https://assets.digital.cabinet-offic...REDL_08-09.pdf
The initial examination of the wreckage, in conjunction
with radar, HUMS, CVFDR and witness data,
determined that a failure within the epicyclic reduction
gearbox module of the MRG resulted in the rupture of
the gearbox case. This allowed the main rotor head,
together with the upper section of the MRG, to separate
from the helicopter.