Fanning Brakes on a Long Downgrade
You are driving a loaded 5-axle combination on a 7% downgrade that is 8 miles long. The speed limit is 45 mph. You entered the grade at 50 mph in 9th gear. To control speed, you have been making quick 2-second brake applications and releases every 10 seconds. After 4 miles, the pedal feels low, the combination is not slowing as it should, and the air gauge has dropped from 118 to 88 psi. You smell hot linings. The engine brake is on low. You see a sign for a brake check area 1 mile ahead and a runaway truck ramp 2 miles ahead. Traffic behind is close. The load is 44,000 lb, and the road is dry.
1. What has the driver been doing wrong, and why is it causing both low air pressure and poor stopping?
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The driver entered the grade too fast and in too high a gear, with the engine brake only on low. The repeated quick 2-second applications every 10 seconds are essentially fanning the brakes: they keep the foundation brakes hot without giving them time to cool, and each application uses air. The compressor cannot keep up with that demand, so the gauge has fallen from 118 to 88 psi. Hot linings also fade-the brakes lose friction-so the pedal feels low and the combination does not slow as it should. The low air pressure and the hot brakes are separate but related problems: poor technique is driving both. The driver should not continue the same pattern and expect the grade to end before the brakes or air supply fail.
2. What should the driver do now, in order?
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First, shift to a lower gear progressively and raise the engine brake to a higher setting, if the speed allows. Then make a firm, controlled brake application to bring the speed down, release it when the speed is reduced, and repeat only as needed-do not continue rapid fanning. Aim for the brake check area one mile ahead to stop and inspect. If the combination will not slow, or if pressure keeps falling, use the runaway truck ramp two miles ahead rather than trying to ride it out. Keep steering and avoid a sudden full spring-brake application at highway speed. At the check area, stop, secure the truck, and let the brakes cool; do not immediately set the parking brake on extremely hot drums without considering that the brakes may not hold until they cool.
3. How should this descent have been set up before entering the grade?
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Before the grade, the driver should have selected a gear low enough that the engine brake and compression hold a safe speed without continuous service braking. The speed at the top should already be at or below the safe descent speed-not 50 mph in 9th gear. The engine brake should be on the setting needed for the load and grade, and the driver should test the brakes at the top. During the descent, use controlled stab braking: apply firmly enough to slow, release when the speed is safe, and reapply as speed builds. Monitor the air gauge and brake feel. If pressure drops abnormally or fade begins, stop at the first safe place; do not wait for the next exit.