Brake Fade on a Six-Percent Grade
You are driving an 80,000-lb tractor-trailer down a 6% grade that runs for 5 miles on Oregon Route 58. You entered the grade at 55 mph in 10th gear and have held the service brake pedal down continuously for about 25 seconds. The speed is now 70 mph and still climbing. The brake pedal feels soft and the truck is not slowing. You smell hot brake lining. The air gauge has dropped from 120 to 95 psi but is holding. A runaway truck ramp is signed one mile ahead. Traffic is moderate, and the right shoulder is narrow. The engine brake is available but is currently switched off. You have not downshifted since entering the grade. The load is sealed and heavy.
1. What mistakes led to this situation?
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The main mistakes were entering a long 6% grade at 55 mph in 10th gear and then holding the service brakes down continuously. That converts speed into heat in the drums instead of letting the engine and engine brake absorb it. The engine brake was off and no downshift was made, so the service brakes had to do all the work. At 80,000 lb on a 6% grade, heat builds quickly; the soft pedal and hot-lining smell indicate brake fade. The air gauge drop from 120 to 95 psi shows the continuous application is also consuming air, which can lead to a low-pressure emergency.
2. What should you do now, in order?
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Release the brake pedal and downshift immediately to a lower gear. Turn on the engine brake. Use the service brakes in short, firm snubs rather than a continuous drag, but only if they still respond. If the speed cannot be controlled, aim for the runaway truck ramp; do not wait until you are past it. Keep steering and use the shoulder only if it is safe. Once stopped, set the parking brake, chock the wheels, and do not continue until the brakes cool and are inspected. A faded brake can recover somewhat after cooling, but the lining may be damaged.
3. Why is holding the pedal down more dangerous than snubbing?
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Continuous light application keeps the shoes against the drums for a long time, so heat has no chance to dissipate. Drum brakes can fade when the lining and drum overheat; the pedal may feel soft or normal while stopping force drops. Snubbing applies the brakes firmly for a short time, then releases them so air can cool the drums. It also lets you check whether the brakes are still there. On a long grade, the correct primary control is a low gear plus engine brake, with service brakes used in short applications to bring speed down in steps.