  For decades, transition to turbulence in viscoelastic parallel shear flows
was believed to require nonlinear instabilities. We provide numerical evidences
for a new wall-mode linear instability that directly triggers the transition to
elastoinertial turbulence in channel flow. The instability is 2D but 3D
features become important as nonlinear effects grow. With larger disturbances,
direct transition to nonlinear instabilities, leading to turbulent states of
both inertial and elastoinertial natures, are observed.
