Basic plane means a plane through the centers of the right and left external ear openings and the lower edge of the eye sockets (Figure 1) of a reference headform (Figure 2) or test headform.
Discrete size means a numerical value that corresponds to the diameter of an equivalent circle representing the helmet interior in inches (±0.25 inch) or to the circumference of the equivalent circle in centimeters (±0.64 centimeters).
Helmet positioning index means the distance in inches, as specified by the manufacturer, from the lowest point of the brow opening at the lateral midpoint of the helmet to the basic plane of a reference headform, when the helmet is firmly and properly positioned on the reference headform.
Impact site means the point on the helmet where the helmet shell first contacts the test anvil during the impact attenuation test.
Midsagittal plane means a longitudinal plane through the apex of a reference headform or test headform that is perpendicular to the basic plane (Figure 3).
Reference headform means a measuring device contoured to the dimensions of one of the three headforms described in Table 2 and Figures 5 through 8 with surface markings indicating the locations of the basic, mid-sagittal, and reference planes, and the centers of the external ear openings.
Reference plane means a plane above and parallel to the basic plane on a reference headform or test headform (Figure 2) at the distance indicated in Table 2.
Retention system means the complete assembly by which the helmet is retained in position on the head during use.
Test headform means a test device contoured to the dimensions of one of the three headforms described in Table 2 and Figures 5 through 8 with surface markings indicating the locations of the basic, mid-sagittal, and reference planes.
Appendix to § 571.218
Table 1-Weight Ranges for Impact Attenuation Test Drop Assembly
Test headform size | Weight range1-lb kg) |
Small | 7.6-8.0 (3.4-3.6) |
Medium | 10.8-11.2 (4.9-5.1) |
Large | 13.2-13.6 (6.0-6.2) |
1 Combined weight of instrumented test headform and supporting assembly for drop test.
49 C.F.R. §571.218