AC 29-2C, Chg 1 2/12/03 (E) Other fixture applications. Test fixtures should provide a flat footrest for an ATD used in tests of passenger and attendant seats. Flightcrew seats associated with special foot rests or foot-operated controls may use simulated footrests. The surface of the footrest should be covered with carpet (or other appropriate material) and be at a position representative of the undeformed floor or control. Test fixtures may also be necessary to provide guides or anchors for torso restraint systems or for holding instrument panels or bulkheads if necessary for the proposed tests. If these provisions are necessary, the installation should represent the configuration of the installation and be of adequate structural strength to withstand the expected test loads.
(iii) Instrumentation. Electronic and photographic instrumentation systems are essential to properly record the information for the tests discussed in this AC.
Electronic instrumentation is used to measure accelerations and forces required for verifying the test environment and for measuring most of the pass/fail criteria and the floor (seat) attach loads. Photographic instrumentation is used for recording the overall qualitative results of the tests, for confirming that the lap safety belt remained on the ATD’s pelvis (no submarining), and that the upper torso restraint straps remained on the ATD’s shoulder, and for recording the relative deformation of the seats as it may influence rapid evacuation of the rotorcraft by the occupants. Paragraph d.(10), of this guidance contains allowable seat deformation information related to an aisle, passageway, access to exits, and so forth.
(A) Electronic instrumentation. Electronic instrumentation should be accomplished in accordance with the Society of Automotive Engineers Recommended Practice SAE J211, Instrumentation for Impact Tests. In this practice, a data channel is considered to include all of the instrumentation components from the transducer through the final data measurement, including connecting cables and any analytical procedures that could alter the magnitude or frequency content of the data. Each dynamic data channel is assigned a nominal channel “class” equivalent to the high frequency limit for that channel, based on a constant output/input ratio vs. frequency response plot which begins at 0.1 Hz (+1/2 to –1/2 db) and extends to the high frequency limit (+1/2 to – 1 db). Frequency response characteristics beyond this high frequency limit are also specified. When digitizing data, the sample rate should be at least five times the –3 db cutoff frequency of the pre-sample analog filters. Since most facilities set all pre- sample analog filters for Channel Class 1,000 and since the –3 db cutoff frequency for Channel Class 1,000 is 1,650 Hz, the minimum digital sampling rate would be about 8,000 samples per second. For the dynamic tests discussed in this guidance, the dynamic data channels should comply with the following channel class characteristics:
(1) Sled or drop tower vehicle acceleration should be measured in accordance with the requirements of Channel Class 60, unless the acceleration is also integrated to obtain velocity or displacement, in which case, it should be measured in accordance with the Channel Class 180 requirements.
(2) Belt restraint system loads should be measured in accordance with the requirements of Channel Class 60.
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(3) ATD head accelerations used for calculating the HIC should be measured in accordance with the requirements of Channel Class 1,000.
(4) ATD femur forces may be measured if desired in accordance with Channel Class 600.
(5) ATD pelvic/lumbar spinal column force should be measured in accordance with the requirements of Channel Class 600.
(6) The full-scale calibration range for each channel should provide sufficient dynamic range for the data being measured.
(7) Digital conversion of analog data should provide sample resolution of not less than 1 percent of full-scale input.
(B) Photographic instrumentation. Photographic instrumentation is used for documenting the response of the ATD and the test items to the dynamic test environment. Both high speed motion picture and still systems are used.
(1) High-speed motion picture cameras that provide data used to calculate displacement or velocity should operate at a nominal speed of 1,000 pictures per second. Photo instrumentation methods should not be used for measurement of acceleration. The locations of the cameras and of targets or targeted measuring points within the field of view should be measured and documented. Targets should be at least 1/100 of the field width covered by the camera and should be of contrasting colors or should contrast with their background. The center of the target should be easily discernible. Rectilinearity of the image should be documented. If the image is not rectilinear, appropriate correction factors should be used in the data analysis process.
