Final answer:
The use of air brakes on a downgrade is only a supplement to engine braking, which is the technique of using the truck's engine to slow down the vehicle. By using both air brakes and engine braking together, the truck can effectively control its speed on a downhill slope.
Step-by-step explanation:
The use of the air brakes on a downgrade is only a supplement to the engine braking.
When a truck goes downhill, the use of air brakes alone may not be sufficient to control its speed due to the large mass of the truck. Engine braking is a technique where the truck's engine is used to slow down the vehicle by shifting to a lower gear and allowing the engine to resist its own momentum.
By using both air brakes and engine braking together, the truck can effectively control its speed on a downhill slope.