How do I set up navigation after a cold-and-dark start?

After a cold-and-dark start, establish stable electrical power, switch on the avionics, allow AHRS or IRS alignment, initialise the FMS present position, load and verify the route, then select the correct GPS/FMS or radio navigation source. Remain parked until attitude, position, active leg and flight-mode indications all make sense.

This is General flight-simulator guidance for MSFS, X-Plane, Prepar3D and comparable platforms. Exact controls vary considerably: a basic GA aircraft may need only its radios and heading indicator, while a detailed airliner may model several inertial units, an independent FMS and realistic alignment delays.

Cold-and-dark navigation setup: the correct order

The dependable order is power, sensors, present position, route, procedures, verification and navigation-source selection. A mistake we see constantly is programming the route first, then discovering that an electrical interruption has reset the avionics or inertial alignment.

1. Establish stable electrical power. Use external power, the APU or the aircraft's normal generator source as appropriate. A battery may illuminate part of the panel without powering every avionics or instrument bus. If the aircraft is not yet electrically ready, complete the power-up and avionics stages of a cold-and-dark start before programming navigation.
2. Power the fitted navigation equipment. This may mean an avionics master in a light aircraft, individual GPS and radio controls, or overhead ADIRS/IRS selectors in an airliner. Check screen brightness as well; an unlit display is not always unpowered.
3. Allow AHRS, IRS or INS initialisation. Keep the aircraft stationary if the simulation models alignment. AHRS-equipped GA panels often initialise automatically, whereas an airliner IRS normally needs its selectors set to NAV and a valid reference position.
4. Set or confirm the present position. Where the FMS requests it, enter or copy a credible gate, airport-reference or GNSS position. Check latitude and longitude hemispheres carefully. A wrong east/west entry can produce a believable route displaced hundreds or thousands of miles from the aircraft.
5. Enter or import the flight plan. Start with origin and destination, then add airways and fixes. Complex add-on aircraft may not read the plan created in the simulator's world map, so use the aircraft's supported import function or enter the route through its CDU or MCDU.
6. Add the departure, arrival and required performance data. Confirm the departure runway and SID against the clearance or intended runway. Weight, cruise altitude, cost index and take-off data may be needed for VNAV and predictions even when lateral route guidance already works.
7. Insert, execute or activate the route. A temporary or modified plan does not control the aircraft until it has been accepted. Step through each leg and inspect the map for duplicate fixes, loops, implausible turns, missing transitions and an unsuitable first leg.
8. Select the navigation source and initial mode. Choose GPS/FMS for flight-plan guidance or VLOC/NAV for a manually tuned VOR or localiser. Set the cleared heading and altitude, then verify the flight-mode annunciator rather than relying on a button light.
9. Perform a final position and guidance check. The displayed runway, airport and first waypoint should be in the correct direction. Do not taxi with attitude or heading flags, an obviously displaced map, an unexecuted route or an unexplained departure discontinuity.

AHRS vs IRS: what is the difference?

AHRS primarily supplies attitude and heading, while IRS supplies attitude, heading, velocity and an inertially calculated position. They are not interchangeable names for the same equipment, and neither is the FMS itself.

The FMS is the complete flight-management system. The FMC is its computing element, while the CDU or MCDU is the cockpit interface used to enter data. Simmers often use these terms loosely, but the distinction helps when troubleshooting.

In Spanish-language aviation material, aviación—often typed aviacion without the accent—AHRS may be described as a sistema de referencia de actitud y rumbo, IRS as a sistema de referencia inercial, and FMS as a sistema de gestión de vuelo. Cockpit labels usually retain their English abbreviations.

What does “AHRS not using configured AHRS type” mean?

AHRS not using configured AHRS type is normally an aircraft, instrument or add-on configuration warning rather than a pilot instruction. Its exact meaning depends on the simulator and aircraft package, but it generally indicates that the active instrument is not using the AHRS implementation requested by the aircraft configuration.

If it appears only in a developer console or log and attitude and heading behave normally, it may be non-critical. If instruments fail, remain flagged or use the wrong heading source, try these checks in order:

• Load the aircraft without panel, avionics or compatibility modifications that could override its instruments.
• Confirm that the correct aircraft variant and panel configuration are selected.
• Restart from a stationary position with stable power and allow the AHRS to initialise again.
• Compare the behaviour with an unmodified default aircraft using similar avionics.
• Repair or reinstall the affected aircraft package if its files may have been replaced or mixed between versions.

There is no universal cockpit switch that fixes this message. Aircraft developers should check the package's AHRS declaration against the instrument implementation it actually loads; ordinary users should not make speculative configuration edits without documentation for that aircraft.

How do I align the IRS in an A320?

To align an A320 IRS, keep the aircraft stationary on stable power, set all three ADIRS IR mode selectors to NAV, initialise a valid present position through the MCDU, and wait until the ALIGN indications and alignment memo clear.

1. Secure the aircraft. Set the parking brake or chocks and do not allow pushback or taxi to begin. Movement can interrupt or invalidate alignment in detailed A320 simulations.
2. Provide uninterrupted electrical power. External power or the APU generator is normally used during the setup. Avoid changing power sources in a way that momentarily depowers the ADIRS.
3. Set IR 1, IR 2 and IR 3 to NAV. The rotary selectors on the overhead ADIRS panel normally have OFF, NAV and ATT positions. NAV is used for a normal full alignment; ATT is a degraded attitude-reference mode, not a quick-alignment shortcut.
4. Enter the flight origin and reference position. On many simulated A320s, entering FROM/TO on the MCDU INIT page makes an ALIGN IRS prompt available. Confirm that the proposed coordinates match the aircraft's actual airport or stand before accepting them. Page names and automation differ between add-ons.
5. Wait for completion. Alignment may be instant, accelerated or take several minutes depending on the add-on's realism setting and modelled conditions. Continue programming the route, but leave the aircraft stationary.
6. Verify the result. The overhead ALIGN lights should extinguish, any IRS alignment memo should disappear, and the flight displays should show valid attitude, heading and map position without associated flags.