A description of photographic calibration boards or scales within the camera field of view, the camera lens focal length, and the make and model of each camera and lens should be documented for each test. Appropriate digital or serial timing should be provided on the image media. A description of the timing signal, the offset of timing signal to the image, and the means of correlating the time of the image with the time of electronic data should be provided. A rigorous, verified analytical procedure should be used for data analysis.
(2) Cameras operating at a nominal rate of 200 pictures per second or greater can be used to document the response of ATD and test items if measurements are not required. For example, actions such as movement of the pelvic restraint system webbing (lap safety belt) off of the ATD pelvis or movement of upper torso restraint webbing off of the ATD’s shoulder can be observed by documentation cameras placed to obtain a “best view” of the anticipated event. These cameras should be provided with appropriate timing and a means of correlating the image with the time of electronic data.
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AC 29-2C, Chg 1 2/12/03
(3) Still image cameras can be used to document the pretest installation and the posttest response of the ATD’s and the test items. At least four pictures should be obtained from different positions around the test items in pretest and posttest conditions. Where an upper torso restraint system is installed, posttest pictures should be obtained before moving the ATD. For the posttest pictures, the ATD’s upper torso may be rotated to the approximate upright seated position so that the condition of the restraint system may be better documented, but no other change to the posttest response of the test item or ATD’s should be made. The pictures should document that the seat remained attached at all point of attachment to the test fixture. Still pictures can also be used to document posttest yielding of the seat for the purpose of showing that it would not impede the rapid evacuation of the airplane occupants. The ATD’s should be removed from the seat in preparation for still pictures used for that purpose.
Targets or an appropriate target grid should be included in such pictures, and the views should be selected so that potential interference with the evacuation process can be determined. For tests where the ATD’s head impacts a fixture or another seat back, pictures should be taken to document the head contact areas.
(iv) ATD. The tests discussed in this guidance were developed using modified forms of the ATD specified by the United States Code of Federal Regulations, Title 49, Part 572 Anthropomorphic Test Dummies, Subpart B – 50th Percentile Male.
These “Part 572B” ATD’s were developed for automobile impact testing and have been shown to be reliable test devices capable of providing reproducible results in repeated testing. However, since ATD development is a continuing process, the standards allow use of “equivalent” dummies. See paragraph c.(2)(iv)(D) of this guidance. Dummy types should not be mixed when the tests discussed in this guidance are performed.
(A) Modification for measuring pelvic/lumbar column load. Since ATD’s have been developed for use in automobile testing to evaluate injury protection in forward, rearward, and sideward impacts, the ATD’s must be modified to measure the spinal load to comply with the § 29.562(c)(7). This load is influenced by a vertical direction component and by upper torso restraints which may produce a downward force component on the shoulders. To measure the load, a load (force) transducer is inserted into the ATD pelvis just below the lumbar column. This modification is shown in Figure 29.562-4. A commercially available “femur” load cell with end plates removed has been adapted to the modified ATD to measure the compression load between the pelvis and the lumbar spine column of the ATD. A “femur” load cell is commonly available to most test facilities and (according to specifications) is insensitive to bending and twisting moments. This feature prevents load transmission through the load cell as it measures the ATD lumbar/pelvis compression forces. To maintain the correct seated height of the ATD, the load cell is fixed in a rigid cup inserted into a hole bored in the top surface of the ATD pelvis, the top flange of which is bolted to the pelvis. If necessary, ballast should be added to the pelvis to maintain the specified weight of the assembly.
Alternative approaches to measuring the axial force transmitted to the lumbar spinal column by the pelvis are acceptable if the method— Page C - 72
(1) Accurately measures the axial force but is insensitive to moments and forces other than that being measured;
(2) Maintains the intended alignment of the spinal column and the pelvis, the correct seated height, and the correct weight distribution of the ATD; and
(3) Does not alter the other performance characteristics of the ATD.
FIGURE AC 29.562-4 – Installation of Pelvic—Lumbar Spine Load Cell In Part 572B Anthropomorphic Dummy.
