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Why can trying to downshift after you are already going too fast on a grade fail?
- A.Lower gears cannot be used once the vehicle is moving
- B.Downshifting is prevented by the air governor
- C.The transmission locks out all gears downhill
- D.The engine speed needed for the lower gear may be higher than the engine can safely reach at that road speed, so the gear may not engage
Correct answer
D. The engine speed needed for the lower gear may be higher than the engine can safely reach at that road speed, so the gear may not engage
Why
Gear selection works by matching engine speed to road speed, and a vehicle that is already too fast may over-rev the engine before the lower gear can take hold. That is why the gear must be chosen before the descent, not rescued midway.
Manual topic: Air Brakes - Downgrade Braking · From the Air Brakes practice bank (AB-190).