Brake Fade on a Wet Mountain Grade
You are driving a 40-foot air-brake coach with 38 passengers down a 6% mountain grade that runs four miles. The pavement is wet and the outside temperature is 38°F. You are in fourth gear at 45 mph; the posted limit for buses is 45. About halfway down you smell hot brake lining, the service brake pedal travels farther than normal, and the air-pressure gauge holds at 105 psi. The engine retarder is available but the selector is in the 'off' position. A curve at the bottom is signed 25 mph. Traffic behind is light. There is a gravel pull-off 300 feet ahead on the right; it is 80 feet long and slopes slightly downhill. You have not used the brakes continuously, but you have made several applications.
1. What is happening, and what should you do immediately?
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The hot lining smell and longer pedal travel point to brake fade from overheated brakes. The steady 105 psi suggests this is not an air leak. Do not wait for the bottom of the grade or for the 25 mph curve. Reduce speed now: downshift to a lower gear, turn on the engine retarder, and make a firm, brief brake application to bring the coach to a crawl. Pull into the gravel pull-off 300 feet ahead.
In the pull-off, set the parking brake, chock wheels if needed, and let the brakes cool. Inspect for smoke, fluid leaks, or damaged components. Notify dispatch. Do not pour water on hot brakes. If the pull-off is too short or the surface is too soft for a 40-foot coach, use the safest available wide spot before the curve rather than continuing at speed.
2. What is the correct descent technique for this grade, and what was wrong with the original approach?
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The original approach was too fast and too dependent on the service brakes. At 45 mph in fourth gear on a wet 6% grade, the coach is already at the posted limit before the brakes are worked hard. The correct technique is to select a gear low enough to hold speed without continuous braking, engage the retarder, and use stab braking: firm applications, then release to let the brakes cool. Keep the speed at a safe crawl, well below the posted limit, and use pull-offs to cool the brakes.
A 25 mph curve at the bottom is a clue that the safe descent speed is much lower than 45 mph. Do not ride the brakes lightly for four miles. Also check the retarder before starting the grade rather than discovering it is off halfway down.
3. If the rear of the coach begins to slide on the wet curve, what do you do?
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Release the service brake and steer in the direction of the skid: look and steer where you want the front of the coach to go. Avoid hard braking and sudden steering. If the engine retarder is contributing to drive-axle slip on the wet surface, turn it off. Do not overcorrect; once the coach straightens, slow gently.
With 38 passengers, smooth inputs matter more than aggressive correction. If the front wheels skid, release the brake and steer straight until traction returns. After the skid, stop at the next safe place, inspect the brakes and tires, and decide whether to continue.