The ICON A5 would seem to be a "conventional" (if we stretch the limits of the term) high-wing monoplane.

But what about these things right here: Image of ICON A5 in flight with "seawings(tm)" circled in red

The protrusions from the hull, circled in red, are called "Seawings(tm)." Their declared purpose, according to the POH, is to stabilize the aircraft in water as well as provide pilot and passenger with a means of convenient in- and egress from the cockpit.

I can't find any confirmation or denial as to whether or not they generate any kind of lift, but they do seem to be shaped in a way that could be a manner of airfoil - and certainly if you're designing an aircraft and have lateral protrusions from the hull and you don't shape them to produce lift you've missed an opportunity.

So for the moment, if we assume they do generate some modicum of lift, does that mean the ICON is no longer properly referred to as a monoplane?

My understanding is that "Biplane" would require the wings to be vertically stacked with each other. Some small portion of the seawings does underlie the main wings, but the vast majority of the seawings' structure is well forward of the main wing.

Would that make it a Tandem-Wing aircraft?

Do either of those terms stop having meaning if the seawings - which are declared as intended to provide stability in water and footing for embarking/disembarking occupants - are instead properly though of as structurally part of the hull - and thus the aircraft is at least partially a lifting body?

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    $\begingroup$ It's not a tandem because both "wings" are at roughly the same position lengthwise. The term "sesquiplane" (one and a half wings, so somewhere between monoplane and biplane) exists, but I had never heard of it before googling it just now. But since I'm not sure that is the correct term (if a correct or well-established one even exists), I leave this as a comment and not an answer. $\endgroup$ May 26, 2022 at 17:21
  • $\begingroup$ @Raketenolli If you mean the wings are at the same position front-to-rear, they're not. The seawings trailing edge is below the leading edge of the main wings, but the vast bulk of the seawings exists well forward of the main wing. (They wouldn't make good steps otherwise.) $\endgroup$ May 26, 2022 at 17:30
  • $\begingroup$ "if you're designing an aircraft and have lateral protrusions from the hull and you don't shape them to produce lift you've missed an opportunity." Brilliant phrasing! $\endgroup$ May 27, 2022 at 1:50

2 Answers 2


A famous aircraft with similar "sponsons" (that's what they are normally called) to keep the plane level in the water is the Boeing 314 Clipper. "Sponsons" were also used (and likely originally pioneered) on Dornier flying boats. Examples include the Dornier Wal, Dornier Do X, and Dornier Do 24.

Re a comment: The term "sesquiplane" is normally applied to an airplane with a much larger lower "wing" than we see on the Icon A5-- such as the Nieuport 11.

Consider: should we call the Fokker Dr.1 a "quadruplane", because of the little "wing" on the axle? (No, we should not.)

It would be a stretch to call the Icon A5, or the Boeing 314, or the Dornier flying boats noted above, anything other than high-wing monoplanes.

  • $\begingroup$ Ah - brief, correct and to the point. +1 $\endgroup$ May 26, 2022 at 19:30
  • $\begingroup$ @PeterKämpf -- "for once" ? !! : ) $\endgroup$ May 26, 2022 at 19:31

Lift from a sponson should be avoided because the long chord but short span will give it a terrible lift/drag ratio, and if more lift is really needed, it is far better to add a little to the span. The higher the aspect ratio of a wing, the more efficient it is aerodynamically - and a sponson is about as low as it is possible to go in both respects. This is also the reason the lifting fuselage aircraft ended up less efficient than conventional aircraft.

They also don't work terribly well to balance the airplane while moving on the water, as dipping a wingtip was a common problem on all of the airplanes that used them, so much so that the wing tips had to be specially sealed to prevent them taking on water, and to ensure they were sufficiently buoyant, and preventable accidents have happened when a tip was dipped at speed. That said, when moored, the aircraft is more stable when level than a conventional hull for loading, and there is no tip float to cause problems when coming alongside a dock, while adding a convenient step.

As for the original question, I have never seen a case where a sponson was regarded as a wing, such as to upgrade a monoplane to a sesquiplane, however, unlike with the Fokker Dr.I, Nieuport monoplanes with extra large airfoils between the undercarriage were described as and named Sesquiplanes, so there might seem to be some precedent to do so, however the lift generated from a sponson is usually minimal, and the one case where the sponson was extended out to form a full sized wing was a failure because any deviation from perfectly level resulted in substantially more drag from the immersed wing.

Here is a photo of a Boeing 314 Clipper prototype dipping a wingtip at speed. Boeing 314 Clipper prototype dipping a wingtip at speed


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