Lemma 76.24.2. Let $S$ be a scheme. Let $f : X \to Y$ and $Y \to Z$ be a morphism of algebraic spaces over $S$. Assume
$X$, $Y$, $Z$ locally Noetherian,
$X$ is flat over $Z$,
for every $z \in |Z|$ the fibre of $X$ over $z$ is flat over the fibre of $Y$ over $z$.
Then $f$ is flat. If $f$ is also surjective, then $Y$ is flat over $Z$.