Linear modal instabilities of flow over untapered wings with aspect ratios
$AR=4$ and 8, based on the NACA 0015 profile, have been investigated numerically over a range of angles of attack,
$\alpha$, and angles of sweep,
$\varLambda$, at chord Reynolds numbers
$100\le Re\le 400$. Laminar base flows have been generated using direct numerical simulation and selective frequency damping, as appropriate. Several families of unstable three-dimensional linear global (TriGlobal) eigenmodes have been identified and their dependence on geometric parameters has been examined in detail at
$Re=400$. The leading global mode A is associated with the peak recirculation in the three-dimensional laminar separation bubble formed on the wing and becomes unstable when recirculation reaches
$\textit {O}(10\,\%)$. On unswept wings, this mode peaks in the midspan region of the wake and moves towards the wing tip with increasing
$\varLambda$, following the displacement of peak recirculation; its linear amplification leads to wake unsteadiness. Additional amplified modes exist at nearly the same and higher frequencies compared to mode A. The critical
$Re$ has been identified and it is shown that amplification increases with increasing sweep, up to
$\varLambda \approx 10^\circ$. At higher
$\varLambda$, all global modes become less amplified and are ultimately stable at
$\varLambda =30^\circ$. An increase in amplification of the leading mode with sweep was not observed over the
$AR=4$ wing, where tip vortex effects were shown to dominate.