What is the difference between an FMC and an autopilot?

In aviation, a flight management computer (FMC) stores the planned route and calculates navigation, performance predictions and a vertical profile; the autopilot moves the flight controls to follow active guidance. The FMC provides the plan and targets, while the autopilot controls the aircraft—and either can operate without the other.

FMC vs autopilot at a glance

For our Aviation & Real-World Flying readers, the practical distinction is guidance versus control.

Autothrottle or autothrust is another distinct system. It may use speeds calculated by the FMC, but it can remain engaged with the autopilot off, and engaging the autopilot does not necessarily activate automatic thrust control.

What does an FMC do in an aircraft?

An FMC calculates where the aircraft should fly and predicts how the flight will progress, but it does not directly move the control surfaces.

• Route management: it stores the active route, waypoints, airways, departures, arrivals and approaches.
• Position and navigation: it combines available navigation inputs to estimate aircraft position and calculate the desired track.
• Performance calculations: it uses entered and sensed data to calculate speeds, climb or descent profiles, fuel predictions and estimated arrival times.
• Guidance targets: it supplies the lateral path, vertical path and speed targets used by the flight-guidance system.
• Cockpit presentation: it provides route and prediction data for the navigation display and CDU or MCDU pages.

The crew still chooses and verifies the route. An FMC accepting a waypoint or displaying a route line does not prove that the flight plan is operationally correct, active or free of discontinuities.

How does the FMC make the autopilot follow a route?

The FMC passes its calculated path to the flight-guidance system, which turns that path into commands the autopilot can follow.

1. Enter and activate the plan. The route, runway, procedures and required performance data must be loaded and verified. Some aircraft require an explicit execute or insert action.
2. Select the required guidance. The pilot arms or engages a mode such as LNAV, NAV, VNAV or Airbus managed guidance. Entering a route alone does not select these modes.
3. Generate flight-director commands. The flight-guidance computers compare the desired path with the aircraft’s position and produce pitch and roll commands.
4. Engage the autopilot. The autopilot follows those commands through the flight-control system. With the autopilot off, the pilot can follow the same flight-director commands manually.
5. Correct deviations. Sensors report the aircraft’s response, allowing the system to keep correcting errors in a closed feedback loop.

The flight mode annunciator, or FMA, is the authoritative indication of which lateral, vertical and thrust modes are active or armed. A lit button on the MCP or FCU can show that a mode was selected without proving it has captured. Our explanation of how sensor feedback and autopilot control commands close this loop covers the control side in greater detail.

Are FMC, FMS and Airbus FMGS the same thing?

FMC, FMS and FMGS are related terms, but they do not technically name the same component.

FMS vs FMGS is principally a difference in scope and aircraft terminology, not a choice between two competing boxes. FMS is the generic term; FMGS is the name used for the integrated Airbus management-and-guidance architecture.

When somebody searches for an “Airbus FMC”, they usually mean the FMGS or the MCDU pages used to program it. Boeing documentation commonly uses FMC and CDU, while Airbus commonly uses FMGS, FMGC and MCDU. Names and architecture vary between aircraft families and generations, so our fuller breakdown of FMC, FMS, CDU and MCDU terminology explains where each label belongs.

Does an autopilot need an FMC?

No; an autopilot can fly selected modes without an FMC or an active flight plan.

Typical independent modes include heading or track hold, altitude hold, vertical speed, flight-path angle and pitch-based IAS or Mach climb modes. Their targets come from the mode-control panel or flight-control unit rather than from a calculated FMC route.

The reverse is also true. An FMC can continue calculating and displaying route guidance while the autopilot is disengaged. If the flight director is on, its command bars can show the pilot how to fly that guidance manually.

When should you use FMC guidance instead of selected autopilot modes?

Use FMC-guided modes for a valid, checked route and profile; use selected modes when you need direct control of heading, track, altitude or climb and descent behaviour.

• Choose LNAV, NAV, VNAV or managed guidance when the flight plan is active, the aircraft supports the required function and you want it to follow the programmed route or profile.
• Choose heading or track mode for vectors, weather avoidance, route interception or when the active leg is wrong.
• Choose a selected vertical mode for an instructed level-off, a deliberately controlled climb or descent, or when the simulated VNAV implementation is unavailable or unreliable.

Selected guidance is not merely an emergency fallback; it is a routine part of operating an autoflight system. A route can remain visible on the navigation display while heading or another selected mode overrides it.

Why is the autopilot not following the FMC route in a flight simulator?

If an autopilot is not following the FMC route, the cause is usually mode selection, route activation or capture conditions—not a failed autopilot.

Across generations of airliner add-ons, a mistake we see constantly is treating a line on the navigation display as proof that the route is controlling the aircraft. Diagnose the problem in this order:

1. Read the FMA first. If HDG, TRK, VS or another selected mode is active, the autopilot is following the panel setting rather than the FMC. An armed mode is only waiting to capture.
2. Check the active flight plan. Confirm that the intended route was executed or inserted where required and that the expected leg is active. A temporary, secondary or modified plan may be displayed without controlling guidance.
3. Inspect discontinuities and active legs. A route break, an incorrect direct-to or a waypoint behind the aircraft can prevent sensible guidance. Do not delete every discontinuity blindly: vector and manual legs may be intentional parts of a procedure.
4. Meet the capture conditions. LNAV or NAV may remain armed if the aircraft is too far from the route or approaching it at an unsuitable angle. Use heading or track mode to establish a reasonable intercept, then arm the route mode again.
5. Verify the navigation source. In aircraft that provide a source selector, GPS or FMS guidance must be selected rather than VOR or localiser guidance. Transport-category implementations differ, so use the aircraft’s FMA and displays rather than assuming a generic GPS switch exists.
6. Check the vertical limits. The MCP or FCU altitude normally constrains an FMC-managed climb or descent. Set an altitude in the intended direction, confirm that the vertical mode has engaged and ensure the required weights, speeds, cruise data and constraints are valid.
7. Confirm what is actually engaged. LNAV or VNAV can command the flight director with no autopilot channel connected. In that state, the pilot must hand-fly the command bars.
8. Check simulator integration and aircraft fidelity. A route loaded through the simulator’s world map, an EFB or another planner may not populate a custom aircraft’s FMC. Import or enter it through the method supported by that aircraft. Some simulated systems also omit coupled VNAV or model it only partially.

If the route uses fixes absent from the aircraft’s navigation database, rebuild the affected segment using waypoints that the simulated FMC recognises. Our guide to active, armed and reversion modes in flight simulators helps decode what the FMA is reporting before you change the route.

Can an FMC perform an automatic landing?

An FMC cannot perform an automatic landing by itself.

Autoland requires an aircraft specifically equipped and configured for it, using a combination of approach-navigation receivers, flight-guidance computers, autopilot channels, radio-altimeter data, automatic thrust and aircraft-specific control logic. The FMC may provide approach data or assist with navigation-aid tuning, but placing a runway or approach in the flight plan does not create autoland capability.

The same caveat applies in a flight simulator: an aircraft may fly an FMC route towards the runway yet lack a modelled autoland system. Check the FMA and the simulated aircraft’s declared capabilities rather than assuming that an engaged approach mode guarantees an automatic touchdown.