  We present the study of the X2-class flare which occurred on the 27 October
2014 and was observed with the Interface Region Imaging Spectrograph (IRIS) and
the EUV Imaging Spectrometer (EIS) on board the Hinode satellite. Thanks to the
high cadence and spatial resolution of the IRIS and EIS instruments, we are
able to compare simultaneous observations of the \xxi~1354.08~\AA~and
\xxiii~263.77~\AA~high temperature emission ($\gtrsim$ 10~MK) in the flare
ribbon during the chromospheric evaporation phase. We find that IRIS observes
completely blue-shifted \xxi~line profiles, up to 200 km s$^{-1}$ during the
rise phase of the flare, indicating that the site of the plasma upflows is
resolved by IRIS. In contrast, the \xxiii~line is often asymmetric, which we
interpret as being due to the lower spatial resolution of EIS. Temperature
estimates from SDO/AIA and Hinode/XRT show that hot emission (log($T$)[K] $>$
7.2) is first concentrated at the footpoints before filling the loops. Density
sensitive lines from IRIS and EIS give electron number density estimates of
$\gtrsim$~10$^{12}$~cm$^{-3}$ in the transition region lines and
10$^{10}$~cm$^{-3}$ in the coronal lines during the impulsive phase. In order
to compare the observational results against theoretical predictions, we have
run a simulation of a flare loop undergoing heating using the HYDRAD 1D hydro
code. We find that the simulated plasma parameters are close to the observed
values which are obtained with IRIS, Hinode and AIA. These results support an
electron beam heating model rather than a purely thermal conduction model as
the driving mechanism for this flare.
