AC 29-2C, Chg 1 2/12/03 Illustration shows a forward facing Test 1 Test 2 seat ATD inertial load shown by arrow:
1030° Min. Vi, fps 30 42 Max. tr. sec. - 031 • 071 Min. G 30 18. 4 Deform floor:
degrees roll 10 10 degrees pitch 10 10 deceleration tr = rise time V1 = impact velocity FIGURE AC 29.562-1. Seat Restraint System Dynamic Tests Normal and Transport Category Rotorcraft Page C - 58
(2) Test facilities. A test proposal is prepared for certification authority approval and should reflect the capability of the facility. It should be noted that a number of test facilities could be used to accomplish dynamic testing. Test facilities can be grouped into categories based on the method they use to generate the impact pulse (i.e., accelerators, decelerators, or impact with rebound) and whether the facility is a horizontal (sled) design or a vertical (drop tower) arrangement. As in all certification compliance tests, a test proposal, which may refer to certain specific or generic test equipment, is approved prior to testing. The test may be conducted anywhere, within certain availability or mutually convenient constraints, as long as the test is conducted in accordance with the approved test plan and properly witnessed.
(i) Facility Characteristics or Features. Each of the facilities has characteristics that may have advantages or disadvantages with regard to the dynamic tests discussed in this guidance. One concern is the rapid sequence of acceleration and deceleration that must take place in the tests. In a landing impact, the acceleration phase (flight) is gradual and usually well separated in time from the deceleration (crash impact) phase. In a test, the deceleration usually closely follows the acceleration.
When assessing the use of a facility for the specific test procedures outlined in the recommendations, it is necessary to assess the possible consequences of this rapid sequence of acceleration and deceleration on the test articles and ATD. The standard accommodates the different facilities that are or may be available for the applicant’s use. That is, the standards dictate the peak acceleration with a tolerance as stated in this AC. The “decay” in deceleration with respect to time is not dictated, thereby allowing for the different test facility equipment characteristics.
(A) Deceleration sled facilities. In an aircraft crash, the impact takes place as a deceleration, so loads are applied more naturally in test facilities that create the test impact pulse as a deceleration. Since it is simpler to design test facilities to extract energy in a controlled manner than to impart energy in a controlled manner, several different deceleration sled facilities can be found. The deceleration sled facility at the FAA’s Civil Aeromedical Institute (CAMI) was referred to in developing the test procedures discussed in this and similar AC’s related to airplanes.
(1) The Acceleration Phase. Sufficient velocity for the test impact pulse acquired in this phase can distort the test results if the acceleration is so high that the test articles or ATD are moved from their intended pre-test position. This inability to control the initial or onset conditions of the test would directly affect the test results.
This can be avoided by using a lower acceleration for a relatively long duration and by providing a coast phase (in which the acceleration or deceleration is nearly zero) prior to the impact. This allows any dynamic oscillation in the test articles or the ATD that might be caused by the acceleration to decay. To guard against errors in data caused by pre-impact accelerations, data from the electronic test measurements (accelerations, loads) should be reviewed for the time period just before the test impact pulse to make sure all measurements are at the baseline (zero) level. Photometry film taken of the Page C - 59
AC 29-2C, Chg 1 2/12/03 test should also be reviewed to make certain that the ATD’s used in the test and the test articles were all in their proper position prior to the test impact pulse.
(2) Orientation of test article. The horizontal test facility readily accommodates forward-facing seats in both tests discussed in this guidance, but problems can exist in positioning the test ATD in Test 1 if the seat is a rear or sidefacing seat. In these cases, the ATD’s tend to fall out of the seat due to the force of gravity and must be restrained in place using breakaway tape, cords, or strings. Since each installation will present its own problems, there is no simple, generally applicable, guidance. Attention should be given to positioning the ATD against the seat back and to proper positioning of the ATD’s arms and legs. It will probably be necessary to build special supports for a breakaway restraint so that the restraint will not interfere with the function of the seat and occupant restraint system during the test. Photos of the test from “side of track cameras” should be reviewed to make sure that the breakaway restraint did break (or become slack) in a manner that did not unduly influence the motion of the ATD or the test articles during the test.
