  Observational evidence revealing the main mechanisms that accelerate quasar
outflows has proven difficult to obtain, due to the complexity of the
absorption features that this gas produces in the spectra of the emission
sources. We build 36 composite outflow spectra, covering a large range of
outflow and quasar parameters, by stacking broad ($>450\,{\rm km\,s^{-1}}$)
absorption line systems in the spectra of SDSS-III/BOSS DR12 quasars. The two
lines of the atomic doublet of CIV, with a separation of $\approx 497\,{\rm
km\,s^{-1}}$, as well as those of other species appear well resolved in most of
our composites. This agrees with broad outflow troughs consisting in the
superposition of narrow absorbers. We also report on the ubiquitous detection
of the radiative-acceleration signature known as line locking in all our
composite outflow spectra, including one spectrum strictly built from broad
absorption line (BAL) systems. This is the first line-locking detection in BAL
composite spectra. Line locking is driven by the CIV atomic doublet, and is
visible on the blue side of most strong absorption transitions. Similar effects
from the doublets of OVI, SiIV, or NV, however, seem to not be present. Our
results confirm that radiation pressure is a prevalent mechanism for
accelerating outflows in quasars.
