  Warm debris disks are a sub-sample of the large population of debris disks,
and display excess emission in the mid-IR. Around solar-type stars, very few
objects show emission features in mid-IR spectroscopic observations, that are
attributed to small, warm silicate dust grains. The origin of this warm dust
can possibly be explained either by a collision between several bodies or by
transport from an outer belt. We present and analyse new far-IR Herschel/Pacs
observations, supplemented by ground-based data in the mid-IR (VLTI/Midi and
VLT/Visir), for one of these rare systems: the 10-16 Myr old debris disk around
HD 113766 A. We improve an existing model to account for these new
observations, and better constrain the spatial distribution of the dust and its
composition. We underline the limitations of SED modelling and the need for
spatially resolved observations. We find that the system is best described by
an inner disk located within the first AU, well constrained by the Midi data,
and an outer disk located between 9-13 AU. In the inner dust belt, our previous
finding of Fe-rich crystalline olivine grains still holds. We do not observe
time variability of the emission features over at least a 8 years time span, in
a environment subjected to strong radiation pressure. The time stability of the
emission features indicates that {\mu}m-sized dust grains are constantly
replenished from the same reservoir, with a possible depletion of
sub-{\mu}m-sized grains. We suggest that the emission features may arise from
multi-composition aggregates. We discuss possible scenarios concerning the
origin of the warm dust. The compactness of the innermost regions as probed by
Midi, as well as the dust composition, suggest that we are witnessing the
outcomes of (at least) one collision between partially differentiated bodies,
in an environment possibly rendered unstable by terrestrial planetary
formation.