(B) Acceleration sled facilities. Acceleration sled facilities, usually based on the Hydraulically Controlled Gas Energized (HYGE) accelerator device, provide the impact test pulse as a controlled acceleration at the beginning of the test. The test item and the ATD are installed facing in the opposite direction from the velocity vector, opposite from the direction used on a deceleration facility, to account for the change in direction of the impact. There should be no problem with the ATD or the test items being out of position due to pre-impact sled acceleration, since there is no sled movement prior to the impact test pulse. Because of this characteristic the applicant may prefer this type of a facility.
(1) Test pulse. After the impact test pulse, when the sled is moving at the maximum test velocity, stop the sled safely. Most of the facilities of this design have limited track length available for deceleration, so that the deceleration levels can be relatively high and deceleration may begin immediately after the impact test pulse.
Since the maximum response of the system usually follows (in time) the impact test pulse, any sled deceleration, which takes place during that response will affect the response and change the test results. The magnitude of change depends on the system being tested, so that no general “correction factor” can be specified. The effect can be minimized if the sled is allowed to coast, without significant deceleration, until the response is complete.
(2) Test results. If the seat or restraint system experiences a structural failure during the test pulse, the post impact deceleration can increase the damage and perhaps result in failures of unrelated components. This will complicate the determination of the initial failure mode and make product improvement more difficult.
One other consideration is that the photometry film coverage of the response to impact test pulse must be accomplished when the sled is moving at near maximum velocity.
Onboard cameras or a series of trackside cameras are usually used to provide film coverage of the test. Since onboard cameras frequently use a wide- angle lens placed Page C - 60
close to the test items, it is necessary to account for the effects of distortion and parallax when analyzing the film. The acceleration sled facility faces the same problems in accommodating rearward-facing or side-facing seats in Test 1 as the deceleration sled facility, and the corrective action is the same for both facilities.
(C) Impact-with-rebound sled facilities. One other type of horizontal test facility used is the “impact-with-rebound” sled facility. On this facility, the impact takes place as the moving sled contacts a braking system, which stores the energy of the impact, and then returns the stored energy back to the sled, causing it to rebound in the opposite direction. This facility has an advantage over acceleration or deceleration facilities in that only one-half of the required velocity for the impact would need to be generated by the facility (assuming 100 percent efficiency). Thus the track length can be shortened, and the method of generating velocity is simplified. The disadvantages of this facility combine the problems mentioned for both acceleration facilities and deceleration facilities. Since one of the reasons for this type of facility is to allow short track length to be used, it may be difficult to obtain sufficiently low acceleration just before or after the impact pulse to resolve data error problems caused by significant pre-impact and post-impact accelerations.
(D) Drop towers. Vertical test facilities can include both drop towers (decelerators) and vertical accelerators. Vertical accelerators, which can produce a longer duration/displacement impact pulse, may not be available. However, drop towers are one of the easiest facilities to build and operate and are frequently used.
(1) Acceleration phase. In these facilities, the pull of earth’s gravity is used to accelerate the sled or guided test fixture and test article to specified impact velocity to avoid the use of a complex mechanical accelerating system. Reproducing the required impact pulse may require extensive development tests for the facility.
Unfortunately, these facilities are more difficult to use for conducting Test 2, particularly for typical forward-facing seats.
(2) Test article. In preparing for (longitudinal) Test 2, the seat should be installed at an angle according to the standards such that the ATD’s tend to fall from the seat due to gravity. The restraint system being tested cannot hold the ATD against the seat unless tightened excessively and will not usually locate the head, arms, or legs in their proper position relative to the seat. Design and fabrication of an auxiliary “break-away” ATD positioning restraint system just for this test are usually a complex task. The auxiliary restraint should not only position the ATD against the seat (including maintaining proper seat cushion deflection) during the pre-release condition of 1 g, it should also maintain the ATD in that proper position during the free fall to impact velocity when the system is exposed to zero g, and then it should release the ATD in a manner that does not interfere with the ATD response to impact. The usual sequence of 1 g/0 g impact, without the possibility of a useful “coast” phase, as done in horizontal facilities, causes shifts in initial conditions for the test impact pulse that can affect the response to the impact. The significance of this undesired movement will depend on Page C - 61
AC 29-2C, Chg 1 2/12/03 the dynamic characteristics of the system under test, and these characteristics are seldom known with sufficient accuracy to achieve the response initially.
