The mathematical model of flight dynamics features a closed-loop, feedback control mechanism. It can be thought of as a computational engine, whose components compute quantities, each relating to a specific aspect of the guidance and control problem. The diagram of Figure 7.1 depicts the operations that a computer program would perform to keep track of the aircrafts state and to control its movement. The rounded boxes appearing in this diagram represent computational processes. Data is represented using the conventional data store symbol (i.e., a data group name between two horizontal lines). This computational view of the mathematical model suggests an organization and sequencing of program logic for implementing the methods encapsulated in the model. It also suggests a rational structuring of data used by the models computational processes.
The computational "engine" uses and produces data. Data has been organized into logically related groups, and include: (a) the flight plan; (b) aircraft state; (c) aircraft state deviations; (d) sensor measurements; (e) guidance parameters; and (f) steering and throttle commands.
The flight plan is a specification of a flight scenario. It describes the route to be flown, the desired aircraft speed along each track leg of the intended route, and environmental conditions, such as wind velocity. The flight plan provides all the information needed by the mathematical model to predict aircraft motion. The flight plan may also be thought of as being lodged in the mind of the pilot, in which case it reflects, at any given time, where he would like to be and how fast he would like to get there.
The aircraft state is a collection of data elements which includes such things as aircraft position, velocity, orientation, and angular rates. Aircraft state is initialized in Stage 0 using sensor measurements, pilot/navigator inputs, and a flight plan if there is one. In Stage 1 the equations of motion and sensor inputs are used to update aircraft state.
Stage 2 program logic compares the flight plan with aircraft state to determine the extent to which desired state (as reflected in the flight plan) deviates from actual aircraft state. The computed flight plan deviations and aircraft state are then used by the Stage 3 process to compute the guidance parameters needed in Stage 4. Guidance parameters provide a means for assessing flight progress and adherence to the flight plan. Guidance parameters include items such as distance to or from a waypoint, bearing to target, or time to a destination.
Finally, the throttle and steering commands computed in Stage 4 are used to drive control surfaces, such as the elevators and ailerons, and to control engine thrust.