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In a total service brake failure on a downgrade, why is shifting to a lower gear helpful but not sufficient by itself?
- A.Engine braking works only in reverse
- B.Engine braking can slow or hold the truck, but at high road speed you may not be able to downshift, and engine force alone may not stop a loaded truck
- C.Lower gears make the truck accelerate
- D.A lower gear disables the parking brake
Correct answer
B. Engine braking can slow or hold the truck, but at high road speed you may not be able to downshift, and engine force alone may not stop a loaded truck
Why
A downshift needs engine speed to match, which is exactly what a runaway truck may not allow. That is why gear selection belongs at the top of the grade, and why ramps and escape routes remain part of the plan when a failure starts halfway down.
Manual topic: Brake Failure - Engine Braking Limits · From the General Knowledge practice bank (GK-243).