Fix Boeing 747-8 FMC KEY NOT ACTIVE on POS INIT with the correct IRS state, line-select key and GPS-position entry sequence.
In a Boeing 747-8 FMC, KEY NOT ACTIVE usually means the selected line cannot accept data in the aircraft’s present state, not that GPS has failed. While stationary, put the IRS selectors in the model’s required ALIGN or NAV state, copy GPS POS to the scratchpad, then insert it at SET IRS POS.
This applies broadly to the 747-8 in Microsoft Flight Simulator 2020 and 2024 and to detailed 747 add-ons, although their IRS modelling differs. The message comes from the CDU/FMC logic; it is not normally a computer-keyboard or controller-binding fault.
How do I enter the GPS position correctly?
The correct sequence is GPS POS → scratchpad → SET IRS POS, with IRS alignment active.
- Keep the aircraft stationary. Park at the gate, set the parking brake and avoid slew or pushback during alignment.
- Establish electrical power. The CDU and inertial-reference equipment must be powered. If the overhead state is uncertain, complete the relevant steps in our 747-8 cold-and-dark start sequence first.
- Start IRS alignment. Move the IRS selectors from OFF to the ALIGN or NAV position required by that aircraft model. Some simulations start alignment when NAV is selected; detailed add-ons may expose a separate ALIGN state.
- Open POS INIT. Reach it through INIT/REF or the FMC index, then enter the reference airport if the implementation requests one.
- Copy the displayed position. Clear any old scratchpad entry with CLR, then press the line-select key beside GPS POS. The coordinates should appear in the scratchpad.
- Set the IRS position. Press the line-select key beside SET IRS POS. Do not try to paste coordinates back into GPS POS, which is a position source rather than the destination field.
- Allow alignment to finish. Wait for the IRS warnings or alignment indications to clear before moving the aircraft.
The same source–scratchpad–destination logic is shown in our worked Boeing POS INIT and IRS alignment explanation, though individual CDU layouts vary.
Why is the SET IRS POS key still inactive?
SET IRS POS is enabled only while the simulated IRS is in a state where it can accept an initial position.
| What you see | Likely cause | What to do |
|---|---|---|
| KEY NOT ACTIVE after an engines-running start | The IRS is already aligned and the position field has been locked. | If the navigation display has a valid position and no IRS flags, continue without entering it again. |
| ENTER IRS POSITION remains displayed | Alignment has not started, or the IRS selectors are still OFF. | Check electrical power and place all required IRS selectors in the correct alignment state. |
| KEY NOT ACTIVE when pasting beside GPS POS | GPS POS is a read-only source field. | Paste the scratchpad coordinates beside SET IRS POS instead. |
| GPS POS is displayed but cannot be copied | That FMC implementation may not make the source line selectable. | Type the displayed position exactly as shown, then insert it at SET IRS POS. |
| The field remains unavailable in a simplified aircraft | IRS alignment may be automatic or only partly simulated. | If the aircraft position is valid and no entry prompt remains, no manual correction is required. |
If manual entry produces INVALID ENTRY or NOT IN DATABASE, that is a formatting problem rather than an inactive key. Preserve the hemisphere letters and digit order shown by the CDU; route-waypoint coordinate formats are not always accepted on POS INIT. Our guide to FMC latitude and longitude formats explains the distinction.
How can I tell whether position entry is unnecessary?
If the aircraft has a valid navigation display and no IRS warning or ENTER IRS POSITION prompt, its position is probably already initialised.
This is common when spawning on a runway or loading an engines-running state. In that situation, cycling the IRS simply to make SET IRS POS active can discard a valid alignment and create a new problem. Only realign if the displayed aircraft position is wrong, the inertial-reference indications are invalid, or the FMC specifically requests a position.
A saved flight, aircraft relocation or slew after initialisation can leave the simulated systems in an inconsistent state. Return the aircraft to a stationary condition and perform a complete alignment; reloading the aircraft state should be the last resort rather than the first fix.
What if LNAV still will not engage?
An aligned IRS does not by itself make LNAV available.
Check that the route has been activated and executed, the first active leg is sensible, and no route discontinuity blocks the path. The remaining route and mode checks are covered in our 747-8 FMC and flight setup guidance for MSFS.