Consideration was given to the problem of control of the longitudinal motion of
the amphibious aeroplane descending with prescribed velocity, altitude, and pitch angle under
incomplete measurements and exogenous disturbances. Decomposition methods of designing
the system for tracking the output variables with stabilization of the tracking errors invariantly
in asymptotics to the exogenous disturbances were developed within the framework of the block
approach. The problem of acquiring current information about the unmeasurable internal and
exogenous variables was solved comprehensively using the state observers in sliding modes which
are constructed on the basis of the control plant model with closed local bonds represented in
terms of the tracking errors. Realization of this approach does without a detailed dynamic
control plant model and its extension by the autonomous dynamic models of the exogenous
actions, which simplifies substantially the controller structure.