Brake Fade on a Six Percent Mountain Grade
You are driving a 5-axle tractor-semitrailer grossing 78,500 lb eastbound on I-70 descending from the Eisenhower Tunnel toward Georgetown. The grade is 6 percent for about 2 miles. You entered at 55 mph in 9th gear; the engine brake is on high. Traffic is light. About halfway down, you smell hot brakes and notice the service brake pedal feels soft. Your speed has crept to 62 mph. The air pressure gauge reads 95 psi and has dropped about 5 psi since the top. You have been using the service brakes in short applications to hold speed, but the last application took longer to slow. The low-air warning has not sounded. The shoulder is narrow, and the next runaway truck ramp is 1.5 miles ahead.
1. What is happening to the brakes, and why is the current descent technique making it worse?
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The smell and soft pedal point to overheated service brakes, not normal wear. On a 6 percent grade, 9th gear and 55 mph are too fast for 78,500 lb; the engine brake cannot absorb enough energy, so the service brakes are doing the work. Short applications at high speed let speed build between applications and keep the drums hot. As the drums heat, friction drops (fade), so the same pedal pressure produces less braking and the pedal may feel soft or long. The 5 psi drop is not yet a low-air emergency, but it means you must stop adding heat and get the combination under control before the next steeper section.
2. What should you do now to control speed without causing a runaway?
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Do not wait for the runaway ramp. Make a firm, controlled brake application to bring speed down toward 35 mph, then select a lower gear before the speed climbs again. Use the engine brake on high and let it hold the lower speed; use the service brakes only in short snubs to stay below the safe speed, then release them to cool. If the pedal continues to fade or speed will not hold, use the next pullout or runaway ramp rather than continuing. Do not ride the brakes down the grade, and do not shift to neutral.
3. If the low-air warning sounds at 60 psi, what is the correct response?
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At 60 psi the low-air warning should sound; that is a warning, not the final spring-brake application, which typically occurs at 20-45 psi. Treat 60 psi as an emergency: stop braking with the service brake if possible, use the engine brake and downshift, and pull off at the first safe place. Do not continue to the bottom of the grade. Once stopped, set the parking brake, chock the wheels, and inspect for a leak or compressor problem. If the pressure continues to fall, the spring brakes will apply and the vehicle may become immobile.