Flight Planning by Flight Management System
Autopilot system is a popular topic recently in which the avionics and other parts combine to fly the airplane without the help of a pilot. But this autopilot system is only a part of the flight management system (FMS) and here we are going to discuss one of the functions of FMS, flight planning.
FMS is a specialized computer system that automates a wide variety of in-flight tasks, decreasing the workload on the flight crew to the point that modern civilian airplane need not carry navigators or flight engineers.
The FMS generally consists of two units; first, a computer unit which provides computing platform and interfaces to other avionics and second, a control display unit (CDU) that provides a primary man-machine interface (MMI) for data entry and information display.
The FMS is typically comprised of interrelated functions: flight planning, navigation, performance computations, trajectory prediction, and guidance.
The flight planning function helps the crew to establish a specific routing for the airplane. The basis of the flight management computers (FMC) flight profile is the route that the airplane is to fly from departure airport to the destination airport.
The FMS flight planning function provides for assembly, modification, and activation of this route data known as a flight plan.
Route data are generally extracted from the FMC navigation database and typically consists of a departure airport and runway, a standard instrument departure (SID) procedure, enroute waypoints and airways, a standard arrival (STAR) procedure, and an approach procedure with a specific destination runway.
Often the destination arrival and approach procedure are not to be selected until the destination terminal area control is contacted.
Once the routing, along with any route parameters and performance selections, are made by the crew, the flight plan is assembled into a “buffer” that is used dominantly by the trajectory predictions in computing the lateral and vertical profile the airplane is hoped to fly from the departure airport to the destination airport.
The selection of flight planning data is done by the crew via menu selections either on the multifunction CDU or navigation display or by data link from the airline’s operational control.
Facilities are provided for the crew to define additional navigation data by means of a supplemental navigation database some of the waypoints of which are abeam, along-track offset (ATO), fix, flight information region (FIR)/ special use area (SUA), latitude/longitude, latitude/longitude crossing, place/bearing/distance off (PBD), place/bearing (PB) and runway extension.
The forward field of view display system shows a presentation of the selected segments of the flight plan as it is being constructed and flown.
The crew can modify the flight plan at any time. Its modification can come from crew selections or through data link from the airline operational communications or air traffic control in reply to a tactical situation.
An edit to the flight plan creates a modified version of the plan that is a copy of the active plan plus any accrued changes made to it.
Predictions of the trajectories are performed on the modified flight plan with each edit and periodically updated, which allows the crew to evaluate the impact of the flight plan changes right before acceptance. When the desired changes have been made to the crew’s satisfaction the modified flight plan is activated.
Flight planning further comprises of four aspects: flight plan construction, lateral flight planning, vertical flight planning and atmospheric modeling.
Flight plan construction is normally done by linking data stored in the navigation database. The data may include any combination of the items; terminal area procedures (SID/ STAR/ approach), airways, pre-stored company routes, fixes (en route waypoints, navigation aids, non-directional beacons, terminal waypoints, airport reference points, runway thresholds) and crew-defined fixes.
These selections may be used together using clearance language, by the menu selection from the navigation database, by specific edit actions, or data link.
Lateral flight planning is done to meet the tactical and strategic flight planning requirements of today’s airspace. The flight planning provides various ways to modify the flight plan at the crew’s discretion.
These various ways are: airway segments, direct-to, direct/intercept, fixes, holding pattern, lateral offset, missed approach procedures and terminal area procedures.
Vertical flight planning consists of a selection of speed, altitude, time constraints at the waypoints (if required), cruise altitude selection, airplane weight, forecast winds, temperatures, and destination pressure as well as altitude bands for the planned use of airplane anti-icing.
Various optimized speed schedules for the various flight phases are generally available. Several airplane performance-related crew selections may also be provided. These selections affect the predicted airplane trajectory and guidance.
Part of the flight planning is to specify forecast conditions for temperatures and winds that will be encountered during the flight. These conditions help the FMS to refine the trajectory predictions to provide more accurate determination of fuel burn, estimated times of arrival (ETAs), rates of climb/descent, and leg transition construction.
The FMS is a highly modernized concept and it needs further development. It is still in its infancy state and more can be hoped for in the coming decades.
Article By: Janak Kumar Tharu
Tribhuvan University, I.O.E
Image Credit: Kiko Alario Salom