Six Percent, Wet Pavement, Empty Trailer
You are descending a 6 percent grade on a wet two-lane highway in an empty 53-foot van. The trailer weighs about 13,000 pounds behind a 3-axle tractor; gross weight is under 40,000. The grade is posted for trucks at 35 mph and runs two miles to a traffic light at the bottom. It began raining twenty minutes ago after three dry weeks, so the pavement is greasy. You are in 9th of 10 gears and using the foot brake in short applications as speed builds past 40. Halfway down, the pedal feels soft and the truck keeps accelerating, so you press harder. The drive tires break traction, the trailer's nose swings left into the oncoming lane, and the steering wheel suddenly feels light in your hands. There is a guardrail on the right and no shoulder.
1. Identify what went wrong in sequence and give the correct immediate response, control by control.
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Sequence: continuous and then harder braking on a long grade overheated the drums and the brakes faded - the pedal goes soft and the truck accelerates despite pressure. Pressing harder locked or nearly locked the drive tires on greasy pavement, and with an empty trailer there is little trailer weight or trailer braking to hold the rear of the combination in line. The tractor's drive axles slid sideways and the trailer kept coming: a drive-axle jackknife developing into a trailer swing.
Immediate response:
- Get off the brake pedal completely. Locked wheels cannot steer; releasing lets the drive tires roll and regain traction.
- Steer in the direction the trailer is going - the trailer is swinging left, so steer left to get back under it. Do not whip the wheel right; that snaps the trailer the other way.
- Do not touch the trailer hand valve. It applies the trailer brakes alone and will pull the trailer further out of line.
- Do not use the engine brake or retarder on this wet surface - it brakes the drive wheels and can re-trigger the same skid.
Once straight, take a lower gear and continue down on light, spaced applications, or crawl in the lowest gear that holds the speed.
2. What should have been done before the crest, and what do you do at the bottom of the grade?
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Before the crest:
- Pull over and do a brake check - a moving test at low speed plus a look at the slack adjusters and pushrod travel, and confirm the trailer brakes are working and in adjustment.
- Select the gear at the top, not after speed builds. On a 6 percent grade the rule of thumb is to descend in a gear no higher than the one you would need to climb it, and if you have to brake more than occasionally, you are in too high a gear.
- Set a target speed below the posted 35 mph and hold it with short, firm applications rather than riding the pedal.
- Recognize the wet-after-dry condition: oil and rubber accumulate on dry pavement and turn slick with the first rain.
At the bottom:
- Do not sit at the light with hot brakes and a hot load. If you must stop, stop, then pull off as soon as safely possible.
- Walk around: check drums and wheels for heat, smell for burning lining, look for a dragging wheel, check tire condition and that the load has not shifted.
- Note the fade event on your paperwork and have the brakes inspected before the next grade. Faded lining that has cooled may still be glazed and weak.