Interpretation ID: nht74-2.22

DATE: 06/12/74

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Southern Railway System

TITLE: FMVSS INTERPRETATION

TEXT: In your letters of January 10, 1974, and May 17, 1974, you have asked if a trailer would comply with S5.8 of Standard No. 121, Air brake systems, ("have a parking brake system. . . when the air pressure in the supply line is at atmospheric pressure") if its parking brake system contained a valve that allows manual release of the parking brake with the supply line at atmospheric pressure but automatically resets itself when the supply line is pressurized, so that the parking brake system operates as specified by S5.8. Such a valve would permit limited motion of trailers on flat cars to cushion shock experienced during transit.

Such a valve does not appear to conflict with the intent of S5.8 that the parking brake system apply when the supply line is at atmospheric pressure. The requirement is not intended to interfere with intentional manual release of the parking brakes after automatic application has occurred. The danger of inadvertent disablement of the parking brake system during subsequent highway travel is avoided by the automatic features which would return the system to normal operation as soon as the trailer is connected to a source of air pressure, i.e., a tractor.

Sincerely,

ATTACH.

May 17, 1974

Richard B. Dyson -- Assistant Chief Counsel for Regulations, National Highway Traffic Safety Administration, U. S. Department of Transportation

Dear Mr. Dyson:

Please refer to my letter dated January 10, 1974, and I also refer to my telephone conversations of May 17 and earlier with Mr. (Illegible Word) of your office, concerning the problems created for trailers to be used on railroad flat cars by the new requirements for parking brakes.

Attached is a memorandum dated May 17, 1974 from Southern Railway's Assistant Vice President - Engineering & Research C. E. Webb, explaining why there is a need for a parking brake system on a trailer which will allow it to move longitudinally along a flat car for a short distance during impact. Read literally, Section S5.8 of the new regulations would require a trailer to be equipped with a parking brake designed so that the trailer would be incapable of this short longitudinal movement on impact. Mr. Webb in his memorandum proposes parameters for a device, to be developed by the manufacturers and approved by DOT if necessary, which would achieve the purpose of the new regulation when the trailer was in over-the-road use, while at the same time meeting the need for a means to "cut out" the parking brake when the trailer is on a rail car. Mr.

Webb envisions a fail-safe device. I might add that by proposing the device described by Mr. Webb, Southern intends to take no exception to a change in the rules which would be more liberal, that is, allow devices to be put on trailers that would also allow the manual disabling of the parking brake in other situations where it may be desirable, e.g., in freight yards during hostling.

As I said in my January 10 letter, we believe @ 5.8 does not forbid installation of a device like that described by Mr. Webb. However, in order to interest manufacturers in developing the device, we feel it important that your agency indicate that it would be proper and permissible under your regulations. Therefore, we would appreciate receiving advice from you to that effect, if that is the view of your agency.

Thank you.

Yours sincerely,

William P. Stallsmith, Jr. Senior General Attorney -- SOUTHERN RAILWAY SYSTEM