Interpretation ID: 77-4.37

TYPE: INTERPRETATION-NHTSA

DATE: 11/15/77

FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA

TO: Wagner Electric Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Wagner Electric Corporation's October 26, 1977, request for confirmation that S5.6.4 of Standard No. 121, Air Brake Systems, does not prohibit the use of a two-valve sequential means to release the parking brakes on a towed vehicle.

I have enclosed for your information an interpretation that addresses this question, stating that a two-valve sequential release is permissible under S5.6.4.

SINCERELY, This responds to your April 23, 1974, question whether Standard No. 121, Air brake systems, is a proposal, whether buses manufactured after January 1, 1975, must conform to Standard No. 121 under all circumstances, what "cut-off date" exists for determination of brake equipment suppliers' ability to provide 121 components on time, and to what extent a bus must be completed to be certified as in compliance with applicable motor vehicle safety standards.

Standard No. 121 has been a final rule since February 27, 1971, and had an effective date of January 1, 1973. In 1972 the effective date was postponed until September 1, 1974. Recently the NHTSA further delayed the effective date for trucks and buses until March 1, 1975, having concluded that suppliers will be able to supply all necessary components by that date.

All buses manufactured after the effective date of an applicable standard must comply with its requirements, under @ 108(a)(1) of the National Traffic and Motor Vehicle Safety Act of 1966 (13 U.S.C. @ 1392(a)(1)), which states "No person shall . . . manufacture for sale . . . any motor vehicle . . . on or after the date any applicable Federal motor vehicle safety standard takes effect . . . unless it is in conformity with such standard. . . ."

You asked whether a trailer which is substantially complete before the effective date but lacks one or more parts due to parts shortages can be certified as conforming although it is equipped with a pre-121 brake system. By analogy with the rules allowing manufacturers to omit "readily attachable" items to be added later in the chain of distribution (Import regulations @ 12.80, Parts 567, 568), the NHTSA will accept a good-faith determination that a vehicle is substantially completed, where only a few parts subject to shortages are missing.

I would also like to answer a technical question raised by R. E. Houser of your engineering staff. In an April 23, 1974, letter he asked for an interpretation of the S5.6.4 language "The parking brake control shall be separate from the service brake control." as it applies to the DD-3 two-step brake release. The S5.6.4 requirement for a separate parking brake control is intended to address the actuation of the brake. We interpret this language not to prohibit the use of a two-step release involving a manual and a foot control.

WAGNER ELECTRIC CORPORATION

October 26, 1977

Office of Chief Counsel National Highway Traffic Safety Administration U.S. Department of Transportation

Re: Request for Interpretation of Section 5.6.4 of FMVSS-121

Section 5.6.4 of FMVSS-121 establishes the need for a parking brake control on a truck or bus to control the parking brakes on the vehicle and of any air braked vehicle that it is designed to tow. This section is reproduced below for your immediate reference.

Section 5.6.4 S5.6.4 Parking brake control trucks and buses. The parking brake control shall be separated from the service brake control. It shall be operable by a person seated in the normal driving position. The control shall be identified in a manner that specifies the method of control operation. The parking brake control shall control the parking brakes of the vehicle and of any air braked vehicle that it is designed to tow.

We request an interpretation which conforms that Section 5.6.4 does not preclude the sequential operation of two valves to release the parking brakes on the towed vehicle.

We have concluded that multiple valve operation is permissible to release the parking brakes on the towed vehicle. Our conclusion is based on the following observations:

1. A towing vehicle has historically been equipped with a separate trailer air supply valve to permit closure of the lines leading to the trailer when the vehicle is operated without an attached trailer. The ability of the parking brake control mentioned in Section 5.6.4 to establish air flow to the trailer is, therefore, dependent upon the operational position of the trailer air supply valve. An adverse interpretation of Section 5.6.4 would, therefore, eliminate the use of this simple disconnect feature and impose a design restriction which apparently was not intended.

