  A common property of topological systems is the appearance of topologically
protected zero-energy excitations. In a superconductor or superfluid such
states set the critical velocity of dissipationless flow $v_{\mathrm{cL}}$,
proposed by Landau, to zero. We check experimentally whether stable superflow
is nevertheless possible in the polar phase of p-wave superfluid $^3$He, which
features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles.
The fluid is driven by rotation of the whole cryostat, and superflow breakdown
is seen as the appearance of single- or half-quantum vortices. Vortices are
detected using the relaxation rate of a Bose-Einstein condensate of magnons,
created within the fluid. The superflow in the polar phase is found to be
stable up to a finite critical velocity $v_{\rm c}\approx 0.2\,$cm/s, despite
the zero value of the Landau critical velocity. We suggest that the stability
of the superflow above $v_{\mathrm{cL}}$ but below $v_{\rm c}$ is provided by
the accumulation of the flow-induced quasiparticles into pockets in the
momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase this
surface has non-trivial topology which includes two pseudo-Weyl points.
Vortices forming above the critical velocity are strongly pinned in the
confining matrix, used to stabilize the polar phase, and hence stable
macroscopic superflow can be maintained even when the external drive is brought
to zero.
