Brake Fade on a Six-Percent Grade
You are descending a 7-km, 6% grade on Highway 17 in northern Ontario. Your 2019 Peterbilt 579 is loaded to 36,500 kg. At the top you are in 7th gear at 50 km/h with the engine brake on high. Halfway down, the speed is 75 km/h and still climbing despite a light, steady 10 psi service brake application. The brake pedal feels soft, and you smell hot lining. The air gauge is at 108 psi. The shoulder is narrow, and a runaway lane is 1.2 km ahead on the right. Traffic ahead is light but moving at 80 km/h. The grade is signed '6% for 7 km' and the posted limit for trucks is 60 km/h. You have already passed the first brake-check area.
1. Explain why the speed is rising despite a constant brake application. What is brake fade?
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The speed is rising because the service brakes are no longer producing enough retarding force to hold the load on a 6% grade. A light, continuous application is the classic cause of brake fade: the drums and linings overheat, the lining friction coefficient drops, and the drums expand. As the drums expand, pushrod stroke increases and the brake force for the same pedal application falls. The smell of hot lining and the soft pedal are warning signs.
The engine brake is helping, but it is not enough for 36,500 kg on a 6% grade in 7th gear. The vehicle is gaining speed because gravity is overcoming the combined retarding force. This is not a situation to solve by holding the same light pedal pressure; the brakes are already too hot.
2. Give the sequence of corrective actions you would take now.
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Release the service brake, select a lower gear, and keep the engine brake on high. Then use snub braking: apply the service brake firmly enough to reduce speed to a safe target, such as 40-45 km/h, then release the brake and let the engine brake and lower gear hold the speed. Repeat only as needed. Do not ride the brakes continuously.
If the speed still climbs after the lower gear and a snub, or if the pedal continues to feel soft, use the runaway lane 1.2 km ahead. Turn on the four-way flashers, warn traffic if possible, and commit fully to the runaway lane rather than trying to continue. Keep watching the air gauge; if pressure falls toward the warning range, the spring brakes may apply and the situation becomes more dangerous.
3. Why would pressing harder or continuing to ride the brakes make the situation worse?
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Pressing harder adds more heat to already overheated brakes. Fade can worsen, the lining can burn or crack, the drum can crack, and a brake fire becomes possible. If the brakes are hot enough, even a hard application may produce less stopping force than a cool brake would.
Riding the brakes also consumes air and keeps the system cycling, which can reduce the reserve available for an emergency stop. The correct response is to use gear selection and engine braking to control speed, with short, firm snubs only when needed. If that does not work, the runaway lane is the safer choice than a continued descent on fading brakes.