  Methyl cyanide is one of the simplest interstellar complex organic molecules,
widely detected in young solar analogues, shocked regions, protoplanetary disks
and comets. CH3CN can be considered a key species to explore the chemical
connections between planet forming disks and comets. For such comparison to be
meaningful kinetics data for the reactions leading to CH3CN formation and
destruction must be updated. We focus on the destruction of methyl cyanide
through collisions with He+. A combined experimental and theoretical
methodology is employed to obtain cross sections (CSs) and branching ratios
(BRs) as a function of collision energy, from which reaction rate coefficients
$k(T)$ are calculated in the temperature range from 10 to 300 K. CSs and BRs
are measured using a guided ion beam set-up. A theoretical treatment based on
an analytical formulation of the potential energy surfaces (PESs) for the
charge exchange process is developed. The method employs a Landau Zener model
to obtain reaction probabilities at crossings between the entrance and exit
PESs, and an adiabatic centrifugal sudden approximation to calculate CSs and
k(T). Rates and BRs differ from those predicted from widely-used capture
models. In particular, the rate coefficient at 10 K is estimated to be almost
one order of magnitude smaller than what reported in the KIDA database. As for
BRs, the charge exchange is completely dissociative and the most abundant
fragments are HCCN+/CCNH+, HCNH+ and CH2+. Our results, combined with a revised
chemical network for formation of CH3CN, support the hypothesis that methyl
cyanide in protoplanetary disks could be mostly the product of gas-phase
processes rather than grain chemistry, as currently proposed. These findings
are expected to have implications in the comparison of the abundance ratios of
N-bearing molecules observed in disks with cometary abundance ratios
