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Trailer Brake Alarm Sounds

TORONTO, Ont. - The Maintenance Council of the Ontario Trucking Association has sounded the alarm over a potentially deadly flaw in the Canadian Motor Vehicle Standard governing the operation of trail...


TORONTO, Ont. – The Maintenance Council of the Ontario Trucking Association has sounded the alarm over a potentially deadly flaw in the Canadian Motor Vehicle Standard governing the operation of trailer brakes – a flaw it believes may have been responsible for at least one fatal and several non-fatal accidents in the recent past.

Problems with CMVS Safety Standard 121 regulations of trailer service brakes were first identified when the British Columbia Trucking Association spoke out in the pages of Truck West in 1996. At that time, the BCTA warned fatal accidents could occur as a result of weaknesses in 121.

It appears the chickens have come home to roost.

This month, the maintenance council came forward to put the blame for at least one recent traffic fatality and several other accidents that could have proven fatal squarely in the lap of 121.

The council did so in two meetings, a special meeting for maintenance managers, government representatives, manufacturers, insurance companies and police officials held June 25, and another for trade media, including Truck News, June 26.

Safety Standard 121 currently requires, in the event of a loss of air pressure in the trailer service brake tanks, that trailer spring brakes must not apply or even drag.

It also says that a loss of trailer service tank pressure must not result in supply (emergency) line pressure dropping below 70 psi.

To meet the requirements, two different systems are currently used.

One gives priority to the spring brake release before filling the service tanks, while the other simultaneously fills the service brake tanks and releases the spring brakes.

The “spring brake priority” system offers quick release of the trailer parking brakes and allows a parked trailer with a failed service tank to be moved to a safer parking area.

With this system it is also possible to couple up to a trailer with empty air tanks, charge the system, release the spring brake and drive away without having any air to operate the service brakes.

The “service brake priority” system does not allow a trailer to be moved until service tanks come up to pressure.

With these systems, a trailer that suffers a service tank failure cannot be moved without manually caging the spring brakes.

However, says Vanderzwaag, if trailer pressure is lost during on road operation with either of these systems, it is possible for the driver to drive for many miles without knowing the trailer has no service brakes.

That’s because air that fills the trailer tanks must pass through a number of restrictions that exist in trailer supply dash valves and trailer spring brake control valves. This feature is essential for meeting 121 requirements.

Because of the minimum 70 psi holdback feature built into the trailer valving, it is possible that even with a major failure in the trailer service tank system, the compressor can keep ahead of the leak rate and the tractor air pressure could come up to near normal, giving the driver no indication of the failed tractor system.

There are a number of causes for loss of service tank air pressure.

Road debris could knock the drain valves off the air tanks and jam the pull cord, air tank valves could come apart, valves could fail or could be improperly installed, says Vanderzwaag.

The potential for loss of all service brakes as a result of a single failure becomes even more serious for train combinations, where booster relay valves are widely used.

If pressure is lost on the lead unit, there will be no service brakes on either unit, because the booster relay will have lost its air supply and cannot signal the brakes on the rear unit to be applied.

Again, the driver may not know that only the tractor brakes are available until the vehicle needs to stop.

The problem is even further exacerbated for multiple-axle trailers, that is trailers with three or more axles (almost 50 per cent of trailers in Canada) which will obviously push tractors much further in the event of a brake failure. (Current 121 standards are practically identical to those developed in the United States, where trailers with three or more axles represent less than three per cent of trailers in the U.S.

According to the maintenance council, this in part explains the failure of 121 standards to set adequate safety requirements for many trailers used in Canada.)

In a discussion paper presented to media, government and industry representatives alike, Vanderzwaag pointed to several incidents, at least one of them fatal, during which the OTA Maintenance Council believes brake performance was compromised in a way that has not been adequately addressed by 121 and provincial standards.

The fatal, in which a pedestrian was killed, occurred in March of this year.

The case involved a 1997 model trailer from which an air tank drain valve became separated. All of the air was lost from one air tank.

In his discussion paper, Vanderzwaag reconstructed the accident as follows: The air drained out of the tank, reducing the pressure in the trailer supply line and drawing air supply from the tractor.

Since the rate of air loss through the open air tank was being restricted by the pressure protection valve in the trailer’s spring brake control valve, the air compressor easily kept up with the demand for air from the trailer.

The air pressure in the tractor dropped to slightly less than 100 psi.

The trailer spring brakes were held off by means of the pressure protection valve in accordance with CMVS 121 S5.8.3.

The vehicle remained apparently stable in this condition and no indication was given to the driver of the impending service brake failure.

When a service brake application was made, the control signal sent to the tractor opened a (FAB) balancing valve that draws air from the failed tank.

