  Wave-particle instabilities driven by departures from local thermodynamic
equilibrium have been conjectured to play a role in governing solar wind
dynamics. We calculate the statistical variation of linear stability over a
large subset of Helios I and II fast solar wind observations using a numerical
evaluation of the Nyquist stability criterion, accounting for multiple sources
of free energy associated with protons and helium including temperature
anisotropies and relative drifts. We find that 88\% of the surveyed intervals
are linearly unstable. The median growth rate of the unstable modes is within
an order of magnitude of the turbulent transfer rate, fast enough to
potentially impact the turbulent scale-to-scale energy transfer. This rate does
not significantly change with radial distance, though the nature of the
unstable modes, and which ion components are responsible for driving the
instabilities, does vary. The effect of ion-ion collisions on stability is
found to be significant; collisionally young wind is much more unstable than
collsionally old wind, with very different kinds of instabilities present in
the two kinds of wind.
