  In this paper, we consider the problem of input-output linearization of the
longitudinal flight dynamics. In longitudinal flight dynamics, inputs are
typically thrust and elevator deflection whereas the outputs are the velocity
and the flight path angle. An input-output linearization-based controller can
be designed to render the multi-input, multi-output system linear; however, the
resulting zero dynamics turns out to be unstable. In this work, we remove the
zero dynamics from the closed-loop dynamics by considering an additional
output. Although the additional output makes the system tall, which, in
general, means that the input-to-output dynamics can not be linearized, we show
that in the case of longitudinal flight dynamics, linearization is possible due
to special geometric properties of the nonlinear terms.
