Brake Fade on a 6% Downgrade With 7,000 Gallons
You are driving a 9,000-gallon tanker loaded with 7,000 gallons of water-based drilling fluid-about 78 percent full-down a 6 percent grade that runs for 4 miles. The posted truck speed is 35 mph. You start down at 40 mph in 8th gear, using the service brakes to hold speed. After 1.5 miles, the brake pedal feels soft and the truck is creeping to 55 mph despite steady pressure. You smell hot brakes. The tank has two bulkheads, but the liquid is still moving. A runaway truck ramp is signed 1 mile ahead. The engine brake is on but set to its lowest stage. The load is not hazardous.
1. What is happening to the brake system, and why does the tanker's load make it worse?
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The brakes are overheating and fading: continuous service-brake application on a 6 percent grade converts speed into heat faster than the drums can shed it, so the friction material loses effectiveness and the pedal can feel soft. The tanker makes it worse because the vehicle is heavy and the liquid can shift forward on the downgrade, adding load to the front axle and reducing the margin on the rear. Even with two bulkheads, 7,000 gallons in a 9,000-gallon tank has a large free surface, so the liquid still moves and can push the truck. A tanker's high center of gravity also makes speed on a grade less forgiving. The engine brake at its lowest stage is not enough retarding force for this grade.
2. What should you do now, in order, given the runaway ramp is 1 mile ahead?
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First, stop riding the brakes. Downshift to a lower gear immediately and raise the engine brake to its highest safe stage. Use snub braking: apply the service brakes firmly to bring speed down, then release and let the engine brake hold it, repeating as needed. Do not turn the engine off or coast in neutral. If speed still climbs toward 55 to 60 mph and you cannot recover, use the runaway truck ramp-do not wait until the last 100 feet. Keep steering steady and warn traffic if possible. After stopping, let the brakes cool and do not continue until the system is checked; hot brakes can fade again.
3. How should you have set up the descent, and how would a half-full tank change your plan?
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Before the grade, I would check the brake system, select a gear low enough that the engine brake can hold 35 mph or less, and enter at or below the posted speed. I would use the engine brake and snub braking from the start, not service brakes continuously. A half-full tank would be worse: there would be more free surface and more surge, so I would reduce speed further and avoid any hard braking. Bulkheads help but do not make a partial load stable. If the load were a flammable liquid, I would also check placards and emergency equipment, but the grade technique would be the same.