  We study the evolutionary conditions for Kelvin-Helmholtz (KH) instability in
a H-alpha solar surge observed in NOAA AR 8227 on 1998 May 30. The jet with
speeds in the range of 45-50 km/s, width of 7 Mm, and electron number density
of 3.83 x 10^{10} cm^{-3} is assumed to be confined in a twisted magnetic flux
tube embedded in a magnetic field of 7 G. The temperature of the plasma flow is
of the order of 10^5 K while that of its environment is taken to be 2 x 10^6 K.
The electron number density of surrounding magnetized plasma has a typical for
the TR/lower corona region value of 2 x 10^{9} cm^{-3}. Under these conditions,
the Alfven speed inside the jet is equal to 78.3 km/s. We model the surge as a
moving magnetic flux tube for two magnetic field configurations: (i) a twisted
tube surrounded by plasma with homogeneous background magnetic field, and (ii)
a twisted tube which environment is plasma with also twisted magnetic field.
The magnetic field twist in given region is characterized by the ratio of
azimuthal to the axial magnetic field components evaluated at the flux tube
radius. The numerical studies of appropriate dispersion relations of MHD modes
supported by the plasma flow in both magnetic field configurations show that
unstable against Kelvin-Helmholtz instability can only be the MHD waves with
high negative mode numbers and the instability occurs at sub-Alfvenic critical
flow velocities in the range of 25-50 km/s.
