Interpretation ID: 2947o
Sr. Supervising Engineer
Oshkosh Truck Corporation
P.O. Box 2566
2307 Oregon Street
Oshkosh, WI 54903-2566
Dear Mr. Thekkanath:
This responds to your letter requesting an interpretation of Safety Standard No. l2l, Air Brake Systems. Section S5.l.l of the standard requires trucks and buses to have an air compressor of sufficient capacity to bring the pressure in the supply and service reservoirs from 85 psi to l00 psi within a specified time. You inquired about the meaning of this requirement in the context of a truck with a trailer behind it. You particularly asked whether the air compressor capacity requirement includes the volume of service reservoirs for the trailer. As discussed below, only the truck reservoirs need to be considered for this requirement.
By way of background information, the National Highway Traffic Safety Administration does not provide approvals for motor vehicles or motor vehicle equipment. Under the National Traffic and Motor Vehicle Safety Act, it is the responsibility of the manufacturer to ensure that its vehicles and equipment meet applicable standards. The following represents our opinion based on the facts provided in your letter.
Sections S5.l and S5.l.l of Standard No. l2l read as follows:
S5.l Required equipment--trucks and buses. Each truck and bus shall have the following equipment:
S5.l.l Air Compressor. An air compressor of sufficient capacity to increase air pressure in the supply and service reservoirs from 85 pounds per square inch (p.s.i.) to l00 p.s.i. when the engine is operating at the vehicle manufacturer's maximum recommended r.p.m. within a time, in seconds, determined by the quotient
(Actual reservoir capacity x 25)/Required reservoir capacity.
The reference in section S5.l.l to "supply and service reservoirs" refers only to the supply and service reservoirs in the truck or bus subject to the requirement. Similarly, the term "actual reservoir capacity" refers only to the actual reservoir capacity of that truck or bus, and the term "required reservoir capacity" refers only to the reservoir capacity required for that truck or bus. Thus, for a truck designed to tow an air-braked trailer, only the truck's reservoirs need to be considered for this requirement. For purposes of testing, the towing vehicle protection system would be activated.
While Standard No. l2l does not specify air compressor capacity for towing vehicles in terms which address towed vehicles, we assume that manufacturers of vehicles designed to tow air-braked vehicles will design them to have sufficient air compressor capacity to ensure safe braking performance under conditions of reasonably forseeable use, including when they are towing air-braked vehicles.
Sincerely,
Erika Z. Jones Chief Counsel
ref: 121 d:9/l2/88