Brake Fade on a Long 6% Downgrade
You are driving a 5-axle tractor-semitrailer at 78,000 lb on a 6% downgrade that is 5 miles long. The posted speed for trucks is 35 mph. At the top, you are at 45 mph in 8th gear with the engine brake on high. The grade steepens after the first mile. To hold 45 mph, you use the service brakes for about 4 seconds every 15 seconds. After 2 miles, the pedal feels soft and travels closer to the floor; the truck accelerates to 55 mph despite the engine brake; you smell hot brake lining. Traffic is behind you, and a runaway ramp is 1 mile ahead on the right.
1. What is happening to the brakes, and why is this descent technique causing it?
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The smell and loss of retardation are signs of brake fade: the drums and linings are overheated, so friction drops and the brakes cannot convert motion into heat as effectively. The pedal change is also a warning that pushrod travel may have increased as the drums expanded, or that the system is not keeping up. The technique is the cause. At 78,000 lb on a 6% grade, applying the service brakes for 4 seconds every 15 seconds keeps the brakes hot without giving them enough time to cool. The engine brake on high is not enough by itself at that weight and grade, and 8th gear is too high to hold 35 mph.
2. What should you have done before starting down the grade?
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Before the descent, check the brakes and air pressure, then select a gear low enough that the engine brake and compression alone hold 35 mph or less. On a 6% grade with 78,000 lb, that likely means a much lower gear than 8th, chosen at the top while speed is low. Use the engine brake and downshift before speed builds. If the engine brake or gear cannot control the speed, do not start down. Use snub braking: apply the service brakes firmly to reduce speed by about 5-10 mph, then release them so they can cool, rather than dragging them lightly. Put on hazard lights and watch for the runaway ramp.
3. What are your immediate actions now that speed is increasing and the pedal feels soft?
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Do not continue riding the brakes. Downshift immediately, one gear at a time, to get more engine braking, keep the engine brake on, and make firm snub applications to bring speed down. If the truck still accelerates or the pedal goes to the floor, steer for the runaway ramp and use it rather than trying to reach the bottom. Do not pump the brakes; that wastes air and can make a low-pressure problem worse. If you must use the parking brake to stop, apply it gradually and be ready for the spring brakes to lock. After stopping, let the brakes cool, inspect them, and do not continue until the cause is corrected.