  Extragalactic jets are generated as bipolar outflows at the nuclei of active
galaxies. Depending on their morphology, they are classified as Fanaroff-Riley
type I (centre-brightened) and Fanaroff-Riley type II (edge-brightened) radio
jets. However, this division is not sharp and observations of these sources at
large scales often show intermediate jet morphologies or even hybrid jet
morphologies with a FRI type jet on one side and a FRII type jet on the other.
A good example of a radio galaxy that is difficult to classify as FRI or FRII
is Hercules~A. This source shows jets with bright radio lobes (a common feature
of FRII type jets) albeit without the hotspots indicative of the violent
interaction between the jet and the ambient medium at the impact region,
because the jets seem to be disrupted inside the lobes at a distance from the
bow shocks surrounding the lobes. In this paper, we explore the jet physics
that could trigger this peculiar morphology by means of three-dimensional
relativisitic hydrodynamical simulations. Our results show that the large-scale
morphological features of Hercules A jets and lobes can be reproduced by the
propagation of a relativistically hot plasma outflow that is disrupted by
helical instability modes, and generates a hot lobe that expands isotropically
against the pressure-decreasing intergalactic medium. We also discuss the
implications that this result may have for the host active nucleus in terms of
a possible transition from high-excitation to low-excitation galaxy modes.
