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Difficulty: HardMountain Driving and Steep Downgrades

You are operating a loaded tractor-trailer combination at 72000 lbs72{}000\text{ lbs} approaching the summit of an unfamiliar 6-mile6\text{-mile}, 8%8\% mountain downgrade. Freezing rain has recently begun, leaving the road surface wet and icy in patches, and the posted advisory for commercial motor vehicles specifies a maximum safe speed of 25 mph25\text{ mph}. What combined operational procedure should you execute to negotiate this downgrade safely without losing traction or experiencing brake fade?

  1. A
    Engage the engine retarder at maximum capacity before the crest to reduce reliance on service brakes, and maintain transmission in a higher gear to prevent engine overspeed.
  2. Downshift to a gear lower than required to climb the grade before starting down, turn off the auxiliary retarder, and apply firm snub braking to reduce speed to 20 mph20\text{ mph} whenever speed reaches 25 mph25\text{ mph}.Answer
  3. C
    Maintain the transmission in neutral or overdrive while applying light, continuous service brake pressure to keep the vehicle travelling at exactly 25 mph25\text{ mph} throughout the entire descent.
  4. D
    Begin the descent in cruising gear to preserve momentum, disengage the clutch through sharp curves, and apply heavy brake applications only when entering switchbacks.

Answer

Downshift to a gear lower than required to climb the grade before starting down, turn off the auxiliary retarder, and apply firm snub braking to reduce speed to 20 mph20\text{ mph} whenever speed reaches 25 mph25\text{ mph}.
Safe negotiation of a steep mountain downgrade under slippery conditions requires selecting the proper low gear before starting down the hill, disabling retarders to prevent drive-wheel skids on slick pavement, and using intermittent snub braking (braking firmly to 5 mph5\text{ mph} below the safe speed and then releasing) to prevent brake fade.

Step-by-Step Solution

1
Determine pre-descent gear selection before beginning the downgrade.
Shift the transmission into a low gear that provides maximum engine braking effect (typically lower than the gear used to climb the hill) before cresting the summit.
Once the downgrade descent begins, gear shifting may be impossible and can lead to a runaway vehicle.
2
Assess road traction and configure the auxiliary braking system (retarder).
Switch off or disable the auxiliary engine retarder.
On wet, icy, or snow-covered surfaces, retarder torque applied exclusively to the drive axle can cause drive wheels to break traction and precipitate an immediate jackknife.
3
Implement the standardized snub braking technique.
Allow engine braking to hold speed; when road speed increases to the safe speed of 25 mph25\text{ mph}, apply service brakes firmly to decelerate to 20 mph20\text{ mph} (about 3 seconds of application), then release the pedal completely.
Snub braking allows brake drums and linings adequate cooling intervals between applications, preventing dangerous heat buildup and brake fade.

Key Concept

Integrated mountain downgrade management: pre-descent low gear selection, retarder deactivation on slick pavement, and the 5 mph snub braking cycle.
Estimated Time:2m 0s
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Topics

Mountain Driving and Steep Downgrades

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