Document ID: NHTSA-2008-0047-0001
Agency: nhtsa
Document Type: Rule
Title: Federal Motor Vehicle Safety Standards No. 121; Air Brake Systems
Posted Date: 2008-03-07T05:00Z

[Federal Register: March 7, 2008 (Volume 73, Number 46)]
[Proposed Rules]               
[Page 12354-12357]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr07mr08-29]                         

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DEPARTMENT OF TRANSPORTATION

National Highway Traffic Safety Administration

49 CFR Part 571

 
Federal Motor Vehicle Safety Standards No. 121; Air Brake Systems

AGENCY: National Highway Traffic Safety Administration (NHTSA), DOT.

ACTION: Denial of petition for rulemaking.

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SUMMARY: This Notice denies a petition by Mr. Wayne Walch of TP 
Trucking in which the petitioner requested three changes to Federal 
Motor Vehicle Safety Standard (FMVSS) No. 121, Air brake systems, 
related to the air compressor operation and low air pressure warning 
system. After reviewing the petition and the available real world data, 
the agency has decided to deny it in its entirety because one of the 
suggested changes is already in the standard, the second would not 
result in any measurable safety benefit, and the third was, among other 
things, not described in sufficient detail for the agency to evaluate 
its function or purpose.

FOR FURTHER INFORMATION CONTACT: For non-legal issues, you may contact 
Mr. Jeff Woods, Office of Crash Avoidance Standards, NHTSA, 1200 New 
Jersey Avenue, SE., Washington, DC 20590 (Telephone: 202-366-6206) 
(FAX: 202-366-7002). For legal issues, you may contact Mr. Ari Scott, 
Office of the Chief Counsel, NHTSA, 1200 New Jersey Avenue, SE., 
Washington, DC 20590 (Telephone: 202-366-2992) (FAX: 202-366-3820).

SUPPLEMENTARY INFORMATION: 

Table of Contents

I. Background
II. General Description of Air Brake Systems and FMVSS No. 121 
Requirements
III. Function of Low Air Pressure Warning and Gauges in Normal and 
Emergency Braking Conditions
IV. Real World Data
V. Agency Analysis and Decision
VI. Conclusion

[[Page 12355]]

I. Background

    The agency received a petition for rulemaking dated October 20, 
2006, from Mr. Wayne Walch of TP Trucking, located in Eagle Point, 
Oregon. The petitioner suggested three improvements related to the air 
compressor operation and low air pressure warning system, which he 
believed would make air brake systems safer, and requested that Federal 
Motor Vehicle Safety Standard (FMVSS) No. 121, Air brake systems, be 
changed accordingly. These suggestions include:
     A warning device that would activate when the air 
compressor begins a new cycle.
     A warning device that would activate if the air compressor 
exceeds a predetermined amount of time to reach the cut-out pressure.
     A warning device that would activate just before the 
beginning of the air compressor cycle.
    In his petition, Mr. Walch describes the typical operation of a low 
pressure warning system in which an audible warning signal is activated 
when the reservoir pressure is at 55 pounds per square inch (psi) or 
below, or one half of the compressor governor cutout pressure, 
whichever is less. The petitioner states that he believes this system 
is not robust as it provides no indication of continual air loss or 
when the compressor is constantly running and this can result in a 
dangerous situation.
    In arguing the merits of the petition, the petitioner describes 
several scenarios in which the recommended systems would operate. 
First, the petitioner describes a scenario in which a system has an air 
leak and the compressor keeps running continuously. As the driver 
applies the brakes, the compressor cannot maintain the needed pressure, 
and the driver loses his brakes. If the truck is traveling down hill, 
the driver could have a serious crash in this situation. The petitioner 
states that even if the spring-operated parking brakes activate, they 
do not have the stopping efficiency as the normal service brakes. The 
petitioner further states that if the parking brakes activate due to 
that condition, the vehicle could stop in an unsafe area, and that most 
drivers will not know how to release the spring parking brakes. As 
such, the petition asks for the above three changes to FMVSS No. 121 to 
make air brakes safer. They are as follows:
    1. Provide an indication to the driver upon air compressor cut-in. 
Thus if the driver is aware that the air compressor is cycling but the 
brakes aren't being used, the driver would be alerted to air system 
leakage.
    2. Set the time on new vehicles for the air compressor to increase 
system pressure from cut-in to cut-out pressure. If the system is 
taking too long to build pressure, then a warning needs to be displayed 
to the driver.
    3. Require a low air pressure warning device that activates just 
before the start of the air compressor cycle. Items 1 and 2 above will 
prevent this.