(B) Figure 29.562-4 shows an acceptable installation of a femur load cell
(d) at the base of the ATD lumbar spine (a). The load cell is in line with the centerline of the lumbar spine and set below the top surface of the pelvis casting to maintain the seated height of the ATD. A rigid adapter cup (e) is fabricated to hold the load cell, and a hole is bored in the ATD pelvis to accept the cup. Provide clearance between the walls of the adapter cup and the load cell and the wires leading from the cell to avoid possible interference loads. The bottom of the load cell is bolted to the adapter cup.
Adapter plates having similar hole patterns in their periphery are fabricated for the lower surface of the lumbar spine (b) and the upper surface of the load cell (c). These plates are fastened to the lumbar spine and load cell with screws through holes matching threaded holes in those components and are then joined together by bolts through the peripheral holes. The flange on the adapter cup has a bolt hole pattern matching that on the pelvis. The cup is fastened to the pelvis using screws to the threaded holes in the pelvis. Spacers (f) may be placed under the flange of the cup to obtain the specified ATD seating height. Additional weight should be placed in the cavity below the adapter Page C - 73
AC 29-2C, Chg 1 2/12/03 cup to compensate for any weight lost because of this modification. The instrument cavity plug (g) is cut to provide clearance for the adapter cup and added weight.
(C) Other ATD modifications. Flailing of the ATD arms often causes the “clavicle” used in the Part 572B ATD to break. To reduce the frequency of this failure, the clavicle may be replaced by a component having the same shape but made of higher strength material. This may increase the ATD weight slightly, but it would be acceptable for the tests discussed in this guidance. Another useful modification is the use of “submarining indicators” on the ATD pelvis. These electronic transducers are located on the anterior surface of the ilium of the ATD pelvis without altering its contour and indicate the position of the lap safety belt as it applies loads to the pelvis. Thus they can provide a direct record that the lap safety belt remains on the pelvis during the test and eliminates the need for careful review of high-speed camera images to make that determination.
(D) Equivalent ATD. The continuing development of ATD for dynamic testing of seat restraint/crash-injury-protection systems is guided by goals of improved biofidelity (human-like response to the impact environment) and reproducibility of test results. The following criteria can be used to assess whether or not an ATD is equivalent to the present Part 572B ATD:
(1) Fabrication in accordance with design and production specifications established and published by a regulatory agency responsible for crash injury protection systems;
(2) Capability of providing data for the measurements discussed in this guidance or of being readily altered to provide the data;
(3) Evaluation by comparison with the Part 572B ATD and shown to generate similar response to the impact environment discussed in this guidance; and
(4) Any deviations from the Part 572B ATD configuration or performance are representative of the occupant of a civil aircraft in the impact environment discussed in this guidance.
(E) Temperature and humidity. Since extremes of temperature and humidity can change the performance of ATD, the tests discussed in this guidance should be conducted at a temperature from 66° F to 78° F, and at a relative humidity from 10 percent to 70 percent. The ATD should have been maintained under these conditions for at least 4 hours prior to the test.
(3) Test Preparation. Preparations for the tests should include selection of the test articles to be used in the tests, determination of the “most critical” conditions for the tests, and installation of the test articles, instrumentation, and ATD on the test fixture.
Preparations pertaining to the normal operation of the test facility, such as safety Page C - 74
provisions and the actual procedure for accomplishment of the tests, are particular to the test facility. These may be included in a test proposal or plan.
(i) Selection of test articles. Many seat designs compose a “family or type” of seats which have the same basic structural design but differ in detail. For example, a basic seat frame configuration can allow for several different seat leg locations to permit installation in different airplanes. If these differences are of such a nature that their effect can be determined by rational analysis, then the analysis can determine the most highly stressed (“most critical”) configuration. The most highly stressed configuration would normally be selected for the dynamic tests so that the other configurations could be accepted by analysis and comparison with that configuration. The HIC depends on head impact (secondary impact after rotor craft ground impact) and is more dependent on seat pitch for multiple row seats and on location for others than on seat structural stress for a given “family” of seats, so that the selection of the most highly stressed seat structure and the most critical seat pitch or location will permit these factors to be evaluated in one dual row test under the conditions of Test 2. Critical pelvic/lumbar spinal column forces are usually found under the vertical impact conditions of Test 1 but are influenced by the upper torso restraint in Test 2. Certain factors should be considered when employing that assumption. For example:
(A) If the test item incorporates some energy absorbing or load limiting design concept necessary to meet the test criteria or other requirement, a less severe loading condition may adversely affect the performance of that design concept as related to the pass-fail criteria. In such a case, it should be shown by rational analysis or additional testing that the design concept would continue to perform as intended even under the lower loads.
