Brake Fade on a 6 Percent Night Descent
A driver is descending a 6 percent grade that runs 4 miles. Gross weight is 79,000 lb. It is 11 p.m., clear, and 34°F. The driver is in 8th gear in a 10-speed manual at 40 mph, using the service brakes in short applications. About 1.5 miles down, the driver smells hot brake lining, the pedal feels firm but the truck needs noticeably more pedal travel to hold 40 mph, and the rear drums are hot at a pull-off. The engine brake is on high. The speed limit is 55 mph; a sign at the top said "Trucks use lower gear." Traffic behind is a line of cars. The next pull-off is 0.8 mile ahead. The tractor-trailer has drum brakes on all axles, and the driver has been on duty for 9 hours.
1. What is happening to the brakes, and what do the firm pedal and hot smell indicate?
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The symptoms point to brake fade from overheating, not a simple air leak. A 79,000-lb combination on a 6 percent grade for 1.5 miles has put a large amount of energy into the drums. Short, repeated applications still heat the brakes; at 40 mph in 8th gear the engine brake and driveline are not holding the load, so the service brakes are doing the work.
- Hot lining smell means the friction material is overheating.
- Firm pedal but more travel/pressure needed means the brakes are losing friction, and heat may be distorting drums or glazing lining.
- Hot rear drums confirm the heat source.
This can progress to a complete loss of service brakes. The sign at the top was the warning; the driver selected too high a gear and too high a speed.
2. What should the driver do in the next 0.8 mile and at the pull-off?
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Immediate actions:
- Put on four-way flashers and reduce speed smoothly; do not make a panic stop that can lock wheels or cause a jackknife.
- Use the engine brake on high and downshift one gear at a time to let the engine hold the load; use the service brakes in one firm, short application rather than riding them.
- Take the 0.8-mile pull-off. Stop, chock the wheels, and let the drums cool. Do not set the parking brake on very hot drums unless the wheels are chocked; follow the manufacturer's guidance.
- Inspect for smoke, cracked drums, glazed lining, damaged hoses, and air leaks. If anything is damaged, do not continue.
- Before restarting, select a gear that will hold descent speed without service brakes. If the truck will not hold in the lowest safe gear, call for help.
3. If the pull-off is full, what is the least-risk action?
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Do not continue down at 40 mph just because the pull-off is full. The least-risk action is to find a safe place to stop-even a wide shoulder-with flashers and reflective triangles, and to stop before the brakes are gone. Continuing with fading brakes on a 6 percent grade risks a runaway. If a runaway ramp is available, use it rather than gamble. Call dispatch, explain the brake fade and location, and request assistance. If the truck can be held at a crawl in a lower gear with the engine brake, move only far enough to reach a safe stopping area; otherwise stay put and protect the scene.