Sergeant Stephan C. Turner
Bus Inspection Unit
Motor Carrier Division
Department of State Police
300 North Clippert
Lansing, MI 48913

Dear Sergeant Turner:

This responds to your recent request for an interpretation of how Federal Motor Vehicle Safety Standard (FMVSS) No. 121, Air Brake Systems applies to school buses. You stated that during a recent inspection, you observed that a fan clutch had twisted off the air line, thereby depleting the secondary air system supply that provides air for the front brakes. You asked about the effect of adding an accessory to the split braking system required by Standard No. 121. Your questions are addressed below.

By way of background information, Congress has authorized NHTSA to issue FMVSSs applicable to new motor vehicles and items of motor vehicle equipment. (Formerly, the authorizing law was the National Traffic and Motor Vehicle Safety Act, which has now 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.

I note that, under Standard No. 121, school buses equipped with air braked systems are effectively required to have a dual braking system that is commonly called a Asplit 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 AEmergency brake system performance,@ requires such school buses 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, AEmergency brake system operation,@ requires the emergency brake system to be operated by a service brake control.

You first ask whether Federal requirements address the installation of accessories, such as a fan clutch, to an existing air brake system. NHTSA does not have any specific regulations addressing the installation of a fan clutch that is connected to an air tank. However, since this device is tied into a vehicle's air brake system, it could affect a vehicle's compliance with Standard No. 121.

If an auxiliary device such as a fan clutch is installed as original equipment on a new vehicle, the vehicle manufacturer is required to certify that, with the device installed, the vehicle satisfies the

requirements of all applicable Federal safety standards. If the device is added to a previously certified new motor vehicle prior to its first sale, the person who modifies the vehicle would be an alterer of a previously certified motor vehicle and would be required to certify that, as altered, the vehicle continues to comply with all of the safety standards affected by the alteration.

Please note that an auxiliary device would not be considered part of the braking system if it were separated from the vehicle's main braking system by a pressure protection valve (i.e., check valve) in such way that the main braking system would not be affected by a leakage failure in the device.

You then ask to which system or side should an accessory be connected for best overall protection. The agency does not typically offer specific guidance with respect to vehicle or equipment design. Nevertheless, we do note that the location of the accessory generally does not matter, provided such an installation does not make the air brake system inoperative. One way to protect the air brake system is to install a check valve between the air source and the accessory.

In response to your next question about why experts differ on whether a check valve should be installed, our technical staff advises that it is a judgment call as to whether such a redundant feature is necessary or worthwhile for a particular air brake system. Since such a failure is already addressed by using a split braking system, some manufacturers apparently believe that it is not cost effective to add such an additional valve. As for cost, the addition of a check valve would increase costs by about $12 to $15 per vehicle.

You also ask whether there is a safety concern that the front brakes of a school bus weighing over 17,000 pounds fail due to a malfunction in an accessory connected to the system. Such a failure is a highly unusual event. Nevertheless, the agency has decided to address the possibility of such a failure in section S.5.7.1 of Standard No. 121. Under this provision, all air braked vehicles are required to meet stopping distance requirements with a leakage failure in their air brake systems.

In response to your final question, NHTSA has never had a requirement prohibiting accessories from being connected to the air brake system or requiring a separate air tank or source of air to power accessories.

I hope this information is helpful. If you have any questions about NHTSA's safety standards, please feel free to contact Marvin Shaw at this address or by telephone at (202) 366-2992.


Samuel J. Dubbin Chief Counsel ref:121 d:3/19/96