Water bomber planes will scout out the nearest large body of water to the fire in question. This will need to be an open source of water, meaning that the planes can easily refill. The pilot will approach the water and lower the plane until the underside skims the water’s surface.
They will then ‘keep it on the step’, meaning that the pilot taxis along the water as fast as possible without returning to the air. This is generally at a speed of around 70 knots. Scoops are then lowered into the water for the plane to collect water to fight the fire. They direct the flow of water up and into the tanks on board.
The pressure created by the plane being kept on the step is what drives the water into the tanks onboard the plane. This means that no pumps are required to collect water. Every plane differs in how rapidly it can uptake water. One of the largest, the Martin Mars, can uptake 1,100 liters or a ton of water per second.
The water is then kept in the tanks through the use of a check valve and vents on the upper side of the tank. This ensures there is no pressurization of the water.