Brake Fade on a 6% Grade With a Tanker
You drive a 78,000-lb five-axle tanker down a 6% grade on I-68 near Cumberland, Maryland. The posted truck speed is 45 mph, and you are at 50 mph in 8th gear at 1,700 rpm. Your engine brake is on high, but the trailer has no retarder. You smell hot brakes and see a faint haze from the left rear trailer wheels. The next runaway truck ramp is 1.2 miles ahead, and the grade continues for 4 miles. Traffic is light. Your air pressure is 105 psi, and the low-air warning has not sounded. You have 30,000 lb of a non-hazardous liquid with baffles. The service brake pedal feels firm, but the truck has gained 5 mph in the last half mile. The left rear trailer tire is not smoking yet. What do you do?
1. Diagnose the problem and state your immediate actions.
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The haze and smell from the trailer wheels, plus a 5-mph speed gain on a 6% grade, point to brake fade, not a normal grade. The service brake pedal is firm and air pressure is 105 psi, so this is not a low-air problem. Do not ride the service brakes; continuous application overheats the drums and makes fade worse.
Reduce speed with the engine brake and a downshift, one gear at a time, while using short, hard stab-brake applications to scrub speed. If the speed continues to climb or the pedal starts to feel soft, treat it as a runaway and head for the ramp. The tanker's baffles reduce surge but do not change the brake-fade risk.
2. Explain the braking and gearing technique you should use on this grade.
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Downshift to a lower gear before the speed climbs too high; at 1,700 rpm in 8th, you have room to drop a gear, but do not skip gears or over-rev. Use stab braking: apply the brakes firmly until speed drops about 5 mph, release, and repeat. This gives the drums time to cool.
Keep the engine brake on high and watch the trailer wheels. If the trailer brakes are fading, the tractor brakes will have to do more work, so reduce speed early. If you cannot hold 45 mph or lower, stop at a safe pullout or use the ramp.
3. How do you decide whether to use the runaway truck ramp?
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The ramp is 1.2 miles ahead and the grade continues for 4 miles. That is a short window. Decide now: if you cannot stabilize speed by the ramp, use it. Do not wait until the brakes are smoking or the pedal is soft.
Before the ramp, turn on hazard lights, keep the steering straight, and do not brake hard on the gravel; the ramp is designed to stop you. Follow posted instructions and expect a long stop. After stopping, set the parking brake, chock if needed, and call for help. Do not try to back out or continue down the grade with faded brakes. Report the brake problem and have the trailer brakes inspected.