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CDL · Brake Fade and Downgrades

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

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

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 Brake only at the bottom
  3. C Select a low gear before the descent and use engine braking as the primary control, supplemented by snub braking — to avoid overheating the brakes
  4. D Coast in neutral

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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About this topic: Brake Fade and Downgrades

Brake fade is the loss of braking power that happens when drums or discs get too hot to convert motion into heat efficiently — the pedal still moves, but the truck stops slowing. On the CDL air brakes test it shows up in questions about long downgrades, riding the brakes, snubbing (controlled braking) and why a lower gear, not the service brakes, is your primary speed control on a hill. This is one of 5 Air Brakes questions on brake fade and downgrades, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

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