Windhaven by George R. R. Martin and Lisa Tuttle is described on Wikipedia as a “science fiction and fantasy” novel. This triggers me, really. As one might know, I endorse a strong demarcation between both genres of speculative fiction. Only almost everyone has his or her own definitions of science fiction and fantasy, and as always with definitions, discussions on this topic cannot be settled in a definite way.
However, I believe that a good working definition of science fiction would be any fiction that involves the extrapolation of real science and technology, or uses plausible technology as essential plot elements. Essential means that one cannot remove said technology or science without destroying the story. For instance if you remove psychohistory from the Foundation series, the whole story simply collapses.
Fantasy, on the other hand, is probably even harder to define. What is possibly the most important feature of fantasy is that authors of this genre dispense to claim any realism in favor of completely fictional worlds. Nevertheless, there are a few recurrent themes in fantasy: fictional races, magic and the battle between good and evil.
So is Windhaven science fiction or fantasy? Though the story is set on a fictional planet called Windhaven, this does not automatically make it science fiction, as both science fiction and fantasy authors might invent fictional planets for their stories. (Discworld is a good example of a fictional “planet” in a fantasy setting.)
Windhaven society is more or less pre-industrial, as is usually the case in fantasy. This is, however, explainable as the people on this planet are the descendants of the survivors of a crashed spacecraft. And its particular geography, many small islands in a vast but hardly navigable ocean, does not really support the development of an industrial society.
Magic is totally absent from the story and the only fictional beings mentioned in the book are the marine Scylla. Though very little is said about these dangerous creatures, one could assume they belong to the original fauna of Windhaven. And though a conflict about social change is a major part of the plot of this novel, it could hardly described as a battle between good and evil. Rather it is political dispute between a conservative group of people and some more progressive individuals.
Despite being technologically less advantaged, Windhaven has one piece of important technology: human-powered aviation. Flyers are people who use artificial wings to fly between islands and are hence an important group in society. The way this technology is described quite resembles hang gliding and therefore I would say it is more than believable. And also if we remove this technology, the very story collapses as the very plot evolves around the life of one flyer.
One final question regards the crash landing. We could wonder whether such crash could be survived by any one at all. Unfortunately we have simply to little information to make a sensible judgement. We do not know what the cause of this crash was, whether Windhaven was the actually the desired destination or just an intermediary stop.
My conclusion is therefore that Windhaven is definitely not fantasy, as the whole story is completely sensible from the perspective of present-day science and technology. And though this novel might not be the most “sexy” example of science fiction, I think it justifiable to consider this work as science fiction.