A flashing ALIGN indication or an MCDU request for IRS position can mean that no valid position was entered, the entered position differs excessively from the sensed or stored position, or the alignment was disturbed. Recheck the coordinates and power state before cycling the selectors; switching an IR unit fully off may restart its alignment.

The A320 procedure should not be copied blindly to Boeing or classic INS-equipped aircraft. For comparison, our Boeing 747 cold-start example shows how IRS alignment and FMC position entry fit into that cockpit.

Can I enter the route while the IRS is aligning?

Yes, most modern airliner simulations allow FMS route and performance entry while the IRS continues aligning. Present-position initialisation is part of the process, so complete it when requested and maintain stable electrical power.

The moving map, groundspeed, LNAV or managed NAV may remain unavailable until sufficient inertial data is present. Route entry being possible does not mean the aircraft is ready to move.

Which navigation source should I select after setup?

Select GPS or FMS for flight-plan tracking, and select VLOC, NAV1 or the appropriate radio source for a manually tuned VOR or localiser. Loading a route does not guarantee that the displays or autopilot are using it.

• Garmin-style GA avionics: the CDI annunciation normally shows GPS for the magenta flight-plan course and VLOC for VOR or ILS signals.
• Conventional HSI installations: select the intended NAV receiver, tune and identify the station, then set the required course. Our explanation of reading an HSI and choosing its navigation source covers the resulting indications.
• Modern airliners: LNAV or managed NAV is a flight-guidance mode, not simply a NAV1/GPS switch. The flight-mode annunciator shows whether lateral guidance is armed or active.
• ILS approaches: use the aircraft's localiser and glide-slope system as specified by its procedure. Some FMS aircraft auto-tune an ILS, but the frequency, course and ident still need checking.

If the relationship between the loaded plan, CDI, flight director and autopilot is unclear, see how GPS and the FMS turn route data into cockpit guidance.

When should I arm LNAV or NAV mode?

Arm route guidance only after alignment is complete, the route is active and the first leg can be intercepted sensibly. Some aircraft allow LNAV or managed NAV to be armed before take-off; others impose altitude, position or intercept-angle conditions.

If the clearance requires runway heading or radar vectors, use heading mode first and engage route guidance when cleared towards an appropriate leg. The flight-mode annunciator is decisive: panel lights show what was requested, while the annunciator shows what is armed and what is actually controlling the aircraft.

Why is the FMS route loaded but not working?

A loaded FMS route provides no useful guidance unless it has been activated, contains a valid active leg and is paired with the correct navigation mode and source.

• No magenta route appears: the route may still be temporary, uninserted or unexecuted. It may also have been loaded into the simulator planner but not the add-on aircraft's own FMS.
• LNAV or managed NAV will not arm: confirm that alignment is complete, an active leg exists ahead of the aircraft and the aircraft satisfies any mode-engagement conditions.
• The autopilot follows a radio course: a GA CDI or HSI may still be set to VLOC or NAV rather than GPS.
• The aircraft turns towards an old waypoint: the previous leg may still be active, or heading mode may be controlling the aircraft. Check the active waypoint before using direct-to.
• A route discontinuity remains: inspect the legs on both sides before deleting it. A discontinuity after a vector or manual leg may be intentional; joining the fixes blindly can create an unsafe reversal or tight turn.
• A SID, STAR or waypoint is unavailable: the planner and aircraft may use different navigation-data cycles. Rebuild the affected section using procedures and fixes available in the aircraft's database.
• LNAV works but VNAV does not: complete the required weight, cruise, performance and altitude entries. Missing performance data often affects vertical guidance without preventing lateral tracking.
• The map is displaced from the airport: verify the entered IRS position and latitude/longitude hemispheres. Do not attempt to repair a position error by repeatedly activating the route.

Does “route unavailable — missing permit” mean the FMS is broken?

Route unavailable — missing permit usually comes from a career, economy, mission or airline-dispatch layer, not from the aircraft's FMS. It means the simulated pilot, company or profile lacks an authorisation required for that mission, aircraft category, airport, region or overflight route.

Open the product's mission or dispatch requirements rather than the MCDU. Obtain or assign the required permit or qualification, change to an authorised aircraft or route, or select a mission available to the profile. If the permit is already shown as owned, reload that career or dispatch session and check for conflicting mission add-ons.

IRS realignment, deleting route discontinuities and changing GPS/NAV sources will not resolve a permit restriction. Only troubleshoot the cockpit route after the external planner has accepted the flight.

What should be checked before taxi?

The aircraft is ready to taxi when its sensors, position, route and selected guidance all agree with the intended departure.

• No unexplained attitude, heading, navigation or alignment flags remain.
• The map places the aircraft at the correct airport and stand area.
• The route is inserted, executed or activated rather than left temporary.
• The departure runway, SID and first active leg match the clearance.
• The first waypoint lies in a sensible direction and no accidental route loop is visible.
• The intended GPS/FMS or radio source is displayed.
• Initial heading, cleared altitude and flight-director modes are set and verified on the flight-mode annunciator.
• Any departure discontinuity has been understood rather than deleted automatically.