Six Percent and the Smell of Hot Linings
A driver is at the top of a four-mile, 6 percent downgrade in dry daylight. The rig is a three-axle tractor pulling a 40-foot flatbed loaded with steel coils; the scale ticket shows 76,000 lb GCW. The first curve is posted 35 mph. The driver enters the grade at 50 mph in 9th gear and begins using the service brakes to hold speed. After about a mile, the driver smells hot brake lining, the pedal feels low and spongy, and the truck has crept to 62 mph even though the driver is pressing harder. Air pressure is 105 psi and not falling. A runaway truck ramp is signed one-half mile ahead on the right. Traffic is behind, and the driver is due at a delivery site in 40 minutes. The engine brake is available but currently switched off.
1. What is happening, and what should the driver have done before starting down the grade?
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Brake fade. The driver is asking the service brakes to absorb the energy of a 76,000-lb rig on a 6 percent grade for miles. Drum brakes overheat, lining friction drops, the pedal can feel low or spongy, and speed climbs even with more pedal pressure. Before the crest, the driver should have slowed to a safe speed below the 35-mph curve posting, selected a low enough gear that engine braking holds the rig back, switched on the engine brake, and used short, firm snubs rather than continuous pressure. The engine brake and gearing do the work; the service brakes are for trimming speed. Entering at 50 mph in 9th gear with the engine brake off put the whole load on the service brakes and set up the fade.
2. What should the driver do now? Consider the ramp, engine brake, and downshifting.
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Treat the rig as an emergency. Get off the service brakes, turn on the engine brake, and downshift one gear at a time only if the tachometer stays in the safe range; the goal is engine braking, not a violent downshift. If speed still climbs toward 62 mph or more and the pedal is low, do not try to finish the grade. Signal, move to the right, and use the runaway ramp one-half mile ahead; it is built for this. Keep the steering straight, follow the ramp surface, and do not jump out while moving. If the truck slows under control before the ramp, pull into a safe wide spot, but do not stop in a travel lane. A 40-minute delivery window is irrelevant against an uncontrolled descent.
3. After stopping, what checks determine whether the rig can continue?
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After stopping, let the brakes cool and inspect. Check for dragging brakes, cracked or glazed linings, leaking wheel seals, low air pressure, and pushrod stroke beyond adjustment limits. Do not pour water on hot drums; rapid cooling can crack them. If the pedal is still low, the lining is damaged, or the rig will not hold on the grade, call for repair and do not continue. If inspection is normal, resume only in a lower gear with the engine brake on and at a speed that lets the engine hold the load. Recheck brake feel and air pressure after a short distance. The fact that the truck eventually stopped does not prove the brakes are fit for the remaining grade.