4) If a seat has vertical or horizontal adjustments, it is tested in the position that produces the most critical loads on the seat structure (typically the highest vertical position). Positions prohibited for takeoff and landing need not be considered. Seat adjustments that do not have a significant effect on structural loading (for example, thigh support angle, lumbar support, or armrest and headrest positions) are tested in the design positions for the 50th percentile male occupant, unless special requirements dictate the positions allowed for takeoff and landing. In addition, height adjustment should be relative to the interior envelope as it relates to the upper contour (ceiling) of the airplane whenever a specific seat design is approved in a particular airplane. For example, the seat need only be raised to the point where the head of the 50th percentile ATD contacts the upper portion of the airplane interior. Height adjustment to a level above the normal 50th percentile male location is intended to validate the strength of the seat and is not related to testing for a range of occupant sizes.
4. TEST ARTICLES.
a. General. In all cases, the test article mu st be representative of the final production article in all structural elements and must include the seat cushions, restraints, and armrests. It must also include a f unctioning position adjustment mechanism and correctly adjusted breakover (if present). Food trays or any other service items or accouterments that are part of the seat design must be representative of the final production item, if they influence seat stiffness or injury criteria. Otherwise, they and any other items of mass that are carried on or positioned by the seat structure need only be representative masses (see paragraph 10g). Examples include weights simulating luggage carried by luggage restraint bars [9.1 kilograms (20 lb) per passenger place], fire extinguishers, survival equipment, and emergency equipment. If these items of mass are placed in a position that could limit the function of an energy-absorbing feature in the test article, they should be of representative shape and stiffness as well as weight.
(1) If items of mass of any significance detach, they could become projectiles or impede evacuation. Detachment of certain items, such as an in-arm ashtray or decorative trim, can be considered inconsequential and should not be grounds for re-test. (The means of restraint should be improved, however.) In any case, detachment of an item of mass should not leave any sharp or injurious edges. Once re tention of an item of mass has been demonstrated, subsequent tests may be conducted with the item secured for test purposes.
(2) Appendix 5 discusses means of compliance in greater detail. This AC does not establish operational requirements for equipment attached to the seat system.
Function of equipment or systems after the test is not required.
b. Critical Components. Components that are critical to the performance of the seat should be assessed according to the test parameter(s) being investigated. What is critical for evaluation of seat structural performance may not be critical for an assessment of HIC. However, design changes made as a result of one performance parameter can 8
influence other performance parameters. Th e following summarizes critical elements relative to the assessment criteria.
(1) The primary load path for structural tests typically includes seat components, such as seat legs, lateral beams (cross tubes), spreaders, cushion supports and cushions, seatbelts and their attachments, attachments between structural members, seat track fittings, and energy absorbers. (Observe th at energy-absorbers are normally integrated into the other seat components.) The strength and defo rmation responses of these members are evaluated during structural tests.
(2) The primary load path for lumbar load tests typically includes bottom cushions, bottom cushion supports, and lateral beams (cross tubes). Also part of this load path are seat legs, spreaders, attachments between structural members, seat track fittings, and energy absorbers.
(3) The primary load path for row-to-row HIC tests typically includes components in the seat assemblies, such as those installed on the seat back (for example, food tray tables, video monitors, and telephones), recline mechanism, breakover devices, seat back energy absorbers, seat back attachment hardware and, in some cases, arm rests.
(4) The primary load path for head/knee path tests is typically the same as that for the structural tests.
(5) Some components affect the positioning of the occupant in the seat place that in turn can influence ATD dynamic response and occupant injury criteria. Examples include seat bottom cushions, bottom cushion support, armrests, and seat backs.
c. Family of Seats.
(1) The concept of a family of seats is a design philosophy. A family of seats is a group of seat assemblies (regardless of the number of seat places) built from equivalent components in the primary load path. By definition, aft and side-facing seats are considered a separate family of seats from forward-facing seats. Most seats are, to some extent, part of a “family.” The intent of th e family concept is to permit a simplified test article selection process. A group of seats can be designed either using the same design concept or as separate entities (non-family members). If the components in the seat design are carefully considered in advance, the baseline testing described in this document may substantiate the majority or all of the seat part numbers for compliance with § 25.562. Additional tests beyond the baseline may be required to substantiate variations in seat design that are beyond the basic family principles. In the context of Appendix 3, the procedures for substantiation depend on a rigorous definition of the “family” and are valid only if we adhere to that definition.
(2) The family of seats is defined based on design characteristics. Criticality assessments determine, in part, the scope of the test program within a family, not between families. Discussions of the definition of a family of seats and determinations of 9
structural criticality are intended to be complementary. Determinations of structural criticality assume that the family of seats has been established and that variations within the family will be substantiated either by tests or analysis. The decision whether to conduct tests or perform analysis is made based on the guidelines given, with the underlying assumption that such choices are made within a well-defined family of seats.
Therefore, a comparison between families to establish that one design is more structurally critical than another is not addressed or recommended by this procedure and is not considered practical.
