  A comprehensive study of the physical parameters of active region fan loops
is presented using the observations recorded with the Interface Region Imaging
Spectrometer (IRIS), the EUV Imaging Spectrometer (EIS) on-board Hinode and the
Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager
(HMI) on-board the Solar Dynamics Observatory (SDO). The fan loops emerging
from non-flaring AR~11899 (near the disk-center) on 19th November, 2013 are
clearly discernible in AIA 171~{\AA} images and those obtained in \ion{Fe}{8}
and \ion{Si}{7} images using EIS. Our measurements of electron densities reveal
that the footpoints of these loops are approximately at constant pressure with
electron densities of $\log\,N_{e}=$ 10.1 cm$^{-3}$ at $\log\,[T/K]=5.15$
(\ion{O}{4}), and $\log\,N_{e}=$ 8.9 cm$^{-3}$ at $\log\,[T/K]=6.15$
(\ion{Si}{10}). The electron temperature diagnosed across the fan loops by
means of EM-Loci suggest that at the footpoints, there are two temperature
components at $\log\,[T/K]=4.95$ and 5.95, which are picked-up by IRIS lines
and EIS lines respectively. At higher heights, the loops are nearly isothermal
at $\log\,[T/K]=5.95$, that remained constant along the loop. The measurement
of Doppler shift using IRIS lines suggests that the plasma at the footpoints of
these loops is predominantly redshifted by 2-3~km~s$^{-1}$ in \ion{C}{2},
10-15~km~s$^{-1}$ in \ion{Si}{4} and $~$15{--}20~km~s$^{-1}$ in \ion{O}{4},
reflecting the increase in the speed of downflows with increasing temperature
from $\log\,[T/K]=4.40$ to 5.15. These observations can be explained by low
frequency nanoflares or impulsive heating, and provide further important
constraints on the modeling of the dynamics of fan loops.
