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Difficulty: MediumMountain Driving and Steep Downgrades

When operating a commercial motor vehicle down a steep mountain downgrade, the primary means of controlling vehicle speed must be the foundation service brakes, with the engine compression braking effect serving only as a secondary backup.

Answer: Answer

Answer

False. Engine braking in a low gear is the primary speed control mechanism on steep mountain downgrades; foundation service brakes are strictly supplementary.
On steep downgrades, gravity continuously pulls a heavy vehicle downhill. The principal retarding force must come from the engine's compression resistance in an appropriately low gear selected before starting down the grade. Foundation service brakes cannot dissipate continuous heat and must only be used as a supplement through proper snub braking to keep the vehicle below its maximum safe speed.

Step-by-Step Solution

1
Identify the primary and secondary mechanisms of vehicle retarding on mountain downgrades according to commercial driving safety principles.
Engine compression in a matched low gear provides continuous retarding power, whereas foundation brakes generate intense friction and heat.
Understanding the physical limitations of friction brakes versus mechanical engine retarding is essential for preventing runaway vehicle scenarios.
2
Evaluate the thermal consequences of using service brakes as the primary descent speed controller.
Continuous or primary reliance on service brakes causes rapid overheating, drum expansion, lining glazing, and total brake fade.
Friction brakes cannot dissipate the massive heat generated by gravity acting on a heavy vehicle over extended mountain grades.
3
Determine the validity of the statement based on CDL downgrade operating standards.
The statement reverses the actual roles by claiming service brakes are primary and engine braking is secondary.
The correct principle requires engine braking to be primary and service brakes to be supplementary via snub braking.

Key Concept

Primary Role of Engine Braking on Steep Downgrades
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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