  We study the set of intrinsic singularities of flat affine systems with $n-1$
controls and $n$ states using the notion of Lie-B\"acklund atlas, previously
introduced by the authors. For this purpose, we prove two easily computable
sufficient conditions to construct flat outputs as a set of independent first
integrals of distributions of vector fields, the first one in a generic case,
namely in a neighborhood of a point where the $n-1$ control vector fields are
independent, and the second one at a degenerate point where $p-1$ control
vector fields are dependent of the $n-p$ others, with $p>1$. We show that the
set of intrinsic singularities includes the set of points where the system does
not satisfy the strong accessibility rank condition and is included in the set
where the distribution of vector fields, introduced in the generic case, is
singular. We conclude this analysis by three examples of apparent singularites
of flat systems in generic and non generic degenerate cases.
