2.3 System Safety and Reduced Braking

System Safety and Reduced Braking

The Reality of Partial System Failure

One of the most significant engineering achievements of the dual air brake system is its ability to handle a partial system failure. A partial failure occurs when a component in either the primary or secondary circuit ruptures, leaks, or fails, resulting in a complete loss of air pressure in that specific circuit. Because the two circuits are isolated by one-way check valves and a dual foot valve, a failure in one circuit does not deplete the air supply of the other.

For example, if a brake hose on the rear axle ruptures, the primary circuit will lose all of its air pressure. However, the secondary circuit, which supplies the front axle brakes, remains completely sealed and pressurized. When the driver presses the brake pedal, the dual foot valve's mechanical contact mechanism will actuate the secondary circuit, allowing the front brakes to apply. This safety feature prevents the total brake failure that was common in older, single-circuit air systems.

Impact of Circuit Failure on Stopping Distance and Steering

While a vehicle with a partial system failure can still be stopped using the service brakes, its braking performance will be severely compromised. The driver must anticipate two major changes in vehicle behavior:

1. Significantly Longer Stopping Distances: Under normal conditions, all wheels on the vehicle contribute to braking. When one circuit fails, only the wheels connected to the surviving circuit will slow the vehicle. In a typical truck, the rear brakes (primary circuit) carry out the majority of the braking work because of the weight distribution of the cargo. If the primary circuit fails, the vehicle must rely entirely on the front steering axle brakes (secondary circuit) to stop. This can increase the stopping distance by two, three, or even four times the normal distance, especially if the vehicle is fully loaded.
2. Altered Steering and Stability Characteristics:
   • Losing the Secondary Circuit (Front Brakes): If the secondary circuit fails, the front steering axle brakes will not apply. While this means the driver maintains full steering response because the front wheels cannot lock up and slide, the vehicle will have very limited stopping power, relying solely on the rear axle brakes.
   • Losing the Primary Circuit (Rear Brakes): If the primary circuit fails, only the front steering axle brakes will apply. Under heavy brake application, the front wheels can easily lock up and slide. Once the steering wheels lock and slide, the driver loses all steering control, and the vehicle will travel straight ahead regardless of which way the steering wheel is turned. Additionally, on an articulated vehicle (such as a tractor-trailer), losing rear tractor brakes while front brakes apply can increase the risk of a jackknife.

Comparison of Circuit Failures

Executing a Controlled Stop Under Failure Conditions

If a driver experiences a partial system failure while driving, they must bring the vehicle to a stop using a technique known as a controlled stop. A controlled stop is designed to maximize braking efficiency while maintaining steering control and preventing the remaining wheels from locking up.

The driver must adhere to the following guidelines:

1. Apply Steady, Progressive Pedal Pressure: The driver should apply smooth, gradual pressure to the brake pedal. Since only one circuit is operating, the pedal may feel different (it may travel further down or feel spongy), but the driver must resist the urge to stomp on the pedal.
2. Do Not Pump the Brakes: Pumping the brake pedal is a common and dangerous mistake. Each time the driver releases the brake pedal, a volume of compressed air is exhausted from the system. If the compressor or the air supply is compromised, this air cannot be replaced. Pumping the brakes will quickly deplete the remaining reservoir pressure. If the surviving reservoir's pressure drops below the 20 to 45 psi (138 to 310 kPa) range, the spring parking brakes will automatically apply. This automatic application is abrupt and cannot be modulated by the driver, which can lead to wheel lockup, skidding, or a jackknife.
3. Maintain Directional Control: The driver must keep both hands firmly on the steering wheel. Because the braking forces are unbalanced, the vehicle may pull strongly to one side or feel unstable. The driver must make steering corrections to keep the vehicle in its lane.
4. Modulate Pressure to Avoid Wheel Lockup: If the driver feels the remaining wheels begin to lock up or slide (indicated by a loss of steering control or a squealing sound), they must slightly release the pedal pressure to allow the wheels to roll again, then reapply steady pressure. Rolling wheels are necessary to maintain steering control.
5. Secure the Vehicle Immediately: Once the vehicle has stopped in a safe location, the driver must immediately pull the parking brake valve to secure the vehicle and call for assistance. The vehicle must not be moved again until the failed circuit is repaired.