Six Percent Grade South of Dickinson
South of Dickinson, you are driving a 78,000-pound tractor-trailer down a 6 percent grade that runs for 5 miles. The posted speed for trucks is 40 mph, and you entered the grade at 58 mph in 9th gear with the engine brake on. To hold 55 mph, you have kept steady pressure on the service brake for about a mile. Near the second mile marker, the pedal feels low and spongy, you smell hot brake lining, and the truck accelerates to 67 mph even though you are pushing harder on the pedal. The engine brake is still on, the trailer brakes are adjusted, and there is an escape ramp 1 mile ahead on the right. Traffic is light. Your air pressure gauge is steady at 105 psi.
1. What is happening, and how did your braking technique contribute?
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You are experiencing brake fade from continuous service-brake use. On a 6 percent grade with 78,000 pounds, the engine brake in 9th gear at 58 mph was not holding the load, so you kept the service brake applied for about a mile. That turns the drums and linings into a heat sink: friction drops as they overheat, the pedal can feel low or spongy, and the truck speeds up even with more pedal force. The steady 105 psi air gauge makes a major air leak less likely; the smell and speed increase point to overheated foundation brakes. Entering at 58 mph in 9th gear was too fast and too high a gear for a 40-mph truck grade.
2. What should you have done before the grade, and what is the correct snub-braking technique?
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Before the grade: check brake adjustment and air pressure, select a gear low enough to hold the posted 40 mph without riding the service brake, enter the grade at or below 40 mph, and use the engine brake. Snub braking: apply the service brakes firmly to reduce speed by about 5 mph, then release completely and let the brakes cool while the engine brake and gear hold speed. Repeat as needed. Do not drag the brakes continuously; that is what caused the heat. If the truck will not hold 40 mph in the chosen gear, stop and choose a lower gear before continuing.
3. What should you do now, with speed increasing at mile two and an escape ramp one mile ahead?
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Do not continue at 67 mph. You still have an escape ramp one mile ahead, and the speed is increasing despite more pedal. Reduce speed with a firm, controlled application while downshifting progressively, keep the engine brake on, and get off the road - use the escape ramp if you cannot get back to 40 mph by the ramp entrance. Continuing risks a runaway with fading brakes. Once stopped, chock the wheels, let the brakes cool, and inspect for damage before moving. Do not try to make up time on the remaining grade.