(B) If different configuration of the same basic design incorporated load-carrying elements, especially joints or fasteners, which differed in detail design, the performance of each detail design should be demonstrated in a dynamic test.
Experience has shown that small details in the design often cause problems in meeting the test performance criteria.
(C) If structural strength is not the critical condition for achieving the performance criteria of the dynamic test, the true critical condition should be evaluated in a dynamic test. For example, if in one of the design configurations the restraint system attachment points are located so that the lap safety belt was more likely to slip above the ATD pelvis during the impact, then that configuration should also be dynamically tested even though the structural loading might be less. In all cases, the test item should be representative of the final production item in all structural elements and should include seat cushions, armrests and armcaps, functioning position adjustment mechanism, and correctly adjusted seat back breakover (if present), food trays or any other service or accoutrements required by the seat manufacturer or customer, and any other items of mass carried or positioned by the seat structure (e.g., weights simulating luggage carried or restrained by luggage restraint bars, fire Page C - 75
AC 29-2C, Chg 1 2/12/03 extinguishers, survival equipment, etc.). If these items of mass are placed in a position that could limit the function of an energy absorbing design concept in the test item, they should be of representative shape and stiffness as well as weight. That is, seat stroking should perform properly when used in rotorcraft interiors.
(ii) Consideration of test criteria. The test proposal or plan should be planned to achieve “most critical” conditions for the criteria that make up each test.
(A) For multiple occupant seat assemblies, a rational structural analysis should be used to determine the number and seat location for the ATD and the direction for seat yaw in Test 2 to provide the most critical seat structural stress. This will usually result in unequally loaded seat legs. The seat deformation procedure should be selected to increase the load on the highest loaded seat leg and to stress the floor track or fitting in the most severe manner. The seat position in Test 2 depends on the upper torso restraint design. See c.(3)(ii)C below.
(B) If multiple row testing is used to gather data for HIC in passenger seats, the seat pitch distance between seat rows should be selected within the allowable range, so that the head would be most likely to contact hard structure in the forward seat row. The effect of the 10° yaw in Test 2 and of any seat back breakover should be considered. Results from previous tests or rational analysis can be used to estimate the head strike path. Upper torso restraints may prevent head strike; however, leg kick loads into the front seat row require use of two rows. This kick load is a seat structural test not an ATD consideration.
(C) If nonsymmetrical upper torso restraints (such as single diagonal shoulder belts) are used in a system, they should be installed on the test fixture in a position representative of that in the aircraft and that would most likely allow the ATD to move out of the restraint. For example, in a forward facing crew seat equipped with a single diagonal shoulder belt, the seat should be yawed in Test 2 in a direction such that the belt passes over the trailing shoulder. This is a part of the pass/fail criteria evaluation.
(D) If a seat has sitting height adjustment, it should be tested in the highest position that could be used by a 50th percentile male occupant in the aircraft installation. See b.(3)(ii) of this guidance.
(E) Floor deformation need not be considered in assessing the consequence of any seat deformation as related to the possible impairment of rapid evacuation of the airplane. After the test, the pitch and roll floor beams can be returned to their neutral position and the necessary measurements of the seat deformation made to determine the effect, if any, on rapid evacuation.