(3) Other facets. In addition, the earth’s gravity will oppose the final rebound of the ATD into the seat back, so that an adequate test of seat back strength and support for the ATD cannot be obtained. The problems in Test 1, or with rear-facing seats in Test 2, are not as difficult because the seat will support the ATD prior to the free fall. However, the zero g condition free fall that exists prior to impact will allow the ATD to “float” in the seat restraint system, perhaps changing position and certainly changing the initial impact conditions if movement occurs. Again, the development of a satisfactory auxiliary breakaway restraint system to assure correct pre-impact condition is difficult.
(ii) Test Fixtures. A test fixture is usually required to position the test article on the sled or drop carriage of the test facility and to represent the aircraft’s structure floor, sidewall, bulkhead, etc. It holds the attachment fittings or floor tracks for the seat, provides the floor and sidewall deformation needed for the test, and provides a floor or footrest for the ATD, and it positions the pertinent interior items, such as instrument panels, sidewalls, bulkheads, a second row of seats, if required, for successful performance of the tests, and otherwise simulates the rotorcraft for the test.
The text fixture is usually fabricated of heavy structural steel and does not necessarily simulate lightweight aircraft design or construction. The details of the fixture will depend upon the requirements of the test articles, but provisions for the specified floor and sidewall deformation are needed.
(A) Purpose of floor or sidewall deformation. The purpose of using pitch and roll deformation for the tests is to demonstrate that the seat/restraint system will remain attached to the airframe and perform properly for the tests, although the structure and seat may be more severely deformed by the forces associated with a particular crash. Typical design deficiencies addressed by the test conditions include, but are not limited to, the following:
(1) Concentrated loads may be imposed on floor fittings (studs) or tracks by seat leg attachment fittings which fit tightly or are clamped to a track or fitting, and which do not have some form of relief (especially lateral roll relief) incorporated in the design. These joint fittings can concentrate the forces on one lip of the floor and sidewall track or stud and may break the joint (track or the fitting).
(2) Similarly, loads can be concentrated on one edge of a floor track or stud fitting having an “I,” “bulb head” or “mushroom” cross section and may prematurely break the flange or the fitting.
(3) Detents, pins, or collars which lock the seat leg fitting to the floor track can become disengaged, or the mechanism which is used to disengage the detents, pins, or “dogs” can be actuated and release the seat as the seat or airframe deforms.
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(4) Seat assemblies that provide an energy absorbing system between a seat “bucket or pan” and a seat frame attached to the floor may not perform properly for a pre-loaded seat frame attached to the floor or sidewall. Deformation of the seat frame may cause the energy absorbers to receive unanticipated loads or cause excessive friction in the guides between the seat bucket and seat frame to lock the energy absorber in place.
(5) Restraint system anchorages attached to the airframe structure may be significantly displaced relative to the seat if the seat deforms during the test, and that displacement may inhibit proper performance of the seat/restraint system. This is especially critical for the necessary vertical stroking or displacement.
(B) Floor Deformation. The pitch and roll displacement is intended to evaluate the track or stud and leg fitting joint (axis) tolerance to angular misalignment and not necessarily axis translational displacement.
(1) For the typical aircraft seat. For a multiple or single person seat, with four seat legs mounted in the airframe on two parallel tracks, the floor deformation test fixture may consist of two parallel beams, a “pitch beam” which pivots about a lateral (y) axis, and a “roll beam” which pivots about a longitudinal (x) axis. The beams can be made of any fairly rigid structural form, box, I-beam, channel, or other appropriate cross section. The pitch beam should be capable of rotating in the x -z plane up to +/- 10° relative to the longitudinal (x) axis. The roll beam should be capable of a +/ - 10° roll about the axis of the seat attachment/fitting joint (centerline of floor track or fittings).
(See Figure 29.562-2 for a schematic of an installation.) A means should be provided to fasten the beams in the deformed positions.
