How do I program a Boeing 737 FMC in a flight simulator?

To program a Boeing 737 FMC in a flight simulator, power the aircraft, initialise its position, enter and activate the route, select the runway and procedures, resolve only genuine route gaps, complete performance and take-off data, then verify every leg on the CDU and navigation display before engaging LNAV or VNAV.

This is the general workflow for Boeing-style 737 add-ons in Microsoft Flight Simulator, FSX, Prepar3D and X-Plane. Exact prompts vary between 737 generations and developers, but the underlying sequence is consistent.

What does the Boeing 737 FMC control?

The FMC calculates the lateral route, predicted fuel and times, speed targets and, in sufficiently detailed aircraft, LNAV and VNAV guidance.

The CDU is the keyboard and display used to communicate with the FMC; the wider system is often called the FMS. The FMC does not physically fly the aircraft by itself. Its guidance must be selected through the flight director, mode control panel, autopilot and autothrottle as appropriate.

Full-fidelity NG-style and MAX-style simulations normally include pages such as POS INIT, RTE, LEGS, PERF INIT and TAKEOFF REF. A simplified 737 may import the simulator's flight plan automatically, omit performance pages or provide only basic lateral guidance.

What do I need before programming the FMC?

You need electrical power, a planned route, an expected departure runway and the aircraft's actual fuel and payload loaded before entering performance figures.

• Stable electrical power: use external power, the APU or another valid source so the CDU does not reset during entry.
• A compatible route: the planned fixes, airways and procedures must exist in the FMC's installed navigation database.
• Runway information: use the runway assigned by ATC or the one appropriate for the simulated wind and airport operation.
• Loaded fuel and payload: entering a zero-fuel weight in the FMC does not necessarily load passengers, cargo or fuel into the aircraft. Use the add-on's loading system first.

If route planning, clearances and procedures are unfamiliar, our practical guide to flying IFR in a simulator explains how the flight plan fits around the FMC work.

How to program a Boeing 737 FMC step by step

The safest sequence is position initialisation, route entry, route activation, procedure selection, LEGS review, performance entry and take-off preparation.

1. Power the CDU and start IRS alignment. Supply stable electrical power and set the inertial reference system controls as required by the aircraft. Route entry can usually continue while alignment runs, but LNAV and reliable map positioning require a valid navigation position.

2. Check IDENT and initialise the position. Confirm that the FMC identifies the expected aircraft and navigation database. On POS INIT, copy a displayed GPS or reference position into the scratchpad and line-select it into SET IRS POS if the simulation requires manual entry. Simplified models may do this automatically.

3. Create the route shell. Open the RTE page and enter the origin and destination ICAO codes. Add the flight number if desired and any company-route identifier only if the aircraft supports the saved route format.

4. Enter the en-route flight plan. On a Boeing-style VIA/TO page, place the airway in the VIA column and its exit fix in the corresponding TO field. A direct waypoint normally appears under TO with DIRECT beside it. Entering every intermediate airway fix manually creates more opportunities for typing errors.

5. Activate and execute the basic route. Select ACTIVATE if offered, inspect the route, then press EXEC. An illuminated execute light means there is still a pending modification; it does not mean the change has already been accepted.

6. Select the departure. Through DEP/ARR, choose the planned runway, SID and transition. Do not manually enter all the SID fixes and then select the same SID from the database, or the route may contain duplicates and turn back on itself.

7. Select the arrival when appropriate. Choose the STAR, transition and approach for the expected landing runway. If the runway assignment is uncertain, leave the final approach until later rather than building and briefing a procedure that is likely to change.

8. Audit every leg before executing procedure changes. Read down the LEGS pages and inspect waypoint order, altitude and speed constraints, discontinuities, vectors and suspiciously large turns. In aircraft that support it, use the navigation display's PLAN mode and the CDU's STEP prompt to examine the route one leg at a time.

9. Complete PERF INIT. Enter or confirm zero-fuel weight, reserve fuel, cost index and cruise altitude using the units shown by the aircraft. A frequent cause of INVALID ENTRY is typing a full weight where the page expects a value expressed in thousands, or mixing kilograms and pounds.

10. Complete thrust and take-off data. Use the appropriate thrust limit and enter the planned flap setting, centre of gravity and V-speeds as supported by the add-on. Transfer the calculated stabiliser trim setting to the aircraft; leaving the value on the CDU does not move the trim. Do not invent an assumed temperature or derate without valid take-off performance data.

