  We consider which is the maximum information measurable from the decay
distributions of polarised baryon decays via amplitude analysis in the helicity
formalism. We focus in particular on the analytical study of the $\Lambda^+_c
\to pK^-\pi^+$ decay distributions, demonstrating that the full information on
its decay amplitudes can be extracted from its distributions, allowing a
simultaneous measurement of both helicity amplitudes and the polarisation
vector. This opens the possibility to use the $\Lambda^+_c \to pK^-\pi^+$ decay
for applications ranging from New Physics searches to low-energy QCD studies,
in particular its use as absolute polarimeter for the $\Lambda^+_c$ baryon.
This result is valid as well for baryon decays having the same spin structure
and it is cross-checked numerically by means of a toy amplitude fit with Monte
Carlo pseudo-data.