(2) Seat and floor interface. The beams should have provision for installing floor tracks or other attachment fittings on their upper surface in a manner that does not alter the above-floor strength of the track or fitting. The track or other attachment fittings should be representative (in above-floor configuration shape and strength) of those used in the rotorcraft. Structural elements below the surface of the floor that are not considered part of the floor track or fitting may be omitted in the installation. The seat having four legs should then be installed on the beams so that the rear seat leg attachment point is near the pitch beam axis of rotation, and the seat positioning pins or locks are fastened in the same manner as specified in the test proposal and as would be used in the rotorcraft, including the adjustment of “anti-rattle” mechanisms, if employed.
(3) Test set-up. The remainder of the test preparations would then be completed (ATD installation and positioning, instrumentation installation, adjustment and calibration, camera checks, etc.). The “floor deformation” would be induced as the final action before the test is accomplished. The roll beam should first be rotated 10° and locked in place, and then the pitch beam should be rotated 10° and locked in place.
The direction of rotation would be selected to produce the most critical loading condition on the seat and floor track or fitting. If the seat is fairly flexible, it may be possible to Page C - 63
AC 29-2C, Chg 1 2/12/03
rot ate the beams by manual effort, perhaps using removable pry bars to gain mechanical advantage. However, rotation of the beams used for tes ting a stiff seat frame is likely to require greater effort than can be accomplished manually, and the use of removable hydraulic jacks or other devices may be necessary. If this condition is expected, provision should be made for appropriate loading points when designing the fixture. This condition is most likely to be encountered when rotating the pitch beam.
The test facility personnel should adhere to appropriate safety provisions during the deformation process. The test fixture may be designed to adjust to fit a wide range of seat designs, including leg spacing, that may be encountered.
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ORIGINAL
POSITION NOTE:
FOR CLARITY, LOAD TRANS
DUCERS ARE
NOT SHOWN
PITCH AXIS
UNDER REAR
LEG ATTACHMENT ., POSITION
AFTER FLOOR
DEFORMATION o/J MINIMUM ~EAT TRACK ~-. 100 ROLL TO BE TlSTED • LEFT OR RIGHT- ~
DETAIL (A)
EXAMPLE OF AIRCRAFT
TRACK BEAM 2/12/03 AC 29-2C, Chg 1 FIGURE AC 29.562-2 – Schematic Floor Deformation Fixture;
Seat Legs Attached at Floor Level Page C - 65
AC 29-2C, Chg 1 2/12/03 (C) Alternative configurations. The preceding discussion described the fixture and floor deformation procedure that would be used for a typical seat that has four seat legs and four attachments to the fuselage floor. These test procedures may be adapted to seats having other designs. Special test fixtures may be necessary for different configurations. The following methods, while not covering all possible seat designs, provide guidance for the more common configurations of seats:
(1) Rotorcraft seats with three legs may have one central leg in front or back of the seat and one leg on each side of the seat. The central leg should be held in its undeformed position as pitch deformation is applied to one side leg and roll to the other.
(2) Seats that are “integral” with the structure without floor or sidewall attachment devices and with continuous attachments such as rows of rivets or screw, etc., are excluded from the deformation, misalignment, or preload prior to test impact.
Similarly bulkhead-mounted seats, solely mounted to a bulkhead, are excluded from the deformation requirement. The test fixture could represent the seat and structure or a rigid bulkhead or an actual bulkhead panel. If a rigid bulkhead installation is used, the test fixture should transfer loads to the seat restraint system through components equivalent to the seat attachment fittings and surrounding bulkhead panel, which exist in the actual installation. Similar guidelines apply to integral seats.
(3) Seats that are attached to both the floor and a bulkhead would be tested on a fixture that positions the bulkhead surface in a plane through the axis of rotation of the pitch beam. The bulkhead surface should be located perpendicular to the plane of the floor (the rotorcraft floor surface, if one were present) in the undeformed condition or in a manner appropriate to the intended installation. Either a rigid bulkhead simulation or replica or an actual bulkhead panel may be used. If a rigid bulkhead simulation is used, the test fixture should transfer loads to the seat restraint system through components equivalent to the attachment fitting and surrounding bulkhead panel that would exist in the actual installation. The seats would be attached to the bulkhead and the floor in a manner representative of the rotorcraft installation, and the floor, as represented in the test, would then be deformed as described in paragraph b.(2)(ii)(B).