5. TEST ARTICLE SELECTION AND RELATED TEST SETUP.
a. General. Test articles are selected in or der to create the critical case condition for the parameters being measured. As a minimum, the most highly stressed configuration is selected for the dynamic tests, so that other configurations could be accepted by comparison with that configuration. For the test article selection to be valid, there must be sufficient design commonality among the seats, so that testing one seat will qualify another. As previously mentioned, this approach is predicated on seats being part of a family of seats. However, the principles of test article selection are valid for individual seats as well as families of seats, so the procedures provided could be applied to a “family” consisting of only one seat. Test article selection for family design concepts is discussed in more detail in paragraphs 5.d. and 5 e. The following additional items should be considered in choosing test articles.
(1) If a multiple-place seat incorporates energy-absorbing or load-limiting features that are necessary to meet the test criteria or other requirements, a partially occupied seat may adversely affect the performance of that seat. In such a case, rational analysis or additional testing must show that the seat will continue to perform as intended, even with fewer occupants.
(2) Experience has shown that small details in the design often cause problems in meeting the test performance criteria. If different configurations of the same basic design of primary load-path members, especially joints or fasteners differ in detail design, the performance of each detail design should be demonstrated in a dynamic test.
(3) Additional dynamic impact testing may be required for a seat with features that could affect the non-structural performance criteria of § 25.562(c)(1-6), even though the test may not be the most critical case based on structural performance. For example, if in one of the design configurations, the restraint system attachment points are located so that the pelvic restraint is more likely to slip above the ATD's pelvis during the impact, that configuration should also be dynamically tested—even though the structural loading might be less than the critical configuration in a family of seats.
(4) Evaluation of typical dress cover materials, including synthetic fabrics, natural fabrics, and leather, has shown that the effect on test results is small, considering other factors such as occupant clothing. Th ese typical materials can be used on a seat without testing more than one material or they can be substituted on an already 10
certificated seat. It is possi ble that some unusual seat surfaces that exhibit very low friction coefficients may require some additional substantiation. An unusual seat surface, for example, would be hard plastic.
(5) Test article selection (whether or not from the family of seats) is dependent on the installation and, in particular, on the seat track. Installation on a different seat track will require substantiation by test or rational analysis.
b. Criteria for Selection. The two main performance criteria used for test article selection are structural strength of the primary load path and injury criteria. In certain cases, emergency egress considerations may be necessary. For example, retention of a footrest may not be critical in the primary load path but could significantly impede egress at an exit if it were to deploy.
(1) Compliance with all parts of § 25.562 can be demonstrated with data obtained from tests using these selection criteria.
(2) To the extent possible, tests are conducted to maximize the amount of useable data and to address more than one performance criterion.
c. Family of Seats When a family of seats is identified using the procedures identified in Appendix 3, a test article selection process based on those procedures can be followed that will minimize the amount of testing required and maximize the usability of the data generated. This process is intended to be integrated with the seat design and not applied to seats post hoc in order to reduce testing once a program is completed or new designs are produced. However, assuming the family definition is robust, modifications to family members can be made or new models can be added to the family following the procedures given. Comparisons between seats in a family should always refer to a seat that was actually tested, rather than two seats that have not been tested. In principle, the seats that have been tested are determined to be critical for the parameters being measured, and any subsequent comparison should be made against the critical configuration. See Appendix 3 for a detail ed discussion of the variation in design intended to be inclusive of a family using this process.
d. Structural Tests for seats with the family design concepts (see §§ 25.562(c)(7) and (8)).
(1) Substantiation of the 16g longitudinal load condition for each family of seats:
Step 1: Determine the 9g forward static in terface loads (or any other standard load) for all seats. It is ge nerally accepted that the interface loads calculated at 0° are sufficient to determine the most critical seat. Special seat design features may require interface load calculations that would take into account the aircraft tapered sections. All occupancy variations and combinations shall be considered for each seat (from unoccupied to fully occupied). The critical test case may be determined by analysis (Finite Element Method [FEM] or static interface loads) or by using test data.
Step 2: Group the seats into two groups: Seats with two legs and seats with more than two legs.
Step 3: For each group of seats (see Figure 5-1):
Step 3a: Compare the aft fitting resultant loads of the seats within each group and identify the seat with the highest load. This seat will be tested.
Step 3b: Subgroup the seats by lateral leg spacing.
NOTE: For groups with more than two legs, there will be multiple lateral leg spacing. The sub-grouping of seats should be based on the leg-pair with the minimum lateral leg spacing.
Step 3c: Identify the subgroup with the minimum (narrowest) lateral leg spacing and identify the seat with the highest seat leg aft fitting resultant load within that subgroup.
Step 3d: If the aft fitting resultant load of the seat identified in step 3c (narrowest leg spacing) is greater than 80 percent of the highest load found on the seat selected for test in step 3a, then the seat identified in step 3c will also be tested, or a single test combining both conditions will be necessary. If the aft-fitting resultant load of the seat identified in step 3c (narrowest leg spacing) is less than 80 percent of the highest load found on the seat selected for test in step 3a, then this seat will only require static substantiation of its ability to tolerate floor deformation. (See Figure 5-2).
