Interpretation ID: aorc3.ogm
Mr. Steven R. Fredin
Chairman
Automotive Occupant Restraints Council
c/o Autoliv N.A., Inc.
1320 Pacific Drive
Auburn Hills, MI 48326
Dear Mr. Fredin:
This responds to your recent letter to Clarke Harper of our Office of Crashworthiness Standards regarding certain provisions of Federal Motor Vehicle Safety Standard (FMVSS) No. 209, Seat Belt Assemblies, and the compliance test procedures adopted by the National Highway Traffic Safety Administration (NHTSA) with respect to that standard.
Sections S4.3(j)(1) and S5.2(j) of Standard No. 209 provide that an emergency-locking retractor must lock before the webbing extends 25 mm (1 in.) when the webbing is subjected to an acceleration of 7 m/s2 (0.7 g) within a period of 50 milliseconds (ms). You ask if the 0.7 g acceleration must be constant, if there is an allowable tolerance for the acceleration, if the 0.7 g must be maintained over a certain duration of time, and whether the acceleration trace is allowed to have a certain level of decay or declining acceleration during the test.
After providing background information with respect to the requirements applicable to manufacturers of motor vehicles and motor vehicle equipment and NHTSA's testing of those items, I will respond to your specific question about the test conditions and procedures applicable to the retractor requirements of Standard No. 209.
Congress has authorized the NHTSA to issue FMVSSs applicable to new motor vehicles and items of motor vehicle equipment (49 U.S.C. 30111). NHTSA, however, does not approve or endorse motor vehicles or motor vehicle equipment. Instead, the statute establishes a "self-certification" process under which each manufacturer is responsible for certifying that its products meet all applicable safety standards (49 U.S.C. 30112, 30115). NHTSA then conducts compliance tests of selected products to determine whether the products, in fact, comply with applicable requirements, usually utilizing test facilities under contract to the agency.
Each FMVSS specifies performance requirements for the vehicles or equipment to which the standard applies. In addition to the test conditions and procedures set forth in the FMVSSs themselves, NHTSA provides instructions, known as "compliance test procedures" or "laboratory test procedures" (TP), for use by the test facilities with which the agency enters into contracts to conduct compliance tests. One purpose of the TP is to provide a standardized testing and data recording format among the various contractors that perform testing on behalf of the agency, so that the test results will reflect the performance characteristics of the product being tested, rather than differences between testing facilities.
The TP, of course, must not be inconsistent with the test procedures and conditions that are set forth in the relevant safety standard. However, the TP generally provides additional detail beyond what is set forth in the relevant FMVSS. In some cases, the TP does not refer to all of the performance requirements established by a standard, and in other cases it may direct the test facility to conduct a test at a lesser stringency than that set out in the standard. Nevertheless, the agency has repeatedly advised both manufacturers and the public that a TP does not limit the requirements of an applicable FMVSS, and each TP reiterates that principle.
While manufacturers are not required to conduct certification tests in any particular manner, any manufacturer that wishes to base its certification of compliance on a test procedure that is different from that included in the standard must necessarily assess whether the results of the alternative test procedure are good predictors of the results of the test procedure specified in the standard.
In the instant case, the agency believes that the provisions of S4.3(j)(1) and S5.2(j) are very clear. Under these sections, an emergency-locking retractor must, when the retractor is subjected to an acceleration of 0.7 g within a period of 50 ms, lock before the webbing extends more than 25 mm (1 in.). Nothing in the standard purports to require a constant acceleration (or a constant rate of increase of acceleration), to establish a specific period during which the acceleration must be maintained, or to prohibit any "decay" after the 0.7 g level is reached. Therefore, each retractor must be able to meet the locking requirements of the standard regardless of the rate of acceleration, the duration of the acceleration, or the extent of any subsequent "decay."
We note that this interpretation of S4.3 and S5.2 is consistent with the purpose of those requirements, which is to ensure that retractors will lock in the event of severe vehicle deceleration, such as that which is experienced in a crash. In the event of an actual crash, it is unlikely that a retractor will be subjected to a constant rate of acceleration. It is also unlikely that the acceleration will occur within a narrow corridor for a specific duration. Moreover, the acceleration experienced by the retractor may decay during emergency events. Nonetheless, in all instances, the retractor must lock if the seat belt is to properly restrain an occupant.
With respect to your inquiry about tolerances, the TP provides that the retractor should be subjected to an acceleration of 0.72 g to ensure that, even if the test facility's instrumentation is slightly inaccurate, the retractor will be subjected to at least 0.7 g. This, in effect, provides a tolerance for the test facility. However, this does not create a tolerance for the manufacturer. As noted above, NHTSA often directs its test facilities to test in a less stringent manner than that set forth in a standard (in this case, at a slightly higher acceleration, which tends to facilitate retractor lock-up) to assure that a test will not have to be repeated in the event of a slight error. However, notwithstanding the TP, each manufacturer must assure that its retractors will lock when tested at the 0.7 g level provided in the standard; a retractor that did not lock until it experienced 0.72 g would be deemed noncompliant.
I hope this information is helpful. Please let me know if you have any further questions or need additional information on this subject.
Sincerely,
Frank Seales, Jr.
Chief Counsel
ref:209
d.2/4/2000