  During the EBEX2013 long duration flight the payload was free to rotate in
azimuth. The observed azimuth motion consisted of a superposition of full
rotations with a period of 10-30 minutes and oscillatory motion with an
amplitude of tens of degrees, average period of 79 s, and period dispersion of
12 s. We interpret the full rotations as induced by slow rotations of the
balloon and the shorter period oscillatory motion as due to torsional
oscillations of the flight line. We derive the torsional stiffness of the
flight line using the bifilar pendulum model and apply it to the flight line of
the EBEX2013 payload. We find a torsional spring constant of 36 kg m$^2$/s$^2$
corresponding to a period of 58 s. We conclude that the bifilar model, which
accounts for the geometry of the flight line but neglects all material
properties, predicts a stiffness and period that are 45% larger and 25% shorter
than those observed. It is useful to have a simple, easy to use, coarse
approximation for the torsional constant of the flight line.
