  The output-error method is a mainstay of aircraft system identification from
flight-test data. It is the method of choice for a wide range of applications,
from the estimation of stability and control derivatives for aerodynamic
database generation to sensor bias estimation in flight-path reconstruction.
However, notable limitations of the output-error method are that it requires ad
hoc modifications for applications to unstable systems and it is an iterative
method which is particularly sensitive to the initial guess. In this paper, we
show how to reformulate the estimation as a collocation problem, an approach
common in other disciplines but seldomly used in flight vehicle system
identification. Both formulations are equivalent in terms of having the same
solution, but the collocation-based can be applied without modifications or
loss of efficiency to unstable systems. Examples with simulated and real-world
flight-test data also show that convergence to the optimum is obtained even
with poor initial guesses.
