HTML
<!--Carousel-->
<div id="carouselExampleIndicators" class="carousel slide" data-ride="carousel">
<ol class="carousel-indicators">
<li data-target="#carouselExampleIndicators" data-slide-to="0" class="active"></li>
<li data-target="#carouselExampleIndicators" data-slide-to="1"></li>
<li data-target="#carouselExampleIndicators" data-slide-to="2"></li>
</ol>
<div class="carousel-inner">
<div class="carousel-item active imagem">
<img class="d-block w-100" src="static/img/aviao.png" alt="First slide">
</div>
<div class="carousel-item">
<img class="d-block w-100" src="static/img/aviao.png" alt="Second slide">
</div>
<div class="carousel-item">
<img class="d-block w-100" src="static/img/aviao.png" alt="Third slide">
</div>
</div>
<a class="carousel-control-prev" href="#carouselExampleIndicators" role="button" data-slide="prev">
<span class="carousel-control-prev-icon" aria-hidden="true"></span>
<span class="sr-only">Previous</span>
</a>
<a class="carousel-control-next" href="#carouselExampleIndicators" role="button" data-slide="next">
<span class="carousel-control-next-icon" aria-hidden="true"></span>
<span class="sr-only">Next</span>
</a>
</div>
<!--Fim do Carousel-->
CSS
html, body {
margin: 0;
padding: 0;
}
.imagem {
height: 400px !impotant;
}