(F) In some cases, it may not be possible to measure data for HIC during the test of the seat and torso restraint system. The design of the surrounding interior, such as the instrument panel, may not be known to the designer of the seat and torso Page C - 76
restraint system, or the system may be used in several applications with different interior configurations. In such cases, it will be necessary to document the head strike path and the velocity along the path. This will require careful placement of photo instrumentation cameras and location of targets on the ATD representing the ATD head center of mass so that the necessary data can be obtained. These data can be used by the interior designer to ensure that head impact with the interior will not take place or that if possible head impact occurs, it will remain within the limits of the HIC. In the event the head impacts the specific interior, the interior under evaluation should be subjected to an individual special test to measure the head impact or HIC. The test is done using a rigid 6.5-inch diameter spherical head form weighing 15 pounds, (which includes necessary mass to represent the neck and a portion of the torso). The center of the head form is guided along the previously determined head strike path so that the form contacts the interior components at the velocity previously determined during the seat and torso restraint system dynamic test. Accelerometers located at the center of the head form would provide the data necessary for the HIC computation. If the interior component to be impacted by the ATD has significant inertial response to the impact environment, it will be necessary to evaluate those features or systems, such as breakover seatbacks or instrument panels designed to move forward, relative to the seat, in a dynamic test program which includes the full ATD occupant/seat/restraint system. See b.(3)(ii) of this guidance for ATD and panel location for adjustable crew seats.
(iii) Use of ATD. ATD used in the tests discussed in this guidance should be maintained to perform in accordance with the requirements described in their specification. Periodic teardown and inspection of the ATD should be accomplished to identify and correct any worn or damaged components, and appropriate ATD calibration tests (as described in their specification) should be accomplished if major components are replaced. Each ATD should be clothed in form -fitting cotton stretch garments with short sleeves, mid-calf length pants, and shoes (size 11E) weighing about 2.5 pounds.
The head and face of the ATD can be coated with chalk dust if it is desired to mark head contact areas on seats or other structure. The friction in limb joints should be set so that the joints barely restrain the weight of the limb when extended horizontally. The ATD should be placed in the seat in a uniform manner for reproducible test results. For the tests discussed in this guidance, the following procedures are adequate:
(A) The ATD should be placed in the center of the seat in as nearly a symmetrical position as possible.
(B) The ATD’s back should be against the seat back without clearance.
This condition can be achieved if the ATD’s legs are lifted as it is lowered into the seat.
Then, the ATD is pushed back into the seat back as it is lowered the last few inches into the seat pan. Once all lifting devices have been removed from the ATD, the ATD should be “rocked” slightly to settle it in the seat.
(C) The ATD knees should be separated about 4 inches.
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AC 29-2C, Chg 1 2/12/03 (D) The ATD hands should be placed on the top of the legs, just behind the knees. If tests on crew seats are conducted in a mockup with aircraft controls, the ATD hands should be lightly tied to the controls. If only the seat and occupant restraint system are tested, the ATD hands should be tied together with a lack cord that provides about 24 inches of separation before the cord becomes tight. This will prevent excessive arm flail during the ATD rebound phase.
(E) To the extent that they influence the injury criteria, all seat adjustments and controls should be in the design position intended for the 50th percentile male occupant. If seat and occupant restraint systems being tested are to be used in applications where requirements (placards) dictate particular positions for landing and takeoff, those positions should be used in the tests.
(F) The feet should be in the appropriate position for the type of seat tested (flat on the floor for a passenger seat or on control pedals or on a 45° footrest for flightcrew systems). The feet should be placed so that the centerlines of the lower legs are approximately parallel, unless the need for placing the feet on aircraft controls dictates otherwise.
(iv) Installation of instrumentation. Professional practice should be followed when installing instrumentation. Care should be taken when installing the transducers to prevent deformation of the transducer body from causing errors in data.
Lead-wires should be routed to avoid entanglement with the ATD or test item, and sufficient slack should be provided to allow motion of the ATD or test item without breaking the lead wires or disconnecting the transducer. Calibration procedures should consider the effect of long transducer lead-wires. Head accelerometer (transducer) should be installed in the ATD in accordance with the ATD specification and the instructions of the transducer manufacturer. The load cell between the pelvis and the lumbar spinal column should be installed as shown in Figure 29.562-4 of this guidance or in a manner that would provide equivalent data.
