United States Department of TransportationInterpretation ID: 10407
Director, Automotive and Support
Equipment Directorate
U.S. Department of the Army
U.S. Army Combat Systems Test Activity
Aberdeen Proving Ground, MD 21005-5059
Dear Mr. Overbay:
This responds to your letter requesting information about Federal motor vehicle safety standard (FMVSS) No. 121, Air brake systems, and NHTSA Test Procedure TP 121-02. You stated that your organization recently tested a vehicle's compliance to the emergency stopping distance requirements in FMVSS No. 121 by disconnecting the service air signal line at the rear service air relay. You further stated that this action "essentially eliminated rear braking during all stops" making the vehicle totally reliant on the front brakes for stopping. According to your letter, the vehicle manufacturer contends that the manner in which you conducted the test is invalid since it was not done in accordance with NHTSA Test Procedure TP 121-02, which specifies rapid bleeding of the vehicle's air reservoirs.
In addition to general questions about FMVSS No. 121 and the NHTSA Test Procedure, you asked whether the removal of the service air signal line (a non-manifold line which is designed to carry compressed air) from the rear air brake relay valve is considered by NHTSA to be a valid test of the emergency system requirements under the provisions in FMVSS No. 121. After providing background information that responds to your general questions about testing of motor vehicles, I will respond to your specific question about test conditions applicable to the emergency stopping requirements.
Congress has authorized NHTSA to issue FMVSSs applicable to new motor vehicles and items of motor vehicle equipment. (Formerly, the National Traffic and Motor Vehicle Safety Act, which has been codified at 49 U.S.C. 30303) 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.
Each of NHTSA's safety standards specifies performance requirements for the vehicle or equipment being tested for compliance with the particular safety standard.
In addition to the test conditions and procedures set forth in the FMVSSs themselves, NHTSA has provided instructions, known as "compliance test procedures," to the test facilities with which the agency enters into contracts to conduct compliance tests for the agency. The compliance test procedures are intended 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, not differences between various testing facilities.
The compliance test procedures must, of course, not be inconsistent with the procedures and conditions that are set forth in the relevant safety standard. However, the compliance test procedures do, on occasion, provide additional detail beyond what is set forth in the relevant FMVSS. These more detailed test procedures and conditions are requirements only for the contractor test facility in conducting tests on behalf of the agency. The test procedures are subject to change and do not always directly reflect all of the requirements of the particular standard for which they are written. The agency has generally stated that the test procedures are not intended to limit the requirements of the applicable FMVSS(s) and that in some cases the test procedures do not include all of the various FMVSS minimum performance requirements.
With this background in mind, let me respond to your specific questions. As for your first question, the requirements in FMVSS No. 121 take precedence over the TP 121-02. As noted above, TP 121-02 contains instructions issued by NHTSA to provide information to agency contractors about how to conduct compliance tests. In contrast, the law requires manufacturers to certify their vehicles to Standard No. 121.
As for your question about FMVSS No. 121's emergency stopping test requirements, those requirements are set forth in section S5.7.1. However, those requirements are not currently applicable to trucks and trailers, as the result of the U.S. Court of Appeals decision in PACCAR v. DOT, 573 F.2d 632, (9th Cir. 1978) cert. denied, 439 U.S. 862 (1978). (see S3 of 49 CFR 571.121). The agency retained the language in S5.7.1 so that those manufacturers that wish to construct their vehicles in accordance with the non-mandatory sections of the standard will have the necessary information to do so.
I hope this information has been helpful. If you have any other questions, please contact Marvin Shaw of my staff at this address or by phone at (202) 366-2992.
Sincerely,
Philip R. Recht Chief Counsel
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