II. General Description of Air Brake Systems and FMVSS No. 121 
Requirements

    The operation of an air brake system relies on compressed air 
stored in reservoirs (tanks) mounted on the vehicle (truck, bus, or 
trailer). By storing compressed air in the reservoirs, the air is 
readily available to make rapid application of the brakes possible. 
When the driver applies the service brakes, the compressed air flows 
from the reservoirs into the service brake chambers that actuate the 
brake mechanism at each wheel. The air in the reservoirs is replenished 
by an air compressor on the engine of the truck or bus, which is 
controlled by a governor that activates the air compressor (cut-in 
pressure) and then turns off the air compressor once the reservoirs are 
fully charged (cut-out pressure). Trailers are also equipped with 
reservoirs, which receive their air supply from a towing vehicle that 
is typically a truck or truck tractor. In the case of multiple trailer 
combination vehicles, the tractor supplies air to all of the trailers 
in the combination.
    As the driver applies the brakes, the air flows from the reservoirs 
into the service brake chambers at a pressure corresponding to the 
position of the brake pedal (treadle valve). Therefore, a light brake 
application would typically result in 10 to 20 psi of compressed air in 
the brake chambers, and a hard brake application would typically result 
in 40 psi or higher pressures in the brake chambers. Since the brake 
chambers are filled with compressed air taken from the reservoirs and 
upon releasing the service brakes the air is vented to the atmosphere, 
the air pressure in the reservoirs becomes slightly depleted whenever 
the brakes are applied. When the reservoir pressure drops to cut-in 
pressure, the governor activates the air compressor to build the system 
pressure back up to the cut-out pressure.
    The process of the air compressor activating at reservoir cut-in 
pressure, then building to reservoir cut-out pressure, is known as 
compressor cycling, and the time between cycles can vary greatly among 
vehicle types and the type of driving that is experienced. The most 
frequent compressor cycling occurs in stop-and-go operations, such as 
experienced by transit buses and refuse trucks, whereas the least 
frequent compressor cycling would typically be on a tractor trailer 
combination vehicle being operated at highway speeds with infrequent 
brake applications.
    The service brake system on air braked vehicles is typically split 
into a primary and a secondary air system. The primary system usually 
controls the brakes on the drive axle(s) and the secondary system 
controls the brakes on the steer axle. Both systems have their own 
reservoirs that are typically fed by a supply reservoir that receives 
air directly from the air compressor. The primary and secondary air 
reservoirs are equipped with check valves for isolation so that a loss 
of pressure in one system does not cause a loss of pressure in the 
other system. In case one system loses pressure, the remaining system 
still provides an emergency braking capability on the vehicle, as well 
as continuing to operate any trailer service brakes, and keeps the 
parking brakes in the released position. Most parking brakes on heavy 
vehicles are of the spring brake design that require adequate brake 
system air pressure in order to release them so the vehicle can be 
moved.
    FMVSS No. 121 has several requirements relating to the reservoirs 
and air compressor systems on trucks, buses, and trailers. The minimum 
size of the reservoirs is specified in FMVSS No. 121 so that an 
adequate reserve of air is available to repeatedly apply the brakes 
without an excessive loss of system air pressure. For trucks and buses, 
S5.1.2.1 requires that the total reservoir volume (combined volume of 
primary, secondary, and supply reservoirs) is at least 12 times the 
combined volume of all of the service brake chambers on the vehicle. 
Slight exceptions are provided in Table V--Brake Chamber Rated Volumes, 
so that vehicle manufacturers can install long-stroke brake chambers in 
place of standard-stroke brake chambers without having to increase the 
size of the reservoirs. For trailers, S5.2.1.1 requires that trailers 
have a reservoir capacity that is at least eight times the combined 
volume of the brake chambers, and again an exception is provided via 
Table V for the use of long-stroke brake chambers.
    S5.1.1 Air compressor requires that an air compressor has 
sufficient capacity to increase the pressure in the reservoirs from 85 
psi to 100 psi within the time, in seconds, expressed by the equation: 
[Actual reservoir capacity x