(4) Seats mounted between fuselage sidewalls or to the sidewall and floor of an airplane should be tested in a manner simulating rotorcraft fuselage cross-section deformation (e.g., from circular or rectangular to flattened circular or rectangular or ellipsoidal shape) during a severe impact. The 10° roll would simulate the change in fuselage shape. Brackets should be fabricated to attach the seat to the sidewall test fixture at the same level above the fixture “floor” that would represent the installation above the rotorcraft floor. The sidewall bracket or rail should be located on the “roll” beam. It is envisaged that the sidewall rolls outward 10° about an axis at the floor and sidewall juncture. Then, as the beams are rotated to produce the critical loading condition, the combined angular and translational deformation would simulate the Page C - 66
deformation at the sidewall attachment during a landing impact. (See Figure 29.562- 3 for a schematic of an installation.)
(5) Seats that are cantilevered from one sidewall without connection to another structure would not be subject to floor deformation. However, sidewall deformation is likely, and should be considered by warping the entire sidewall attachment plane, or the attachment points of the seat, 10° to represent the most likely fuselage sidewall deformation. This is intended to evaluate a critical condition for seat attachment or seat and occupant restraint system performance. Either a rigid sidewall simulation or an actual sidewall panel may be used. If a rigid sidewall simulation is used, the test fixture should transfer loads to the seat through components equivalent to the attachment fitting as well as the surrounding sidewall to replicate the actual installation.
(6) Side-facing seats, occupiable for takeoff and landing, are subject to the specified dynamic test conditions. Compliance with the structural requirements should be demonstrated for side-facing seats using the same conditions for the test and pass/fail criteria as for fore- and aft-facing seats. The seat should be loaded in the most critical case structurally. Means of restraining the ATDs may need to be adapted to ensure adequate retention during the test. The application of floor distortion will need to be assessed on an individual basis, depending on the design and the method of attaching the seat.
(7) A seat assembly for multiple occupants may have more than two pairs of legs. If the assembly uses a uniform cross section, deformation of only the outer leg assemblies is sufficient. The inner leg pairs may be maintained in the normal, undeformed position for the dynamic tests.
(D) Multiple Row Test Fixtures. In tests of passenger seats normally installed in rows in a rotorcraft, head impact conditions should be evaluated by tests using at least two rows of seats. This allows direct measurements of the head injury data if secondary head impact occurs and demonstrates the effect of the interaction loads between rows; e.g., due to occupant contact with the front row. (That is, ATD leg contact does not overload the front row.) These conditions are usually critical only on Test 2. The single seat row fixture used for the test should be used to position the front (first) seat row and provide appropriate floor deformation to that row. The test is critical for the first row strength. An additional simple fixture may position the second seat row in the proper location and need not provide floor deformation. The second row should be fully occupied unless it is not as critical a condition for the first seat row.
Representative seat cushions and torso restraint systems should be used on both seat rows. The allowable seat pitch (longitudinal spacing) can be determined by analysis of previous test data or limited by type design data and information for the most critical condition for head or leg impact against relatively stiff structure in the first seat row.
Operational limitations that specify the allowable seat pitch of the seats in rotorcraft may be considered also. No impact surface such as seats, bulkheads, etc., may be needed Page C - 67
AC 29-2C, Chg 1 2/12/03 for the ATD in the first seat row unless such a surface is within the expected head strike envelope whenever the seats are installed in rotorcraft.
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ORIGINAL
POSITION NOTE:
FOR CLARITY,
LOAD TRANSDUCERS
ARE NOT SHOWN
PITCH AXIS
UNDER REAR
LEG ATTACHMENT "' POSITION
AFTER FLOOR
DEFORMATION
REPRESENTATION
SEAT TO SIDEWALL AT'fACHMENT
IGID TEST
STRUCTURE 2/12/03 AC 29-2C, Chg 1 FIGURE AC 29.562-3 – Schematic Test Fixture;
Side Wall Mounted Seat Page C - 69