Six Percent Grade in the Rain
You are driving a 68,000-pound GCW tractor-trailer down a 6% grade that runs for four miles. The posted truck speed is 35 mph. At the top you are at 40 mph, the transmission is in D with the engine brake set to automatic, and the pavement is wet from a light rain. You figure the engine brake and automatic will hold you. Halfway down, speed climbs to 48 mph. You apply the service brake steadily for about 20 seconds, slowing to 42 mph, then release. A mile later you smell hot brake lining and the pedal feels slightly lower. The engine brake is still on, but speed is creeping up again. There is a runaway truck ramp two-tenths of a mile ahead on the right.
1. What went wrong with your descent plan, and what should you have done before entering the grade?
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The descent plan was wrong at the top. A 6% grade for four miles with 68,000 pounds needs a gear that holds 35 mph without continuous service brake, and the top is where you choose it - not halfway down. D with the engine brake on automatic let speed build to 48 mph, and then a 20-second continuous application put heat into the brakes. Before entering the grade I would have slowed to the posted 35 mph or lower, selected a lower gear (for many tractors, one or two gears below the gear that would hold 35 on flat ground), confirmed the engine brake was on high, and checked that the trailer brakes were working. On wet pavement I would use gentler snubs but still control speed; wet does not mean coast and hope. The smell of hot lining and a lower pedal are signs of fade, not normal braking.
2. Explain the correct braking technique for the remaining grade and why continuous light application is harmful.
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Use snub braking: apply the service brake firmly enough to drop about 5 mph, then release completely and let the brakes cool while the engine brake and gear hold speed. Repeat as needed. Continuous light application is harmful because it keeps the shoes and drums hot for a long period, which can glaze the lining, boil moisture out of the drum, and cause fade - the pedal goes down and braking force drops. It also uses more air over time. Keep the engine brake on, stay in the lower gear, and do not fan or pump the pedal. If the speed will not hold even with snubs, shift down again before the rpm gets too high, and be ready to use the escape ramp.
3. The pedal is lower and speed is rising. What are your actions, and when would you use the ramp?
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A lower pedal and rising speed mean the service brakes are fading. Do not pump the pedal and do not continue down the grade hoping they recover. Immediately downshift to the next lower gear, keep the engine brake on, and make one firm, straight-line brake application to reduce speed as much as possible. If the pedal goes to the floor or the truck keeps accelerating, the escape ramp is the correct choice; use it before your speed is too high to enter safely. Steer straight, keep the wheels rolling, and do not attempt to turn off at speed. The ramp is designed to stop you; a runaway truck is not. After stopping, set the parking brake, chock, and do not move until the brakes are inspected.