[[Page 12356]]

25] / [required reservoir capacity], with the engine at maximum 
recommended r.p.m. Thus if a truck had minimum-sized air reservoirs, 
the compressor must be able to reach 100 psi from 85 psi within 25 
seconds with the engine at maximum recommended rated speed. S5.1.1.1 
Air compressor cut-in pressure requires that the governor cut-in 
pressure is at least 85 psi for a bus and at least 100 psi for a truck.
    S5.1.4 Pressure gauge requires a pressure gauge that is visible to 
the driver for each service brake system. In a typical split air brake 
system there are two independent air subsystems (primary and secondary) 
that each have a reservoir or series of reservoirs. The air pressure 
gauge has two pressure indicators (pointers)--one for the primary 
system, and one for the secondary system, or, two separate gauges can 
be used with one gauge provided for each system.
    S5.1.5 Warning signal requires a low air pressure warning signal 
that is either visible to the driver, or if it is not directly in front 
of the driver, is both visible and audible. The warning signal must 
activate when the pressure in any reservoir system is below 60 psi and 
the vehicle's ignition is in the ``on'' position.

III. Function of Low Air Pressure Warning and Gauges in Normal and 
Emergency Braking Conditions

    During normal driving, the reservoir systems are automatically 
recharged by the air compressor, and the driver can monitor the air 
pressure gauges to see that the air pressure in the reservoirs is 
staying between the cut-in and cut-out pressure limits. Most drivers of 
air-braked vehicles are aware of the function of the low air pressure 
warning signal and air pressure gauges on heavy vehicles. The vast 
majority of drivers of air-braked vehicles have commercial drivers 
licenses (CDL's). In order to obtain a CDL with an endorsement to drive 
vehicles with air brakes, drivers are required to demonstrate that they 
possess the knowledge and skills to operate a vehicle equipped with air 
brakes. After starting the engine, the air brake system builds pressure 
in the primary and secondary systems as indicated by the gauges, and 
the low pressure warning turns off indicating normal system operation. 
However, the minimum pressure for the low air pressure warning system 
activation as required in FMVSS No. 121 is ``below 60 psi'' which is 
slightly higher than stated by the petitioner (55 psi, or one-half the 
compressor governor cut-out pressure, whichever is less). The 
petitioner cited the North American Standard Out-of-Service Criteria 
for the low pressure warning device published by the Commercial Vehicle 
Safety Alliance and these air pressure values are slightly lower than 
required by FMVSS No. 121 that applies to the manufacturers of new 
vehicles. To ensure compliance with the ``below 60 psi'' requirement in 
FMVSS No. 121, the actual low pressure warning typically activates 
slightly above 60 psi when measured on vehicles.
    There are several common types of brake system failures that can 
cause the low pressure warning signal to activate. To begin, minor 
leaks in the system can often be overcome by the capacity of the air 
compressor to re-supply air to the brake system. However, this 
discussion focuses on substantial leaks and failures that the air 
compressor cannot overcome, as well as failures of the air compressor 
itself.
    A substantial leak in a brake hose supplying a service brake 
chamber, or in a service brake chamber (e.g., due to a failed 
diaphragm), will result in leakage whenever the brake pedal is applied. 
If the leak is sufficiently large and the brake pedal is applied for a 
long duration, the pressure in either the primary or secondary 
reservoir may become sufficiently low to activate the warning signal, 
which is required to activate when the air pressure in the service 
reservoir system is below 60 psi. However, the remaining service brake 
system (secondary or primary) will remain intact and provide for an 
emergency braking capability, and will continue to keep the parking 
brakes released. The driver would be able to determine by viewing the 
air pressure gauges the rate of pressure loss and whether the loss was 
in the primary or secondary system.
    Failures or leaks can also occur in the air supply portion of the 
system, including the governor, air compressor, compressor discharge 
hose, and the air dryer located between the air compressor and the 
service reservoirs. Whether the compressor does not cut-in, or its 
discharge air is vented to atmosphere because of a hose failure 
downstream of the compressor, the result is that as the driver depletes 
air in both reservoir systems during the application of the service 
brakes, the pressure in both the primary and secondary systems 
continues to drop until the low-pressure warning system activates. 
Typically, the primary system will activate the low pressure warning 
signal first while the secondary system will have a higher pressure. At 
this point the vehicle is in emergency braking mode and the driver has 
the ability to pull off the roadway. If for some reason the brakes were 
repeatedly applied, the pressure in both the primary and secondary 
systems would become further depleted and the spring brakes would 
eventually apply automatically which would also bring the vehicle to a 
stop. Truck drivers with CDLs are generally knowledgeable about these 
aspects of air brake system failures and the importance and meaning of 
low pressure warning signals.

