Air Pressure Falling on Jellico Mountain
Marcus is driving a 2019 tractor with a three-axle semitrailer, gross weight 78,000 lb, southbound on I-75 approaching the Jellico Mountain grade near the Kentucky-Tennessee line. The sign says 6% grade for 4 miles. At the top, his air pressure gauge reads 110 psi. He starts down at 55 mph in 9th gear, using the service brakes steadily to hold 50 mph. Three miles down, the gauge reads 80 psi and is falling; the low-air warning has not sounded, and he smells hot brake lining. Traffic is light, but the right shoulder is narrow and there is a runaway ramp 1 mile ahead. His engine brake is on but set to the lowest setting. He has 45 minutes to reach a delivery appointment.
1. What should Marcus have done before starting down the grade, and what should he do now with pressure at 80 psi and falling?
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Before the descent he should have checked the air-brake system, selected a gear low enough to hold 50 mph without the service brakes, set the engine brake to a high setting, and planned to use short snubs rather than steady pressure. Now, at 80 psi and falling with a hot-lining smell, he is overusing the service brakes and consuming air. He should stop the continuous brake application, downshift to a lower gear, increase engine-brake retarding, and make controlled snubs to reduce speed to a crawl. If he cannot hold speed or pressure keeps falling, he should use the runaway ramp one mile ahead rather than continue. At 60 psi the warning will sound, and at about 45 psi the spring brakes will begin to apply, so he has little margin. He should not wait for the warning to act.
2. Explain what happens as pressure continues down from 80 psi to 20 psi, and why pumping the brake pedal to conserve air is wrong.
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At 60 psi the low-air warning buzzer and light must activate. At roughly 45 psi the spring brakes begin to apply, and by about 20 psi they are fully applied; the vehicle will stop and may not be able to move until pressure is restored. Pumping the pedal releases a burst of air on every application, so it drains the reservoirs faster and reduces the air available for braking. It also adds heat to already hot brakes. The correct response is to use engine braking and gears, not to fan or pump the brake. If pressure reaches 45 psi on the grade, he should steer to the shoulder or ramp and stop; the spring brakes will help hold the vehicle.
3. At the runaway ramp, he stops and sees smoke from a brake drum. What should he do and not do?
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Set the parking brake, chock the wheels, keep people away, and let the drums cool. He should not pour water on hot drums or crawl under an unblocked vehicle. He should inspect for fire, call for repair, and not continue until the brake system is checked. If flames appear, evacuate and call 911. Overheated brakes can fade or catch fire; the smoke indicates severe overheating, so treating it as a minor smell would be dangerous.