Interpretation ID: nht94-4.45

TYPE: INTERPRETATION-NHTSA

DATE: October 4, 1994

FROM: Larry W. Overbay -- Director, Automotive and Support Equipment Directorate, U.S. Dept. of The Army

TO: John Womack -- NHTSA

TITLE: None

ATTACHMT: ATTACHED TO 2/17/95 LETTER FROM PHILIP R. RECHT TO LARRY W. OVERBAY (A43; STD. 121)

TEXT: We are requesting a National Highway Traffic Safety Administration (NHSTA) written response to several questions we have concerning the Federal Motor Vehicle Safety Standard (FMVSS) No. 121 and NHSTA Test Procedure TP 121-02. These questions were discuss ed in a telephone conversation between Mr. Dwayne Perrin, NHSTA, and Mr. Richard Kimball, this office, on September 19, 1994.

U.S. Army Combat Systems Test Activity (CSTA) is a test agency for the U.S. Army Test and Evaluation Command (TECOM). One of the missions of CSTA is to test developmental tactical vehicles. Recently we were requested by U.S. Army Tank-Automotive Com mand (TACOM) to test for a vehicle manufacturer's compliance to FMVSS No. 121.

The vehicle under test is one variant of the Family of Medium Tactical Vehicles (FMTV). The vehicle is a three axle truck with a five ton payload capacity and a GVW of 32,000 lbs. According to our test results, the vehicle fails to conform to the re quired emergency, stopping distances. Our test procedures for assessing the performance of the emergency stopping distances involved disconnecting the service air signal line at the rear service air relay. A pneumatic system schematic is provided as an enclosure. This essentially eliminated rear braking during all stops. The vehicle then became totally reliant on the front brakes for stopping.

Preliminary results were reported to TACOM who queried the manufacturer about their nonconformance. The response from the manufacturer stated that the test was invalid since the testing had not been conducted in accordance with the NHTSA Test Procedu re TP 121-02, which recommends rapid bleeding of the vehicle's air reservoirs. TACOM requested CSTA to solicit a NHTSA position on the issue.

We therefore request your position on the following:

a. Is the NHTSA Test Procedure TP 121-02 the governing document for single point failure testing or does FMVSS No. 121 take precedence?

b. What is the intended purpose of the NHTSA Test Procedure TP 121-02?

C. Does NHTSA consider the removal of the service air signal line (a non-manifolded line which is designed to carry compressed air) from the rear air brake relay valve a valid test of the emergency system requirements under the provisions in FMVSS No . 121.

Due to test schedule constraints, a response within 60 days is requested. We look forward to your reply. The points of contact for our organization are Mr. Richard B. Kimball or Roger C. Link, (410) 278-5152 and (410) 278-4857, respectively.