Final answer:
Fanning the brakes during downgrades is not recommended because it prevents the brakes from cooling properly, which can lead to brake fade and reduced effectiveness.
Step-by-step explanation:
You should NOT fan the brakes on and off during long downgrades because it does not allow brakes to cool.
Fanning the brakes," — repeatedly tapping the brake pedal — can lead to heat buildup because the brakes do not have sufficient time to cool between applications. This excessive heat can lead to brake fade, where the brakes become overheated and less effective. It is better to use a technique called "engine braking" along with applying steady pressure to maintain control and give brakes time to cool on long downgrades. Also, fanning brakes uses more air pressure, which can deplete the air needed for proper brake functioning.