Final answer:
Fuel trucks and airplanes need to be grounded before pumping fuel to prevent the risk of static electricity buildup, which can cause fires or explosions. Static charges can accumulate during the refueling process, and without proper grounding, the discharge of these charges can be dangerous.
Step-by-step explanation:
Fuel trucks and airplanes need to be grounded before pumping fuel due to the risk of static electricity buildup. When fuel is being transferred, there is a possibility for static charges to accumulate on the vehicles and the fuel itself. If the vehicles are not properly grounded, these charges can cause sparks, leading to potential fires or explosions.
For example, during the refueling process, the fuel flowing through pipes can generate static electricity due to friction. If the truck or airplane is not grounded, this static charge can build up, and when the fuel comes into contact with a grounded surface, like the fuel tank or the nozzle, it can discharge and cause a fire or explosion.
Another example is when aircraft are being fueled. The fuel pump generates static electricity, and if the aircraft is not grounded, the static charge can build up on the plane's metal structure or any other conductive parts, creating a hazardous situation.