2. Section 6.1.14 of FMVSS-121 defines the venting of the lines leading to the trailer when testing the towing vehicle emergency brake system per Section 5.7.3(a). The venting of the trailer supply line could appreciably reduce the pressure available on the towing vehicle to effect the emergency stop. Many vehicle manufacturers have elected to use a trailer air supply valve, which, under these conditions, will automatically trip to the closed position to preserve a high pressure in the tractor service brake resorvoir system.

These tripping air supply valves respond in a similar manner when the parking brake control required by Section 5.6.4 is utilized to park the combination vehicle. It is, therefore, necessary to manually restore the trailer air supply valve to the depressed position before air flow to the towed vehicle can be restored. An adverse interpretation of Section 5.6.4 would prohibit the use of these systems.

3. A clarification proposed in Docket 75-16 Notice 4 defines the desired interaction of the various push-pull valves provided on a tractor to control the tractor parking brakes and the flow of air to the trailer. We refer you specifically to Section 5.6.1(c)(1) and Section 5.8.1.2, which are reproduced below for your convenience.

Section 5.8.1.2

S5.8.1.2 Truck-tractor service brake system-criditional requirements. The service brake system of a truck designed to tow an air-brake-equipped vehicle shall, under the conditions of S6.1.15, be capable of modulating the air pressure in the control line of the control trailer by means of the service brake control. The service brake system shall be equipped with a means of opening and closing the connections from the service brake system of the towing vehicle to the supply and control lines of the control trailer, and simultaneously to open and close a vent to the atmosphere in the supply line to the control trailer. This means shall close automatically in all cases before automatic application of the parking brake system occurs, and before air pressure in all reservoirs of the truck service brake system drops to a level chosen at the option of the top manufacturer that is more than 20 psi, but less than 45 psi. This means shall also operate by utilization of a manual control that does not override automatic operations consisting of a red octagonal knob that, when pulled, closes the air supply to the towed vehicle and vents the trailer supply line to the atmosphere, and when pushed, opens the air supply line to a towed vehicle and permits presentation of the trailer supply (Illegible Word). The following legend shall appear on, above, or below the knob in block capital letters at least one-eighth of an inch in height:

TRAILER AIR SUPPLY

FUEL TO (Illegible Words)

Section 5.6.1.6(c)(1)

(c) In the case of a truck designed to tow an air-brake-equipped vehicle, be equipped with two parking brake system controls that meet the requirements of (1) and (2):

(1) The tractor and trailer parking control shall consist of a yellow diamond-shaped knob that, when pulled, applies the parking brake system of the towing vehicle and vents the trailer supply line to the atmosphere, and, when pushed, releases the parking brake system of the towing vehicle and permits pressurization of the trailer supply line. The following legend shall appear on, above, or below the knob in block capital letters at least one-eighth of an inch in height:

TRACTOR-TRAILER PARK PULL TO APPLY PUSH TO RELEASE:

Note in Section 5.6.1.6(c) that the tractor trailer park valve when pushed releases the tractor parking brakes and "permits pressurization of the trailer supply line". In Section 5.8.1.2, the trailer air supply valve when pushed opens the air supply line to a towed vehicle and "permits pressurization of the trailer supply line". The term "permits" accurately describes the present interaction of the two valves in question. The tractor trailer park valve cannot pressurize the trailer air supply line unless the trailer air supply valve is positioned to deliver air to the trailer supply line. Similarly, the trailer air supply valve cannot pressurize the trailer air supply line if previously vented by the tractor trailer park valve. The driver must restore the particular valve(s) to the appropriate delivery position to effect repressurization of the trailer air supply line. The sequence of valve restoration is dependent on the type of tractor protection system provided on the vehicle.

In conclusion, the apparent intent of the present Section 5.6.4 of FMVSS-121 is to insure the provision of a single (common) control means to apply the parking brakes on all of the vehicles in the combination and does not preclude the sequential operation of two valves to release the parking brakes on the towed vehicle. Interpretation of Section 5.6.4, which confirms this intent, will clarify this matter.

John W. Kourik, Chief Engineer Brake Products