Wet 6 Percent Grade, Trailer Hand Valve, and a Jackknife
You are descending a 6 percent grade on a two-lane mountain highway in a 5-axle combination, grossing 78,000 pounds, with the trailer loaded with gravel. The road is wet from a morning rain and the temperature is 34°F. You are in 7th gear at 42 mph using the service brakes to hold speed. About halfway down you feel the trailer push into the tractor, the steering wheel goes light, and the rear of the tractor swings toward the left; the trailer's left rear tires are smoking in the mirror. The engine brake is on 'high,' the trailer hand valve has been used twice to help slow down, and you have 1.5 miles of grade left with a 25 mph advisory curve at the bottom. There is a gravel shoulder about 8 feet wide on the right and no guardrail.
1. Name what is happening and give your first three actions.
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This is a tractor (drive-axle) jackknife. The drive tires locked under braking on wet pavement, lost lateral grip, and the loaded trailer's mass pushed them sideways, folding the combination around the kingpin.
First: get off the foot brake completely. A locked drive axle is the pivot the jackknife rotates around, and it will not steer until it rolls again. Second: steer in the direction the rear of the tractor is sliding - here, to the left - to bring the tractor's nose back in line with the trailer. Counter-steering away from the skid makes the fold worse. Third: as the tires regain grip and the rig straightens, reapply the brakes lightly and progressively, drop a gear or two, and let the engine brake hold the lower speed.
Do not touch the trailer hand valve. It is not a tool for slowing a descent, and in this scenario it is part of why you are in a skid.
2. Explain how the hand valve contributed to this jackknife.
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The trailer hand valve applies the trailer's brakes only. When only the trailer brakes are applied, the trailer slows and the tractor does not - so the trailer's momentum keeps coming and pushes the tractor's drive axles around the kingpin. On a wet 6 percent grade with a loaded gravel trailer, that push is exactly the force needed to break the drive tires loose.
Speed on a downgrade should be controlled by the gear you are in, steady gentle service brake applications (snubbing 5-6 mph, then releasing to let the drums cool), and the engine brake. If the engine brake has a trailer-brake feature, that is the correct way to get trailer braking proportional to the tractor's. The hand valve is for holding a parked trailer and for testing - not for descent.
3. You have control again at 35 mph with 1.5 miles of grade left and a 25 mph curve at the bottom. Plan the rest of the descent.
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Get the speed down now, not at the curve. Downshift into the gear you should have started in - likely 4th or 5th in a loaded rig on 6 percent - set the engine brake to a level that holds 25 to 30 mph, and use short firm brake applications rather than riding the pedal.
Treat the 8-foot gravel shoulder as a planned escape route: if the speed will not hold, put the right wheels onto it early and brake straight to a stop, instead of arriving at the curve hot. With the temperature at 34°F, the wet surface can be icy on bridges and in shaded cuts, so every application needs to be smooth.
Before the curve, be at or under 25 mph. At the bottom, pull into a safe area and check the drums for heat, smell for burning lining, and check slack adjuster travel. Then write up the jackknife and the hand valve use - that is a lesson the next grade will test again.