IV. Real World Data

    The petitioner cites two scenarios for truck crashes that it states 
are related to the inadequacy of the currently required low air 
pressure warning system. The first is runaway trucks on downgrades that 
the petitioner claims is caused by air leaks. The petitioner provided 
no data to support this conclusion. Similarly, the agency is not aware 
that this is a prevalent crash mode. Our experience indicates that 
runaway truck crashes are most often due to brake fade from overheated 
and/or out-of-adjustment S-cam drum brakes that result in a loss of 
brake effectiveness, often exacerbated by excessive speed on a 
downgrade. We have no indications that runaway truck crashes are being 
caused by air leaks or contributed to by inadequate low pressure 
warning systems.
    The other crash scenario presented by the petitioner was a truck 
stopping in an unsafe area because of an air leak that caused the 
parking brakes to apply and most drivers would not know how to move the 
vehicle. While it is true that this can happen, the agency has no 
indications of widespread problems with trucks being stranded on 
roadways or in unsafe areas due to loss of air pressure in the brake 
system and being involved in crashes. The petitioner also did not 
provide such data. Additionally, as we have previously stated, the 
current low-pressure warning system already alerts the driver of a 
substantial loss of air pressure and the truck's braking system can be 
operating in the emergency braking mode. As such, the driver can still 
make several brake applications to safely bring the truck to a stop off 
of a travel lane.

V. Agency Analysis and Decision

    The first requested change made by the petitioner is:

    There needs to be a way to make the driver aware of when the air 
compressor is starting a new cycle. This lets the driver know there 
is a loss of air in the system. If he is not using the brakes and 
the air compressor is cycling he should stop the vehicle and do an 
inspection for an air leak or call for repairs to the air system 
before continuing on or before a possible accident on a downhill 
grade.

[[Page 12357]]

    The agency believes that this change would mean that a lamp on the 
instrument panel would illuminate (or some other type of indicator 
would signal) every time that the air compressor cycled on at cut-in 
pressure. Since cycling of the compressor occurs during normal 
operation of a vehicle equipped with an air brake system, the agency 
believes that most truck drivers would find this to be a nuisance, 
particularly when driving at night. The agency's fleet evaluation 
experience in the early 1990's with antilock brake systems (ABS) 
warning lamps was that drivers would sometimes remove the bulb or cover 
it with opaque tape because of a perceived nuisance (when in fact it 
was indicating a malfunction in the ABS that, under hard braking, could 
result in a loss-of-control crash). A warning system that activates 
during normal operation may have a limited safety benefit, and 
activations are more effective when they only occur when there is a 
condition that warrants some type of intervention by the driver. 
Therefore, we do not believe it would be appropriate to adopt the 
petitioner's first request. However, we note that neither FMVSS No. 
101, Controls and Displays, nor FMVSS No. 121 prohibits the addition of 
a compressor cycling lamp, if a truck operator chooses to have such a 
system installed.
    The second requested change is:

