  We compute the $O(\beta_0 \alpha_s^2)$ QCD corrections to the Standard Model
decay $t\rightarrow W^+ b$ as well as the non standard decay $t\rightarrow H^+
b$. We then use our results to compute the BLM scale for these decays, and
study the dependence of the BLM scale on the mass of the boson in the decay. We
find that the BLM procedure gives extremely small scales when top decays into
heavy bosons. When we try to improve the BLM scale by reexpressing rates as a
functions of the top quark running mass, we find the BLM scale exhibits
unnatural dependence on the boson mass, casting doubts on the applicability of
the BLM procedure to these processes.
