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Difficulty: MediumMountain Driving and Steep Downgrades

You are driving a commercial motor vehicle with a gross combination weight of 78,000 lbs descending a steep 6% grade with a posted truck safe speed of 35 mph. Which of the following procedures describes the correct braking technique to maintain control and prevent brake fade?

  1. A
    Maintain continuous, light pressure on the service brake pedal throughout the entire descent to hold vehicle speed steadily at exactly 35 mph.
  2. Apply the brakes firmly until your speed drops to 30 mph, then release the brakes completely and repeat the process when your speed reaches 35 mph.Answer
  3. C
    Shift the transmission into neutral to coast and apply maximum air brake pressure in short pulses whenever vehicle speed reaches 40 mph.
  4. D
    Keep the engine retarder engaged at maximum power while continuously feathering the foot brake to prevent speed from exceeding 35 mph on wet pavement.

Answer

Apply the brakes firmly until speed drops to 30 mph (5 mph below the safe speed), then release them completely and repeat when speed returns to 35 mph.
The established FMCSA standard for steep downgrade braking is the snub braking technique: once the vehicle reaches its safe speed, apply the brakes with enough force to reduce speed by 5 mph in approximately 3 seconds, then fully release the pedal so the brakes can cool until the vehicle accelerates back to the safe speed.

Step-by-Step Solution

1
Select and shift into the proper low gear before starting down the steep grade.
Engine braking provides the primary retarding force needed to resist gravitational acceleration.
Attempting to downshift after starting down a steep grade can result in missing the gear and losing all engine braking control.
2
Allow the vehicle speed to reach the established safe speed of 35 mph.
The vehicle is operating at the designated safe threshold for the weight and grade.
Braking too early or irregularly prevents establishing a consistent speed management rhythm.
3
Apply the service brakes with steady, firm pressure until speed drops to 30 mph (approximately 5 mph below the safe speed), taking about 3 seconds, then release completely.
The vehicle slows down efficiently while allowing brake drums and linings to cool when released.
This proper snub braking cycle prevents brake fade by dissipating heat between applications.

Key Concept

Proper snub braking technique on steep mountain downgrades
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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