    They need to set the time on new vehicles at the factory on how 
long it takes the air compressor at the start of its cycle to meet 
the cut off pressure. If it is taking to[o] long or continuous 
running occurs there needs to be something to warn the driver there 
is a major problem. This is a very unsafe situation and should have 
a priority warning to the driver.

    Regarding the requested change by the petitioner to set the 
required time for air pressure build time, we note that this facet of 
air brake systems is addressed in the previously discussed section 
S5.1.1 in FMVSS No. 121, which requires the air compressor to have 
sufficient capacity to increase the air system pressure from 85 to 100 
psi in the specified amount of time. However, this requirement allows 
for some variation in the amount of time needed to charge the air 
system. Under FMVSS No. 121, the time for charging the air system is 
measured with the engine at maximum rated speed, so the actual charging 
time during normal driving can vary based upon actual engine speed and 
gear selection. Compared to charging time with the engine at maximum 
rated speed, the charging time would be longer when the truck is 
sitting at idle. Other factors, such as the frequency of brake 
application, number of towed units, air being supplied to increase air 
suspension pressure, etc., would cause air to be depleted at the same 
time the air compressor is charging the system. Therefore, these would 
also affect the charging time, and we believe that requiring a warning 
to activate when a constant time period has elapsed is an impracticable 
requirement, given the variable nature of the charging period under the 
current regulatory scheme. We note that our safety standard already 
regulates performance in the area of air pressure charging time, but we 
believe that it does so more appropriately than the proposed change. 
For this reason, we are not adopting the petitioner's second request.
    The final requested change is:

    It would be some help to have a low air pressure warning device 
that comes on just before the start of the air compressor cycle. 
When this low air warning comes on the vehicle is in a dangerous 
situation. Number 1 and 2 will prevent this.

    The third requested change in the petition is not clearly defined 
for the agency to fully evaluate. The statement ``just before the start 
of the air compressor cycle'' has two meanings. The first meaning is a 
pressure slightly above the cut-in pressure, e.g., approximately 105 to 
110 psi. The second meaning is a pressure slightly below the cut-in 
pressure, e.g., approximately 90 to 95 psi. Based upon the information 
in the petition, the agency does not understand the concept of this 
warning lamp, and how its operation differs from the currently-required 
low pressure warning signal required in FMVSS No. 121, other than being 
set to activate at a higher air pressure. It also seems nearly 
identical to/redundant with the petitioner's first requested change, as 
this warning would activate just before the start of a new air 
compressor cycle, and then the warning from the first request would 
activate when the compressor began that new cycle. Furthermore, we note 
that activation of a warning signal at either of these pressures would 
result in the warning being activated extremely frequently, including 
during normal driving operations. Given these reasons, we are denying 
the petitioner's third requested change.

VI. Conclusion

    Based upon this review of the petition, the agency is denying it. 
In summary, it appears that one or two warning lamps would be required 
to activate upon each cut-in of the compressor cycle, and this would 
not provide additional information to the driver beyond the information 
that is already available from the existing air pressure gauges. In 
addition, we believe that warning systems that activate frequently 
during normal driving conditions can be perceived as a nuisance, and 
may have limited safety effect. Finally, we are not aware of any known 
safety problems not addressed by the existing low pressure warning 
signal requirements in FMVSS No. 121.

    Issued: March 3, 2008.
Stephen R. Kratzke,
Associate Administrator for Rulemaking.
[FR Doc. E8-4460 Filed 3-6-08; 8:45 am]

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