(A) An upper torso restraint is required by §29.785(b). The tension load should be measured in a segment of webbing between the ATD’s shoulders and the first contact of the webbing with hard structure (the anchorage point or a webbing guide). Restraint webbing should not be cut to insert a load cell in series with the webbing, since that would change the characteristics of the restraint system.
Commercially available load cells can be placed over the webbing without cutting. They should be placed on free webbing and should not contact hard structure, seat upholstery, or the ATD during the test. They should not be used on double- reeved webbing, multiple-layered webbing, locally-stitched webbing, or folded webbing unless it can be demonstrated that these conditions do not cause errors in the data. These load cells should be calibrated using a length of webbing of the type used in the restraint system. If the placement of the load cell on the webbing causes the restraint system to sag, the weight of the load cell can be supported by light string or tape that will break away during the test.
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(B) Loads in restraint systems attaching directly to the test fixture can be measured by three-axis load cells fixed to the test fixture at the appropriate location.
These commercially available load cells measure the forces in three orthogonal directions simultaneously, so that the direction as well as the magnitude of the force can be determined. If desired, similar load cells can be used to measure forces at other boundaries between the test fixture and the test item, such as the forces transmitted by the legs of the seat into the floor track. It is possible to use independent, single axis load cells arranged to provide similar data, but care should be taken to use load cells that can withstand significant cross-axis loading or bending without causing errors in the test data, or use careful (often complex) installation to protect the load cells from cross-axis loading or bending. Since load cells are sensitive to the inertial forces of their own internal mass and to the mass of fixtures located between them and the test article, as well as to forces applied by the test article, it may be necessary to compensate the test data for that inaccuracy if the error is significant. Data for such compensation will usually be obtained from an additional dynamic test replicating the load cell installation but will not include the test item.
(v) Restraint system adjustment. The ATD should be sitting in the normal upright position. Care should be taken not to tighten the restraint system beyond the level reasonably expected in use and do not lock any emergency locking device (inertia reel) prior to the impact. Automatic locking retractors should be allowed to perform the webbing retraction and automatic locking function without assistance. Care should be taken that emergency locking retractors sensitive to acceleration do not lock prior to the impact test because of pre-impact acceleration applied by the test facility that is not present in a landing impact. If “comfort zone” retractors are used, they should be adjusted in accordance with instructions given to the user of the system. If manual adjustment of the restraint system is required, it should be sufficient to remove slack in the webbing, but it should not be adjusted so that it is unduly tight. Since the force required to adjust the length of the webbing can be as high as 11 pounds, a preload of 12-15 pounds is commonly recommended. This load is too small to be accurately measured by transducers selected to measure the high loads encountered in the impact test, so it should be measured manually as the restraint is being adjusted. Special gauges are commercially available to assist in this measurement. The preload should be checked and adjusted, if necessary, just prior to the floor deformation phase of the test.
(vi) Repetition of tests. It may be necessary to repeat the tests discussed in this AC if accurate data are not collected in critical data channels or if some other error occurs (e.g., cameras fail to operate, impact pulse inadequate, etc.). Preparation for a repeated test should follow the same steps as for the initial test. The seat should be removed from the fixture, and its attachment fittings or floor track examined and replaced, if necessary, to correct any damage. The ATD should be carefully examined and repaired or adjusted, if necessary. It is usually preferable to use a new seat and restraint system for all repeated tests to preclude system failures due to undetected damage. A new seat and restraint system should be used if there is any detectable variation from the intended design configuration.
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AC 29-2C, Chg 1 2/12/03
d. Data Analysis And Compliance With The Criteria
(1) General. All data obtained in the dynamic tests should be reviewed for errors. Baseline drift, “ringing,” and other common electronic instrumentation problems should be detected and corrected before the tests. Loss of data during the test is readily observed in a plot of the data vs. time and is typically indicated by sharp discontinuities in the data, often exceeding the amplitude limits of the data collection system. If these occur early in the test in essential data channels, the data should be rejected and the test repeated. If they occur late in the test, after the maximum data in each channel has been recorded, the validity of the data should be carefully evaluated, but the maximum values of the data may still be acceptable for the tests described in this guidance. The HIC does not represent a maximum data value, but represents an integration of data over a varying time base. The head acceleration measurements used for that computation should not be accepted if errors or loss of data are apparent in the data at any time from the beginning of the test until the ATD and all test articles are at rest after the test.