Step 3e: Conduct a 16g longitudinal dynamic test of each seat selected in steps 3a and 3d from each group. If the seats selected in steps 3a and 3d do not include the seat with the most critical beam load, that seat should be tested as well.
Step 3f: The occupancy that produced the highest calculated seat leg resultant tension reaction in the aft fitting is used for the test, unless the load of the fully occupied seat is within 10 percent of the highest seat leg load. Due to the statically indeterminate nature of seat structure, there are assumptions used to calculate interface loads, which will result in some uncertainty. Data indicate that calculated reactions within 10 percent of one another are effectively equivalent. In such cases, a fully occupied seat will impart an overall greater load than a partially occupied seat. Therefore, if the fully occupied seat leg load is within 10 percent of the highest loaded seat leg, test the seat fully occupied.
Step 3g: Select yaw, pitch, and roll for test setup.
NOTE: If any seat in the family is intended to be installed on canted seat tracks, the yaw angle for the test for those seats should be 10 degrees plus or minus the aircraft installation cant angle (if it is more critical) depending on which yaw angle maximizes the calculated reaction. (A test 12
yaw angle greater than the minimum required may be used to accommodate the test fixture adjustment capability.) Cant angles of 2 degrees or less may be neglected.
Step 3h: The mass of baggage, life vests, and literature pocket contents should be installed at each seat place, regardless of seat occupancy.
Step 3i: Retention of a specific item of mass, including emergency equipment, need only be demonstrated once during the 16g longitudinal load condition, if suitable retention using the production means of attachment is shown by dynamic test. The item of mass may be restrained for all other 16g longitudinal tests.
Family of Seats More Than Two Legs Narrowest Leg Spacing All Other Leg Spacings Select Critical Seat(s) 16g Test Group Sub-Group Two Leg Seats Narrowest Leg Spacing All Other Leg Spacings Select Critical Seat(s) 16g Test Group Sub-Group See next figure Figure 5-1. Decision Chart—16g Test 13
Narrowest Spacing All Other Spacings Determine the highest loaded leg.
Determine the highest loaded leg.
B Compare the highest loaded legs.
Is the seat with the narrowest spacing the one with the highest loads in this group?
NO Does the seat with the narrowest leg spacing have an interface load more than 80% of the highest interface load of this group?
NO A YES YES Test the seat with the narrowest leg spacing only.
Test both
OR
Ballast/modify seat narrow leg spacing to make
it more critical.
C D Test the highest loaded leg
AND
Perform a “pitch and roll” test on the seat with the narrowest leg spacing.
Figure 5-2. Structural Test Article Decision Tree for Seat Family NOTE: A “Pitch and Roll” test is a static floor deformation test used to substantiate the flexibility of the seat with the narrowest leg spacing.
Pass/fail criteria for this test are the same as for any static or dynamic structural test. However, no injury or other performance criteria are assessed by this test.
E 14
A. 9g static interface loads are the generally accepted indicator of seat structure criticality. Based on this analysis, the hi ghest loaded leg will indicate the most critical configuration to test. (Other features may drive criticality, e.g., beam bending, in addition to the interface loads analysis.) Using 9g static interface loads, identify the highest loaded leg for the seats with the narrowest leg spacing sub-group and the highest loaded leg for the seat from the wider spaced-leg subgroup(s).
B. Compare the interface loads of the highest loaded seat leg from the sub-groups(s).
It is generally accepted that the seat with the narrowest leg spacing will exhibit the highest pre-loads during “pitch & roll.”
C. By testing the seat with the narrowest leg spacing, the test covers the highest loaded leg and the highest pre-load from “pitch & roll.”
D. If the static interface loads for the seat with narrowest leg spacing is within 20% of the seat with the wide leg spacing, it cannot be easily determined which is the more critical to test. The pre-load from “pitch & roll” contributes more to the criticality of the seat with narrow leg spacing. Since the most critical seat cannot be easily determined, either test both seats or create the critical case, using the narrow seat so that it has the more critical interface loads and the highest pre-load from “pitch & roll.”
Test only the narrow seat. The critical case can be created one of two ways, either by ballasting the seat so that the interface loads are increased to match those of the absolute critical seat or by modifying the seat to increase the structural criticality. These are two different approaches, and will change the load distribution on the seat, which is acceptable as long as it maintains criticality.
NOTE: Modification of the seat should be limited to relocating seat legs along a beam to create a more critical overhang. Modifications should not change seat hardware.
E. If the highest static interface load on the seat with the narrowest leg spacing is less than 80% of the highest loaded seat in this group, it can be assumed that the narrow seat with the higher pre-load not be more critical. Only the seat with the highest leg load need be dynamically tested. A check must still be made on the narrowest leg spacing seat to ensure the structure has enough flexibility to accommodate floor warpage. This seat should be placed on a static test fixture and the floor warpage applied. No dynamic test of this configuration is required.
This is a test of the primary structure. No ATDs or other additions to test article/set-up are required.