To finalize this problem, note that a real cable has stiffness
in addition to tension. Stiffness tends to return a wire to its
original straight-line shape even when it is not under tension.
For example, a piano wire straightens if released from
a curved shape; package wrapping string does not.
Stiffness represents a restoring force in addition to tension,
and increases the wave speed. Consequently, for a real
cable, the speed of 60.0 m/s that we determined is most
likely associated with a tension lower than 1.92 - 103 N and
a correspondingly smaller acceleration of the helicopter.