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CDL · General

On a long downhill grade with air brakes, what is the proper way to control speed?

Correct answer

Select a low gear before the descent and use engine braking as the primary control, supplemented by snub braking — to avoid overheating the brakes

  1. A Ride the brakes continuously the whole way down
  2. B Select a low gear before the descent and use engine braking as the primary control, supplemented by snub braking — to avoid overheating the brakes
  3. C Coast in neutral
  4. D Brake only at the bottom

Why this is the answer

On a long downgrade, select a LOW GEAR BEFORE starting down and use ENGINE BRAKING as the primary speed control, supplemented by SNUB BRAKING (apply the brakes firmly to slow about 5 mph below your target speed, then release to let them cool; repeat). CDL air brakes. This prevents BRAKE FADE from continuous overheating. AVOID riding the brakes (continuous application overheats them) and never coast in neutral (loss of control). The proper gear should be selected before the descent (you may not be able to downshift once moving too fast). Knowing the low-gear + engine-braking + snub-braking technique for downgrades is commonly tested and safety-critical.
Source: FMCSA CDL Manual — Air Brakes, Downgrade Control

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