Brake Fade on a Six-Percent Grade
You are driving a 45-foot, 40,000-pound coach with 42 passengers down a 7-mile, 6% grade on a two-lane mountain highway. The posted speed is 55 mph. The coach has an automatic transmission with a retarder and an engine brake. You entered the grade at 50 mph in a high gear. Two miles down, speed has climbed to 68 mph despite brake applications about every 30 seconds. You smell hot brakes, the pedal feels soft and travels farther, and the air gauge reads 95 psi and is slowly falling. There is a runaway truck ramp 3/4 mile ahead. The temperature is 70°F and the pavement is dry.
1. What do you do in the next 30 seconds?
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Act immediately. Apply firm, steady pressure to bring the speed down to about 35–40 mph, select a lower transmission range, and put the retarder and engine brake on full. Do not pump the brakes. Tell passengers to stay seated and brace. If the speed does not fall within a few seconds, if the pedal goes soft to the floor, or if the air pressure keeps dropping, commit to the runaway ramp 3/4 mile ahead. Do not wait until the ramp is past. Keep both hands on the wheel and steer straight; do not try to turn back onto the highway once you are on the ramp.
2. What descent technique should have been used, and what went wrong?
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The descent started in too high a gear. On a 6% grade with 40,000 pounds and 42 passengers, the correct setup is to select a gear low enough to hold a safe speed without service brakes, engage the retarder/engine brake before the grade, and use snub braking: apply the brakes firmly to cut about 5–6 mph, then release and let them cool. The driver applied brakes every 30 seconds while the speed climbed, which is riding the brakes-heat builds faster than it can dissipate, the drums glaze, and the pedal goes soft. An automatic transmission should be downshifted manually; never coast in neutral. The speed should have been controlled at the top, not at 68 mph.
3. How do you decide whether to use the runaway ramp?
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Use the ramp now. The warning signs are all present: speed still climbing after repeated applications, hot-brake smell, a soft pedal with longer travel, and air pressure slowly falling. At 68 mph, 3/4 mile is about 40 seconds-enough if you commit immediately. If you pass the ramp and the brakes fail completely, you have no better option on a 7-mile grade. On the ramp, keep the wheel straight, keep the bus upright, and brake as firmly as the surface allows. After stopping, set the parking brake, evacuate if there is fire or smoke, and call 911. Then report the brake fade to maintenance before the bus moves again.