Brake Fade on a Six-Percent Grade
Your 80,000-lb combination is descending a 6% grade on I-70 west of Salina. The grade is posted 2 miles long. You entered at 55 mph in 9th gear with the engine brake on high. Traffic slows, so you hold the service brake continuously for about 40 seconds. The pedal begins to feel soft and the truck accelerates to 68 mph despite the engine brake. You smell hot brake lining. The trailer is a dry van with no trailer brake on the engine brake, and the tractor has a single rear drive axle. The next exit is 1.5 miles ahead; an escape ramp is posted 0.7 mile ahead on the right. Your trailer hand valve is within reach.
1. Diagnose what is happening and explain why continuous braking caused it.
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The brakes are fading from heat. At 80,000 lb on a 6% grade, speed is being converted into heat faster than the drums can shed it. A continuous 40-second application overheats the linings and drums; the lining loses friction and the drums expand, so the pushrods travel farther and the air pressure is used up trying to maintain contact. That is why the pedal feels soft and speed climbs even though the engine brake is on high. The engine brake is only on the tractor, and a single drive axle gives limited retarding, so it cannot hold the load alone. The hot-lining smell confirms the system is past its thermal limit. Continuing to hold the pedal will make the fade worse, not better.
2. What do you do now, in order, and when do you commit to the escape ramp?
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Release the foot brake immediately so the drums can cool. Downshift to a lower gear to use engine compression, keep the engine brake on, and begin stab braking: apply firmly until speed drops about 5 mph, release, and repeat. Do not fan the pedal or hold light pressure. Warn traffic if possible and move toward the right lane. If speed continues to climb past roughly 70 mph, or if the pedal goes to the floor, commit to the escape ramp at 0.7 mile; do not wait for the exit at 1.5 miles. Enter the ramp under control with wheels straight and expect deep gravel. The ramp is a one-time decision; trying to save the load by passing it risks a runaway.
3. Why is the trailer hand valve the wrong tool here, and what should have been done before the descent?
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The trailer hand valve applies only the trailer brakes. It does not slow the tractor, and it can cause the trailer to swing or jackknife because the tractor keeps rolling while the trailer drags. It is a test device, not a downhill brake. Before the descent you should have selected a gear low enough to hold speed without service brakes, confirmed the engine brake was working, checked the trailer brake connections, and reviewed the escape ramp location. You should also have tested the brakes at low speed before the grade and kept a safe following distance. Once fade begins, the correct response is to reduce heat input and use a lower gear, not to add more brake pressure or grab the hand valve.