11. Perform the final cockpit cross-check. Confirm that the route begins ahead of the aircraft, the first active leg is sensible and no unintended gaps remain. Set the MCP altitude, heading and speed required for departure, then arm LNAV or VNAV only when their engagement conditions are satisfied.

For a page-by-page visual example, our screenshot-based PMDG 737NG FMC walkthrough uses an older simulator but retains useful Boeing route-entry and performance-page logic.

Should I use airways, individual fixes or an imported route?

Airway entry is usually the cleanest choice for a normal IFR flight, while individual fixes are better for short routes and repairs.

Imported plans often exclude SIDs and STARs deliberately. That is usually preferable because the procedures can then be selected inside the FMC for the assigned runways. If the imported plan already contains procedure fixes, selecting the procedure again may create duplicate legs.

Should I load the approach before departure?

Load the approach before departure when the destination runway is reasonably certain; otherwise, load only the confirmed arrival elements and add the approach after ATC or the weather settles.

After changing a runway, STAR or approach, inspect the entire modified section before pressing EXEC. A runway change can alter transitions, duplicate a common fix or leave a discontinuity where the old and new procedures meet.

Selecting an ILS approach in the FMC does not guarantee that every simulated 737 will tune the radios or set both course selectors automatically. That behaviour depends on the aircraft model, and our guide to setting the 737 course knob correctly explains when the CRS selectors matter.

How should I handle route discontinuities and vectors?

Keep deliberate vector legs and close only those discontinuities that represent a genuine, cleared connection between two route segments.

A ROUTE DISCONTINUITY is not automatically an error. It may mark the join between a SID and the en-route plan, a radar-vector segment on an arrival, or a procedure that expects the pilot to intercept the next leg manually.

How do I close a genuine route discontinuity?

Close a genuine gap by moving the next valid waypoint over the discontinuity, then inspecting the resulting path before execution.

1. Confirm the intended connection. Check that flying directly to the next fix matches the clearance or the route you intend to simulate.
2. Copy the next waypoint. Select it into the scratchpad and place it over the discontinuity using the appropriate line-select key.
3. Inspect the modified leg. Check the navigation display for a sharp turn, loop, track reversal or shortcut through terrain.
4. Execute only when satisfied. Press EXEC after confirming that the modified route is safe and correctly ordered.

Do not close a vectors-to-final leg merely to make the LEGS page look tidy. Fly the assigned heading or use HDG SEL, then intercept or activate the appropriate approach leg when cleared.

Why is my Boeing 737 FMC route not working?

Most FMC route problems come from unexecuted changes, mismatched navigation data, duplicate procedure fixes, unresolved discontinuities or an active leg the aircraft cannot sensibly intercept.

Why was my waypoint or airway rejected?

A rejected waypoint or airway usually has the wrong spelling, does not exist in the installed database, has multiple regional matches or uses an exit fix that is not part of that airway.

Check the scratchpad message, verify the identifier and try the fix individually. If a planner and the aircraft use different navigation data, adapt the route to what the FMC database contains rather than repeatedly forcing an invalid airway segment.

Why does the route double back or draw a loop?

A loop normally means the route contains a duplicate fix, the wrong procedure transition or a waypoint that lies behind the aircraft.

Step through the LEGS page and remove only the incorrect occurrence. This often happens when procedure waypoints were imported with the en-route plan and the same SID or STAR was then selected through DEP/ARR.

Why will LNAV not follow the route?

LNAV will not follow a route that is inactive, unexecuted, interrupted by a discontinuity or unsuitable for interception from the aircraft's present heading and position.

Use heading select to maintain a safe path, choose a sensible active leg or create a direct-to modification, inspect the new track and execute it before trying LNAV again. Also check IRS alignment, flight-director status and the add-on's LNAV engagement conditions.

Why is VNAV blank, late or obviously wrong?

VNAV calculations become unavailable or unreliable when performance data are missing, weight units are wrong, the cruise altitude is unrealistic or the route contains broken or contradictory constraints.

Check PERF INIT, the active route and the MCP altitude. During descent, VNAV cannot command a lower altitude if the MCP altitude selector has not been lowered as required. Some simplified 737s model only part of Boeing VNAV, so speed, level change or vertical-speed modes may still require manual management.

Does programming the FMC make the 737 fly automatically?

No; programming the FMC creates guidance, but the pilot still has to configure and monitor the flight-director, autopilot, autothrottle and MCP modes.

LNAV follows the active lateral leg, while VNAV uses the route, performance entries, constraints and selected MCP altitude. The heading and course knobs serve different purposes, and turning a CRS selector will not make LNAV follow a missing or invalid FMC leg.