Brake Fade on a Six Percent Grade With a Loaded Van
You are driving a 5-axle tractor-semitrailer at 78,000 lb gross eastbound on a 6% downgrade 2.8 miles long. At the top a sign reads trucks 35 mph, and a runaway ramp is marked near the bottom. You crest the grade at 35 mph in 9th gear with the engine brake on high, air pressure at 118 psi, and outside temperature at 95 F. About a mile down, you smell hot brakes and see your speed has climbed to 48 mph even though you have been braking firmly every 15-20 seconds. The shoulder is narrow, and a loaded tanker is about 100 feet behind you. The engine brake is still on high and the tachometer is near the top of its operating range.
1. What should you have done before crossing the crest, and why does entering at 35 mph in 9th gear matter?
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Before the crest: check brake adjustment and air pressure, do a brake test (apply and feel for pull or delay), confirm the engine brake is working, read the grade and length signs, and select a gear low enough that engine braking holds the truck at or below the posted 35 mph without service brakes. Entering at 35 in 9th gear matters because a diesel with the engine brake on high can hold a steady speed on a 6% grade only if the gear keeps engine rpm in its effective range. You cannot fix a too-high gear choice halfway down by braking harder. The crest is the last place where you can slow down cheaply; after that, every mph you add must be removed by the brakes, which converts speed into heat.
2. Halfway down, you smell hot brakes and the speed is climbing to 48 mph. What is happening, and what are your actions in order?
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You are in brake fade. Repeated or continuous service brake applications have overheated the drums and linings, friction is dropping, and that is why speed climbs despite braking. Actions in order: release the service brake fully, downshift one or two gears to raise engine rpm so the engine brake works harder; if the engine brake alone will not hold, use snub braking, a firm short application that drops speed by about 5 mph, followed by a full release to let the drums cool; get speed back to 35 mph or below and keep it there. If speed continues to climb, or the pedal feels long or the truck will not slow, commit to the runaway ramp rather than ride the brakes to the bottom. Do not hold light continuous pressure. That is what caused the fade.
3. Your air gauge has dropped from 118 to 95 psi during the descent. How do you interpret that, and does it change your plan?
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A drop to 95 psi is not yet an emergency, since the low-air warning is about 60 psi and spring brakes begin to apply in the 20-45 psi range, but it warns that your brake applications are consuming air faster than the compressor is replacing it. On a descent the compressor should be turning at high rpm and recovering quickly; a steady decline suggests either an air leak or an application pattern that never lets the system recover, such as long, light, continuous applications. The plan changes in two ways. Switch to snub braking with full releases so the compressor has recovery time, and lower your threshold for using the ramp. If the gauge keeps falling toward 60 psi while you still need service brakes, stop relying on brakes as your primary control and take the escape ramp. Never continue a descent hoping the compressor catches up.