Brake Fade on a 6 Percent Grade
On a 6 percent downgrade in north Georgia, a driver is descending a 3-mile grade with a loaded tractor-trailer weighing about 40,000 lb. The speed limit is 45 mph. The driver has been applying the service brakes lightly and continuously to hold 45 mph. About halfway down, the speed rises to 55 mph, the air pressure gauge still reads 100 psi, and there is a hot-brake smell. The service brake pedal now feels soft and requires more travel, but the low-pressure warning has not activated. The engine brake is available but has not been used; the transmission is in a high gear. The shoulder is narrow, and there is a runaway truck ramp 1 mile ahead. The driver must decide what to do immediately and what should have been done at the top of the grade.
1. The gauge still reads 100 psi and the warning has not activated. Is the soft pedal and speed increase caused by low air pressure? Explain.
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No. The gauge is in the normal range and the low-pressure warning has not activated, so the immediate cause is not an air supply failure. The hot-brake smell, the speed increase on a downgrade, and the soft/long pedal point to brake fade from overheating. Continuous light brake application keeps the drums and linings hot without giving them time to cool. As the linings heat, friction drops and the brakes lose effectiveness. The pedal may feel different because the pushrods are traveling farther as the drums expand, but the driver should not mistake that for a normal pedal. The correct diagnosis is overheated, fading brakes-not a low-air condition.
2. What should the driver do right now? Address gear, engine brake, brake application, and the runaway ramp.
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The driver should act immediately to reduce speed and stop the heat buildup. Downshift to a lower gear to increase engine braking, and turn on the engine brake if the vehicle is equipped and conditions allow. Apply the service brakes firmly enough to drop speed by about 5 to 10 mph, then release them completely to let the drums cool; repeat as needed rather than riding the pedal. Do not use the parking brake at highway speed. If the speed continues to climb and the brakes cannot hold it, use the runaway truck ramp rather than continuing down the grade. The driver should also turn on hazard lights and warn traffic. The goal is to get the speed down before the fade becomes a no-brake condition.
3. What should the driver have done at the top of the grade, and how does snub braking differ from what was done?
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At the top, the driver should have checked the brakes, selected a gear low enough to hold a safe speed without continuous braking, and used the engine brake as the primary retarder. The safe speed should have been set before the descent, not discovered halfway down. Snub braking means applying the brakes firmly to reduce speed by about 5 to 10 mph, then releasing them so air can cool the drums, and repeating as speed builds. Continuous light application-what this driver did-keeps the brakes hot and causes fade. A lower gear reduces the amount of energy the service brakes must absorb and gives the driver a reserve if the grade steepens. The engine brake should have been on before the speed climbed, not after the smell of hot brakes appeared.