Pressure Falling on a 6 Percent Grade
A 79,000-lb five-axle combination starts down a 6 percent grade posted "Trucks 35 mph," five miles long, in eastern Tennessee. The load is 40,000 lb of steel coil secured behind a bulkhead. Pressure at the top: 120 psi primary, 122 secondary. The driver is in 7th and has the engine brake on high; the retarder is rated for this weight. One and a half miles down, the primary gauge reads 86 and the secondary 90. There is a light, continuous hiss from the left rear of the tractor. Speed has crept to 42 mph. The low-pressure warning has not sounded. A sign reads "Runaway Truck Ramp - 1/4 mile." The right lane is open, traffic is visible in his mirrors, and the shoulder is narrow with a guardrail.
1. Order the driver's next actions and justify the order.
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The governing fact is that pressure is falling on a grade and the service brakes are the only thing keeping the truck from running away. Order: first, downshift now while 42 mph is still manageable - 6th or lower - so engine braking carries the load without spending air. Second, keep the engine brake on high; the pavement is dry and the coil is behind a bulkhead, so there is no traction reason to back it off. Third, signal, move right, and take the ramp a quarter mile ahead. The justification for the order is outcome quality: a runaway ramp is engineered to stop a heavy truck and is a known, controlled result, while continuing with an undiagnosed leak and 86 psi is a bet that the compressor stays ahead of it. It probably will not, and the failure mode is spring brakes locking at speed. Diagnosis happens after the truck is stationary.
2. What happens automatically if he keeps rolling, and why is an automatic spring-brake application at highway speed dangerous?
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At 60 psi the low-pressure warning sounds. Somewhere in the 20-to-45 psi band the tractor protection valve closes, halting further loss from the tractor reservoirs and cutting supply to the trailer; the trailer's spring brakes then apply because their supply is gone. The tractor's own spring brakes apply as pressure falls into that same band. At 40-plus mph that is a full, unmodulated spring-brake application on the drive and trailer axles; wheels lock, the trailer comes around, and a jackknife on a downgrade is a likely result. Even if it somehow stayed straight, the truck stops in a live lane of a 6 percent grade, which invites a secondary collision. That is the reasoning behind choosing the ramp at 86 psi rather than gambling to the bottom.
3. Suppose the ramp had been closed. How should he make a controlled stop on the shoulder, and what should he avoid?
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Do it while pressure remains, and do it deliberately. Downshift first to bring engine braking in, then apply the service brakes progressively - firm, steady pressure, no fanning or pumping, which dumps reservoir air and gives nothing back. Aim for the widest shoulder or an unpaved pull-out rather than the guardrail. As pressure drops into the spring-brake range, use the graduated parking-brake control (the blue knob) to bring the spring brakes on gradually instead of letting them slam on; the same control lets him ease off and roll clear if he stops in a bad spot. Once stopped, set the parking brakes, chock the wheels, place warning triangles, and call for help. Do not crawl under the truck on a grade to hunt for the leak or open any air line.