I saw a couple of CL-415s near my lake house a few days ago, and it got me wondering, how do they pick up water? What method do they use to get water into the water tanks? How long does it take to fill up the water tanks? And do the tanks get filled up fully?
They scoop it up! Making contact with the water, a probe/scoop is extended, which allows water to gush into the internal tanks:
Image source: "Bombardier 415 Probe" by Wikimedia Commons user Aliano43 under CC BY-SA 4.0
The method is therefore simply a combination of aircraft forward speed and an adequately shaped metal duct. Here is another close-up good picture of the scoop, and a video showing the process from a good angle.
As for the duration, one source mentions that
“The water tanks can be filled in 12 seconds by the scoops.”
(which is what a large number of videos of this process seem to confirm).
As stated above, either the flying boat superstructure, or the pontoon floats on the aircraft are fitted with “scoops” - ram intakes just fed of the step, which direct water up into a holding tank, mounted inside the aircraft. The aircraft makes an approach onto a lake or river with the scoops deployed, then maintains a high speed step taxi over about 10 to 20 seconds. The ram pressure of water hitting the scoops forces it up into the holding tank. Once filled, the aircraft will take off again and retract the scoops prior to heading out for another drop. Alternatively, the water tanks can be filled from trucks on the ground, or filled up with a mixture of water and chemical retardant prior to take off.
The TV series Ice Pilots: NWT featured a good segment in the episode below starting at time index @30:55, where Buffalo’s chief pilot Arnie trains Turkish Air Force pilots on how to scoop water and makes water drops in a CL-215.
When scooping up water in a high-speed step taxi, care must be taken to maintain a constant attitude with the appropriate increase in power to compensate for the quick increase in weight due to filling the water tanks. If you get it wrong, you can start causing the water to slosh around in the tanks, resulting in a pilot induced oscillations, and potential risk of an accident.
For both of CL215/415 and the Air Tractor Fire Boss, the scooping operation typically takes place in the neighborhood of 85 to 90 knots, depending on the air frame.