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It’s far worse than landing a land plane on land with the gear up. Landing on water with the gear down is a very bad thing.
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Just side stepping the runway in favor of the grass is usually sufficient to protect the keel.
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Landing on grass with wheels retracted on amphibious floats is also an option if the gear won’t extend and there’s no suitable body of water around. The keel of the floats is plenty strong to handle the landing as long as the surface won’t scrape the keel down to a nub. One possible reason being a float plane not equipped with landing gear might land on the grass if taking the plane in for service at a shop on a land locked airport. Unlike land lubber aircraft, intentionally landing a float plane without landing gear on land (sod) isn’t an uncommon practice. Of course, landing on water with the gear up is the norm and a good thing. The indicator was photo bombed by the paddle on the opposite float I took this picture to show the mechanical landing gear position indicator.
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