Airport utility workers are usually outdoors in all kinds of weather. They may direct incoming and outgoing aircraft near the terminal, using hand or light signals. When the aircraft is stationary, they operate the service vehicles that fill the fuel and water tanks. They add cooling-system and hydraulic fluids and check tires for specified air pressure. They also replace waste system chemicals and remove liquid waste. During the winter they remove ice and prevent ice buildup on the wings of aircraft by applying deicing chemicals.