Interpretation ID: nht94-6.27

DATE: April 18, 1994

FROM: John Collins -- Senior Vice President, Government Affairs, American Trucking Associations (Alexandria, VA)

TO: John G. Womack -- Acting Chief Counsel, NHTSA

TITLE: None

ATTACHMT: Attached to letter dated 9/2/94 from John Womack to John Collins (A42; Std. 121)

TEXT:

This letter is a request for an interpretation of the test procedure specified in paragraph S5.8.2, "Supply line retention", of Federal Motor Vehicle Safety Standard (FMVSS) 121, "Air Brake Systems". The last sentence in this paragraph states:

"A trailer shall meet the above supply line retention requirement with its brake system connected to the trailer test rig shown in Figure 1, with the reservoirs of the trailer and test rig initially pressurized to 100 psi, and the regulator of the test rig set at 100 psi."

Our interpretation of test conditions specified in this sentence and Figure 1 (copy attached) is that:

1. The test rig remains connected to the shop air, as shown in Figure 1, for the duration of the test.

2. The shut-off valve of the test rig remains open for the duration of the test.

3. The pressure in the test rig's 1000 cu in. reservoir is maintained at 100 psi for the duration of the test.

The basis for our interpretation are:

a. S5.8.2 does not say that the test rig is to be separated from the shop air supply for the test.

b. S5.8.2 does not say to turn the shut-off valve to the "off" position for the tests. The valve is considered to be normally "open" for tests since it is "open" during the tests for which the test rig was originally designed (brake apply and releasing timing tests in paragraphs S5.3.3 and S5.3.4 of FMVSS 121) and these paragraphs do not say to have the shut-off valve "open" the tests.

c. There would be no reason for S5.8.2 to specify setting the regulator valve at 100 psi if air was not flowing through the regulator valve. Air can only flow through the valve in this test when air is coming from the shop air supply and going out through the shut-off valve.

d. When a combination vehicle is moving, the vehicle's air compressor is operating to maintain the air system pressure within a specified range.

Whenever the trailer supply line pressure is less than the tractor reservoir pressure, air is flowing through the supply line.

We would very much appreciate an early answer to this request as The Maintenance Council of the American Trucking Associations is planning tractor- trailer air systems tests during the first week of May. The objective of these tests is to develop information which will help manufacturers and owners build/modify equipment in such a way as to effectively live with the requirements of S5.8.2 as we interpret the.

Attachment

Figure 1. - Trailer Test Rig. (Graphics omitted.)