Interpretation ID: 23189ogm

Mr. Mark Doody
Technical Manager
Timoney Group
Gibbstown
Navan, Co. Meath
Ireland

Dear Mr. Doody:

This responds to your recent electronic mail message requesting an interpretation of how Federal Motor Vehicle Safety Standard (FMVSS) No. 121, Air Brake Systems, applies to truck air brakes. Specifically, you ask whether it would be "legal" under the National Highway Traffic Safety Administration (NHTSA) standards for a 6 x 6 vehicle to have an emergency braking system using two "L-split" circuits. Your message describes the "L-split" as two circuits with each circuit braking one rear axle and one front wheel.

By way of background information, Congress has authorized NHTSA to issue Federal motor vehicle safety standards (FMVSSs) applicable to new motor vehicles and items of motor vehicle equipment. 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.

NHTSA has issued several standards applicable to brake systems. Standard No. 121 establishes performance and equipment requirements for braking systems on vehicles equipped with air brake systems, and applies to almost all new trucks, buses, and trailers equipped with air brake systems. I note that, under Standard No. 121, trucks equipped with air braked systems are effectively required to have a dual braking system that is commonly called a split braking system as the result of the requirements in S5.7.1 and S5.7.2. Section S5.7.1, which is referred to as "Emergency brake system performance," requires air braked trucks to comply with a performance requirement that sets forth the distances in which they must stop if there is a leakage failure in the brake system. Section S5.7.2 requires the emergency brake system to be operated by a service brake control.

Although the performance requirements of Standard No. 121 are usually met by use of a "split circuit" emergency braking system, the Standard does not explicitly require such a system or declare that any "split circuit" system have a particular configuration or design. What is required is that the emergency brake system meet all applicable performance requirements.

I hope this information is helpful. If you have any questions, please feel free to contact Otto Matheke at this address or by telephone at (202) 366-2992.

Sincerely,

John Womack
Acting Chief Counsel

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