(2) Impact pulse shape. Data for evaluating the impact pulse shape are obtained from an accelerometer that measures the acceleration in the direction parallel to the line of inertial response shown in Figure 29.562-1 of this guidance. The impact pulse intended for the tests discussed in this guidance has a symmetrical (isosceles) triangular shape. Since this ideal pulse is considered a minimum test condition, it is possible to evaluate the actual test pulse by comparing it with the ideal triangular pulse.
The ideal pulse can be drawn to scale on the data plot of the test sled or carriage acceleration vs. time. The test pulse is acceptable if the plotted data are equal to or greater than the ideal impact pulse. This method can lead to a practical necessity of exceeding the ideal pulse by a significant degree, unless the test facility has precise control in generating the test pulse. A graphic technique may be used to evaluate test impact pulse shapes that are not precise isosceles triangles. A graphic technique is contained in paragraph f. (1) of this guidance.
(3) Head Injury Criterion (HIC). Data for determining the HIC need to be collected during the tests discussed in this guidance only if the ATD’s head is exposed to secondary impact. The HIC is a method for defining an acceptable limit; i.e., the maximum values of the HIC should not exceed 1,000 for head impact against broad interior surfaces in a crash. The HIC is reported as the maximum value, and the time interval during which the maximum value occurs is also given. Most facilities will make this computation if requested. The HIC is calculated by computer -based data analysis systems because manual attempts to use this method with real data are likely to be tedious. The HIC is calculated according to the following equation:
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2.5 MAX Where: t 1 and t2 are any two points in the time range during the head impact. The range should not exceed 0.050 seconds, and a(t) is the resultant head acceleration at the center of gravity (expressed in g’s) during the head form impact.
(i) Data collection. The HIC is commonly based on data obtained from three mutually perpendicular accelerometers installed in the head of the ATD in accordance with the ATD specification. Data from these accelerometers are obtained using a data system conforming to Channel Class 1,000 as described in SAE Recommended Practice J211. For the tests discussed in this guidance (both ATD and head form), only the data taken during secondary head impact with the aircraft interior need be considered. Head impact is often indicated in the data by a rapid change in the magnitude of the acceleration. Alternately, a film of the test may show head impact which can be correlated with the acceleration data by using the time base common to both electronic and photographic instrumentation, or simple contact switches on the impacted surface can be used to define the initial contact time.
(ii) HIC methodology. The following discussion outlines the basic method for computing the HIC. The magnitude of the resultant acceleration vector obtained from the three accelerometers is plotted against time. Then, beginning at the time of initial head contact (t1), the average value of the resultant acceleration is found for each increasing increment of time (t2 – t1), then integrating the curve between the range of t1 and t2 and then dividing the integral value by the time (t2 – t1). This calculation should use all data points provided by the minimum 8,000 samples per second digital sampling rate for the integration. However, the maximizing time intervals need be no more precise than 0.001 seconds. The average values are then raised to the 2.5 power and multiplied by the corresponding increment of time (t2 – t1). This procedure is then repeated, increasing t1 by 0.001 seconds for each repetition. The maximum value of the set of computations obtained from this procedure is the HIC. The procedure may be simplified by noting that the maximum value will only occur in intervals where the resultant magnitude of acceleration at t1 is equal to the resultant magnitude of acceleration at t2 and when the average resultant acceleration in that interval is equal to 5/3 times the acceleration at t1 or t2.
(iii) Limitations. HIC does not consider injuries that can occur from contact with surfaces having small contact areas or sharp edges, especially if those surfaces are relatively rigid. These injuries can occur at low impact velocities, and are often described as “cosmetic” injuries; however, they can involve irreversible nerve damage and permanent disfigurement. While there is no generally accepted test procedure to provide quantitative assessment of these injuries, a judgmental evaluation of soft tissue injuries can be made by assessing tears or cuts in a synthetic skin placed Page C - 81