Final answer:
Fanning brakes during long downhill runs can lead to rapid overheating due to insufficient heat transfer, posing risks to vehicle safety.
Explanation:
Experts do not recommend fanning (on-again off-again braking) for long downhill runs because the temperature increase may occur too fast for sufficient heat to transfer from the brakes to the environment. When a truck heads downhill and brakes, the brakes must convert the gravitational potential energy of the truck into internal energy, leading to a rapid temperature increase in the brakes. This overheating can be dangerous and is why techniques like engine braking or regenerative braking are preferred for managing speed on downhill runs.
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