Examkin

Question

Difficulty: MediumSpeed on Curves and Downgrades

A commercial driver operating a loaded five-axle combination vehicle approaches the summit of a long 6%6\% downgrade with an established safe speed of 35 mph35\text{ mph}. What sequence of actions must the driver take to maintain vehicle control and prevent brake fade throughout the descent?

  1. A
    Descend in cruising gear while applying continuous light pressure to the service brakes to keep the vehicle steady at the target speed.
  2. B
    Shift the transmission into neutral and rely primarily on the engine retarder to control downhill momentum on the slope.
  3. Downshift to a low gear before starting down the grade, let speed reach 35 mph35\text{ mph}, apply the brakes firmly to reduce speed to 30 mph30\text{ mph}, and release them until speed climbs back to 35 mph35\text{ mph}.Answer
  4. D
    Maintain highway speed until mid-descent, then downshift into lower gears and apply heavy continuous braking if speed exceeds safe limits.

Answer

Downshift to a low gear before starting down the grade, let speed reach 35 mph35\text{ mph}, apply the brakes firmly to reduce speed to 30 mph30\text{ mph}, and release them until speed climbs back to 35 mph35\text{ mph}.
The primary method of speed control on steep downgrades is the braking effect of the engine in a low gear selected before starting down the grade. When supplemented by the proper snub braking technique—applying firm brake pressure to reduce speed 5 mph5\text{ mph} below the safe speed and then completely releasing the pedal—the brakes have adequate time to dissipate heat between applications, preventing dangerous brake fade.

Step-by-Step Solution

1
Select the proper low gear before cresting the hill and beginning the downgrade descent.
The vehicle engages engine compression braking as the primary speed-retarding force.
Downshifting on a downgrade is extremely difficult or impossible once speed builds, risking transmission lockout in neutral.
2
Monitor the speedometer until road speed reaches the designated safe speed of 35 mph35\text{ mph}.
The driver identifies the exact threshold for initiating service brake application.
Establishes a controlled baseline speed matched to grade steepness and vehicle weight.
3
Apply the service brakes firmly to reduce vehicle speed by 5 mph5\text{ mph} (down to 30 mph30\text{ mph}) over approximately 33 seconds, then release the brake pedal completely.
Speed drops safely below the threshold and brake components are given time to cool while speed gradually rebuilds.
Snub braking prevents continuous thermal buildup and eliminates brake fade on long downgrades.

Key Concept

Proper downgrade speed management relies on pre-descent low gear selection and intermittent snub braking (5 mph5\text{ mph} reduction cycle) to prevent thermal brake fade.
Estimated Time:1m 15s
Rate this question

Topics

Speed on Curves and Downgrades

More questions from this topic