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Difficulty: EasyMountain Driving and Steep Downgrades

A commercial motor vehicle driver is preparing to travel down an extended steep mountain downgrade. In what chronological order should the driver execute these procedures to maintain proper vehicle control before and during the descent?

  1. 1Determine the maximum safe descent speed based on gross vehicle weight, grade steepness, and road surface conditions.
  2. 2Reduce speed and shift the transmission into the appropriate low gear before reaching the crest of the hill.
  3. 3Cross the summit and rely on engine compression in the selected low gear as the primary braking force.
  4. 4Apply firm service brake pressure to reduce speed to 5 mph5\text{ mph} below the safe speed whenever the threshold is reached.

Answer

1. Determine safe descent speed -> 2. Downshift to low gear before cresting -> 3. Cross summit using engine compression -> 4. Apply intermittent snub braking when safe speed is reached
Safe mountain descent begins with planning the safe speed, followed immediately by downshifting into the correct low gear before cresting the hill. After cresting, the driver utilizes engine compression braking as the primary speed control, applying intermittent snub braking (slowing by 5 mph5\text{ mph} and releasing) only when the safe speed is reached.

Step-by-Step Solution

1
Calculate the safe operating speed before starting down the hill based on load weight, length and steepness of the grade, weather, and road conditions.
A clear, predetermined safe maximum speed threshold is established.
Safe speed selection dictates gear choice and prevents runaway risk before gravitational forces take over.
2
Decelerate and shift the transmission into a low gear before cresting the summit or starting down the grade.
The drivetrain is securely engaged in a low gear that provides maximum engine retarding power.
Federal CDL regulations require shifting before the descent because shifting modern transmissions on a steep downgrade can cause transmission lock-out in neutral.
3
Begin the downgrade descent letting engine braking control the vehicle's speed.
The engine absorbs the vehicle momentum, minimizing the work required by the friction service brakes.
Engine compression is the primary mechanism for controlling speed down mountain grades.
4
Execute the snub braking technique: when vehicle speed increases to the safe speed, apply moderate-to-firm brake pressure to slow 5 mph5\text{ mph} below safe speed, then release.
Brakes cool between short application cycles, preventing thermal brake fade.
Intermittent snub braking keeps brake drum and lining temperatures low while maintaining speed within safe boundaries.

Key Concept

Pre-crest low gear selection and intermittent snub braking on mountain downgrades
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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