Since there was no air to send to the subsequent relay valves and there was no means for air to flow through the valve no service brake action took place.

According to Vanderzwaag, certain pilot relay valves (also called balancing valves) address the problem by means of a bypass orifice and the use of a check valve to allow the control signal to flow through to subsequent valves in the event of such a failure. But not all valves have this feature.

This example of unexpected trailer service brake failure was just one of 14 actual incidents described in Vanderzwaag’s paper.

According to the OTA Maintenance Council, these are problems that could and should be addressed in a revision of 121 requirements for trailer brake systems; revisions called for when the BCTA first sounded the alarm in 1996.

At that time, according to Vanderzwaag, some revisions were made, but not the proposed revisions that would have alleviated the problems described in his paper.

These revisions were not made due to what the authorities called insufficient evidence that the current standards were posing a safety risk, Vanderzwaag says.

It seemed that federal and provincial authorities here in Canada preferred to mirror their U.S. counterpart (the National Highway Traffic Safety Administration) which dropped safety provisions, such as split braking systems and low air warning requirements from their rule-making in 1992 (remember, less than three per cent of U.S. trailers have three or more axles, while 50 per cent of Canadian trailers do.)

The OTA Maintenance Council is hoping that raising the issue and publishing Vanderzwaag’s paper will encourage provincial and federal authorities to create additional CMVSS requirements for trailer brakes that address the issue for Canadian carriers (the Canadian standards are currently essentially a copy of U.S. 49 CFR 571.121).

The maintenance council proposes 121 standards be revised for trailer brake systems with three or more axles, in order to allow vehicles that are designed to common interjurisdictional (cross-border, cross-provincial) configurations to be unaffected.

Changes suggested in the paper include:

Require low air pressure warning on trailer air tanks. (ABS communication protocols allow this functionality to be added with very l
ittle additional hardware.)

Require dual service system on trailers.

This could involved three air line connections between the tractor and the trailer, and minor modifications to the tractor.

Use of dual control relay valves would ensure if one circuit failed, control signals would still reach each relay valve, or axle by axle split systems could be used.

Require all relay valves to have back up air sources or other means of redundancy.

Install a low air warning system into the tractor protection system that is set to 95 psi.

This would sense air pressure drops resulting from air pressure being lost from a failed air tank or failed air hose, and would provide warning when a trailer’s air tanks have not yet filled.

Even though the loss of air is not visible on the tractor air pressure gauges, this sensor would detect the problem.

Adding supplementary fill lines for each tank by “tee-ing” from the trailer supply line to each air tank using a one-way check valve. (This would not comply with current 121 specs but would ensure rapid and complete tank fill and a noticeable air loss rate if the tank fails.)

Revise trailer-valving requirements to respond to loss of service tank air by applying the trailer spring brakes.

This may also require a protected air tank to allow releases of the spring brakes in an emergency.

Revise requirements for rapid exhausting of the trailer supply line.

The OTA Maintenance Council also recommends revising provincial standards for:

More intensive inspection of air brake systems during periodic safety inspections.

This would require a sufficient level of technician knowledge to effectively inspect the systems.

Requiring vehicles to be subjected to performance-based brake testing at periodic intervals.

Standards for emergency brake application timing on combination vehicles that consistent with the timing requirements in 121.

Separate licensing of technicians to repair air brake systems.

Of course, these revisions have yet to be enacted.

“We have produced this paper and now we intend to submit letters asking for revisions of the provincial and federal standards,” Vanderzwaag explains.

In the meantime, fleet maintenance managers, drivers and technicians will just have to make the best of it, by adopting best business practices to deal with the inherent problem. The OTA Maintenance Council recommends the following to carriers and drivers:

Improve driver education and awareness in methods of responding to the loss or malfunction of trailer brakes. (According to the OTA, the only recourse available to a driver who has lost trailer service brakes and needs brakes in an emergency situation is to apply the trailer emergency brakes by means of the trailer supply valve (the red button).

Most drivers are not trained to do so and given the short period of time they have to respond when faced with such an emergency that has potentially disastrous consequences, most will not think of it in time.

Drivers are also often averse to taking such severe action for fear of a jackknife, rollover or other serious consequences.

According to the OTA, these fears may not be valid, (spring brake applications are not as severe as many drivers believe and in many cases do not cause wheel lock up) but most drivers have never applied the trailer emergency brakes while the vehicle is in motion. They have no experience of how the vehicle will actually respond.

This information has not typically been found in driver training manuals, according to Vanderzwaag. Ontario’s air brake program now includes this information, but it needs to be reinforced by employers and the training community.

Improve technician training to ensure technicians are able to correctly inspect air brake systems, fully understand their functionality and the differences between the operations of critical brake valves.

Conduct more intensive inspection of crashes involving trailers.


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