Tier (aircraft)

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Seaplane with single-stage floats ( Cessna 208 )
Single-stage hull flying boat ( Be-12 )

In aircraft construction, a step is the distinctive height recess of the planking or outer skin below the waterline that is present on the underside of a float of a seaplane . The underside of the hull of a flying boat, along with any support floats, can also have steps. The construction can be designed in one or more stages. The step helps the aircraft get from displacement travel to plane travel more easily . Behind the step, which is usually in front of half the waterline length of the swimmer, the cross-section of the mostly keeled swimmers is considerably reduced.

Mode of action

Due to the design, the rear part of the stepped hulls and floats has a smaller cross-section and thus a lower buoyancy volume than the front part. The lower buoyancy at the rear makes it possible to press the rear float area a little deeper into the water against the buoyancy in order to achieve the required angle of attack during take- off and not to hinder the aircraft's pitching movement required for take-off too much.

At the same time, the step acts as a tear-off edge, so that from a certain speed in the rear area through the step, the water flow is released from the hull skin and ambient air from the sides gets under the swimmers, whereby they get on plane and only the front, shorter area comes into contact with water Has. This “going on the step” causes a reduction in the cohesive forces , ie the “suction effect” of the water, as well as a reduction in the drag resistance, which makes it possible to achieve a sufficient take-off speed. In the event of overloading or if there is no wind and the water surface is smooth, a hollow space known as a trough can arise during the take-off process under the float part located behind the step, in which a sometimes considerable negative pressure can build up, as the water does not come off the chimney edges . The suction acting in this case also makes it impossible to take off, but it can be overcome by changing the direction of the take-off run in relation to the wave pattern of the water surface or by driving in a circle causing waves and starting across the self-generated waves . This real “sticking” to the water surface occurred in particular in the whale flying boat equipped with stubby swimmers, which is why a copy of the successor model Do 18 was tested with two ventilation pipes with a diameter of 70 mm for static pressure equalization, which ran from the bottom of the keel to the top of the boat and could be opened and closed. This step ventilation was used in the 1930s at the German Research Institute for Aviation, in some cases successful start-up attempts under a wide variety of weather conditions.


  • Wilfried Copenhagen : Transpress-Lexicon aviation. 4th, revised edition, Transpress, Berlin 1979, pp. 540/541 (keyword: level).
  • Heinz AF Schmidt: Meyers Taschenlexikon aviation. 2nd, improved and expanded edition, Bibliographisches Institut Leipzig, 1967, pp. 324/325 (keyword: level).

Individual evidence

  1. ^ Hans-Jürgen Becker: Seaplanes, flying boats, amphibians, float planes (= Die deutsche Luftfahrt, Volume 21). Bernard & Graefe, Bonn 1994, ISBN 3-7637-6106-3 , pp. 20/21.