Interpretation ID: nht95-3.52
TYPE: INTERPRETATION-NHTSA
DATE: July 25, 1995
FROM: John Womack -- Acting Chief Counsel, NHTSA
TO: Larry W. Overbay -- Director, Automotive And Support Equipment Directorate, U.S. Department of the Army
TITLE: NONE
TEXT: Dear Mr. Overbay:
This letter follows up a telephone conversation between Mr. Edward Glancy of my staff and Mr. John Hretz of the U.S. Department of the Army in which Mr. Hretz requested a clarification of a February 17, 1995, letter that we sent to you. In that letter, w e discussed the testing of air braked vehicles under Federal Motor Vehicle Safety Standard (FMVSS) No. 121, Air Brake Systems. We explained that, as a result of a court decision, the emergency stopping test requirements, set forth in S5.7.1 of the standa rd, are not currently applicable to trucks and trailers.
As Mr. Glancy explained, subsequent to that letter, the National Highway Traffic Safety Administration (NHTSA) issued a final rule reinstating emergency stopping tests in FMVSS No. 121. The amendments reinstating these tests take effect on March 1, 1997 , for truck tractors and March 1, 1998, for other medium and heavy vehicles that are equipped with air brakes. Once these amendments take effect, the provisions in S5.7.1 will again be applicable to air braked vehicles.
Mr. Hretz asked whether, in conducting the emergency stopping distance tests, 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. Your question is addressed below.
Section S5.7.1 states that
When stopped six times for each combination of weight and speed specified in S5.3.1.1, except for a loaded truck tractor with an unbraked control trailer, on a road surface having a PFC of 0.9, with a single failure in the service brake system of a part designed to contain compressed air or brake fluid (except failure of a common valve, manifold, brake fluid housing, or brake chamber housing), the vehicle shall stop at least once in not more than the distance specified in Column 5 of Table II.
In describing the failure conditions for which stopping distance requirements must be met, S5.7.1 broadly specifies "a single failure in the service brake system of a part designed to contain compressed air or brake fluid," except for certain listed part s. It is our opinion that the failure mode Mr. Hretz described in which one disconnects the service air signal line at the rear service air relay comes within this language. Therefore, a vehicle would not comply with FMVSS No. 121 if it did not meet th e specified stopping distance requirements after disconnection of the service air signal line at the rear service air relay.
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.