Interpretation ID: 22692

Mr. Jeffrey D. Gonneville
Senior Project Technical Manager
Massachusetts Bay Transportation Authority
80 Broadway
Everett, MA 02149

Dear Mr. Gonneville:

This is in reply to your recent letter concerning the requirements of Standard No. 121, Air brake systems, as they apply to large passenger buses equipped with a liquid crystal display (LCD) information panel. Your letter describes the LCD panel as a microprocessor controlled programmable display having the ability to display a number of conditions in the vehicle, including the air pressure present in the primary and secondary brake systems. You further state that the LCD display would be programmed so the default mode would be to show the primary and secondary brake system pressure. However, in the event that a malfunction or abnormal condition is detected by the vehicle's monitoring system, a message or warning would be displayed on the LCD in place of the brake pressure gauges until the vehicle operator acknowledges the fault and resets the display. Once the display is reset, the LCD will again go to the default mode and display the primary and secondary brake system pressure. In addition to the LCD, you describe the buses as having a traditional warning light and buzzer that will activate in the case of low brake system air pressure.

I am happy to have this opportunity to discuss the role of the National Highway Traffic Safety Administration (NHTSA) in this matter. Under its statutory authority to issue Federal motor vehicle safety standards applicable to motor vehicles and motor vehicle equipment, NHTSA issued Standard No. 121, "Air brake systems," which specifies minimum performance requirements for trucks, buses, and trailers equipped with air brake systems. Among other things, Standard No. 121 requires that vehicles be equipped with a pressure gauge for each service brake system (S5.1.4) and a warning device that gives continuous warning to a driver when the ignition is in the "on" or "run" position and the service reservoir system pressure is below 60 psi (S5.1.5).

The requirements for pressure gauges are found in S5.1.4 of Standard No. 121. Paragraph S5.1.4 requires a pressure gauge to be "readily visible" to a person seated in the normal driving position. It is the agency's position that in the context of Standard No. 121, "readily visible" means visible whenever the driver wants to see it. It does not mean that the air pressure level should be continuously visible. As we understand your system, the air pressure gauges will be "readily visible" unless a fault indicator or warning message appeared on the LCD. If this occurs, the driver could determine the air pressure at any time by pushing the "reset" button. We have concluded that this operation satisfies S5.1.4 and that your system would be permissible under that section.

A low pressure warning signal is required by paragraph S5.1.5 and must be separate from the pressure gauge. You state that the buses will have a separate warning light and an audible alarm. This would appear to conform to S5.1.5.

I hope that this is responsive to your inquiry. If you have any questions, please contact Otto Matheke of my staff at (202) 366-5253.

Sincerely,

John Womack,
Acting Chief Counsel

ref:121
d.7/16/01