Learn what an aircraft FMS does, how to programme and verify a route in a flight simulator, and how to fix LNAV, VNAV and discontinuity errors.
An aircraft flight management system (FMS) combines a navigation database, the planned route, aircraft performance data and inputs from position sensors to calculate guidance, speeds, fuel and arrival predictions. In a flight simulator, initialise it, enter and check the route and performance data, then select the lateral or vertical guidance modes that use its commands.
What does an aircraft FMS actually do?
The FMS turns position, route and performance data into instructions and predictions for the pilots, displays and flight-guidance systems. In real-world aviation and detailed flight simulators, it can sequence waypoints, calculate lateral and vertical paths, estimate fuel at destination and suggest economical speeds.
The terminology varies between aircraft, which causes needless confusion:
| Term | Meaning | What you interact with |
|---|---|---|
| FMS | The complete flight-management function or system | Route, performance, predictions and guidance |
| FMC | The flight management computer | The computer performing many FMS calculations |
| CDU or MCDU | The control and display interface | Keys, line-select buttons or a touchscreen used to enter data |
| Autopilot and flight director | Systems that follow selected guidance | LNAV, VNAV, NAV, managed or selected modes |
Boeing documentation commonly refers to the FMC and CDU, while Airbus aircraft often use MCDU and FMGS terminology. The underlying purpose is similar, but the pages, keys and activation logic are not interchangeable. Our explanation of how navigation systems combine position sources and route guidance covers the wider system around the FMS.
What information does an FMS need?
An FMS needs accurate position, route and performance inputs before its guidance and predictions can be trusted. Depending on the aircraft, those inputs include:
- Origin, destination, runway and alternate airport.
- Departure, en-route waypoints, airways, arrival and approach.
- Initial position and data from GNSS, inertial systems or radio navigation.
- Zero-fuel weight, fuel, reserves and aircraft loading.
- Cruise altitude, cost index or equivalent economy setting, and forecast winds.
- Speed and altitude constraints attached to procedures or entered by the crew.
Not every simulated FMS models all of these. A basic unit may provide only route navigation, while a study-level airliner may calculate take-off data, climb profiles and detailed fuel predictions. Predictions are only as reliable as the aircraft model and the values entered.
How do you use an FMS in a flight simulator?
Use the aircraft's own checklist and interface rather than applying one universal key sequence. The normal workflow is broadly the same across modern airliners:
- Power and initialise the system. Supply the required electrical power, enter or confirm the aircraft position and complete any inertial-reference alignment. Some simulator aircraft automate this; others require gate coordinates and several minutes of simulated alignment.
- Set the flight-plan basics. Enter the origin and destination, then confirm the correct runway and any route identifier required by the model. Check that the FMS navigation database contains the procedures you intend to use.
- Build the route. Add the departure, en-route waypoints or airways, arrival, approach and transitions that match the planned or cleared route. Do not choose a similarly named waypoint merely because the intended one is absent.
- Deal with discontinuities carefully. A route discontinuity is a break between legs. Join the legs only when the route is supposed to be continuous; a discontinuity may deliberately represent radar vectors, a manual leg or the end of a procedure.
- Enter performance data. Supply the weights, fuel, reserves, cruise altitude and economy setting requested by that aircraft. Complete take-off or approach pages where the simulation models them, but do not invent values just to clear a warning.
- Verify every leg. Compare the legs page and navigation display with the expected route, checking waypoint order, tracks, altitude constraints and procedure transitions. Look especially for doubled-back tracks, large route gaps and waypoints with the same identifier in different regions.
- Activate the plan. Execute, insert or activate the temporary route using the command provided by that FMS. Until this is done, the edited route may remain only a proposal and will not drive guidance.
- Select and monitor guidance. Set the cleared altitude and appropriate flight-guidance modes. After departure, confirm on the flight mode annunciator that LNAV, VNAV, NAV or managed guidance is armed or active; the magenta route alone does not mean the aircraft will follow it.
For the page-by-page logic of a common airliner implementation, our worked Boeing 737 FMC programming sequence shows how initialisation, route entry, performance setup and activation fit together.
Does the FMS fly the aircraft automatically?
The FMS calculates guidance, but the autopilot or flight director must be placed in a mode that follows it. You can also hand-fly using flight-director commands generated from the FMS route.
Lateral guidance is normally selected through modes such as LNAV, NAV or managed heading. Vertical guidance may use VNAV or managed climb and descent, but its authority depends on the aircraft: the selected altitude, thrust mode and speed controls can all limit what it does. Always read the flight mode annunciator rather than assuming a button press produced the expected mode.
Should you enter the route manually or import it?
Manual entry is best for learning the FMS and catching route errors; importing saves time on long plans but still requires full verification. An imported flight plan may omit procedures, use different navigation data or populate the simulator's map without programming the aircraft's own FMS.
If you change a runway, SID, STAR or approach in the cockpit, do not assume an external planner or the simulator's flight-planning screen has updated with it. Treat the cockpit route as the authoritative version for aircraft guidance, then check every changed leg.
Why will the FMS not follow the route?
An FMS usually fails to follow the intended route because the plan is inactive, the wrong leg is active, guidance has not been selected or the navigation data does not match the planned procedure.
| Symptom | Likely cause | Practical fix |
|---|---|---|
NOT IN DATABASE or similar | Mistyped identifier, missing waypoint or different navigation-data cycle | Check the spelling, region and procedure data; never substitute an unverified waypoint |
| Route discontinuity | A gap, vectors leg or incompatible procedure transition | Check the chart or planned route before connecting legs; some gaps are intentional |
| LNAV or NAV will not engage | Route not activated, unsuitable intercept angle, wrong active leg or wrong navigation source | Verify the active leg and mode, then use heading or a verified direct-to command to establish an intercept |
| VNAV is unavailable or behaves unexpectedly | Missing performance data, conflicting altitude selection, impossible constraint or limited simulation | Complete the performance pages, check constraints and use selected vertical modes when the model cannot calculate the requested path |
| Aircraft turns backwards | Wrong waypoint, transition or leg became active | Select the correct next leg only after checking its track and distance on the display |
The mistake we see most often is deleting every discontinuity without checking why it exists. This can connect two legs that were never meant to join and produce an abrupt turn or a path through unsuitable terrain.
What changes when using the FMS for an approach?
Approach use requires the correct runway, procedure, transition, constraints, minima and guidance source to be checked before interception. An RNAV approach follows different mode logic from an ILS, and simply loading an approach does not guarantee that it is active or that the autopilot will descend.
Confirm the final approach track, distance, altitude restrictions and missed-approach legs against the procedure you intend to fly. For the simulator-specific sequence after route programming, see our GPS and RNAV approach workflow for Microsoft Flight Simulator.