GPS PRIMARY LOST on the A320 means GPS is no longer the FMGS's trusted source. Learn what remains available, how to check accuracy and when to go around.
On the Airbus A320, GPS PRIMARY LOST means the FMGS can no longer use GPS as its primary position source because the data is unavailable or fails integrity checks. The aircraft retains inertial and, when available, radio-updated navigation, but crews must verify accuracy before relying on managed guidance.
For our Aviation & Real-World Flying readers, the key distinction is between losing the preferred GPS update and losing the entire navigation solution. Exact indications and permitted responses vary with A320 standard, equipment fit and operator procedure; simulator behaviour also depends on how deeply the aircraft add-on models the FMGS.
What does GPS PRIMARY LOST actually mean?
It is a loss of the preferred GPS position update, not a declaration that every navigation system has failed.
The Flight Management and Guidance System normally maintains an FM position using inertial data and the best accepted update source. Depending on the aircraft standard and available aids, its navigation modes can include GPS/IRS, DME/DME-IRS, VOR/DME-IRS and IRS-only navigation. Radio updating also depends on aid coverage, tuning and equipment fit.
The autopilot does not necessarily disconnect, and NAV mode may continue following the route. Inertial or radio-updated position can remain accurate for some time, so the message must be considered alongside the navigation-accuracy indication on the MCDU PROG page.
A HIGH indication generally means the FMGS estimates that position accuracy satisfies the active required navigation performance. LOW means it does not. Confirm that the required value itself is appropriate: a manually entered or incorrectly modelled RNP value can make the HIGH/LOW comparison misleading.
| Indication | What it tells you |
|---|---|
GPS PRIMARY LOST | GPS no longer meets the criteria for use as the primary position source; a fallback source may still be accurate. |
NAV ACCUR DOWNGRAD or LOW accuracy | The estimated FM position no longer satisfies the required accuracy. This directly limits reliance on managed navigation. |
FM/GPS POS DISAGREE | The GPS and FM position estimates differ significantly and must be cross-checked against independent information. |
| GPS receiver fault | A receiver, power supply or data path has failed. Depending on the installation, one remaining valid receiver may preserve GPS primary. |
Depending on the A320 standard, the message may appear on the navigation displays and/or through the MCDU rather than as a conventional ECAM system warning. Our guide to reading the A320's PFD, ND and ECAM indications explains where these displays fit into the crew's cross-check.
Why does GPS PRIMARY LOST appear?
The message appears whenever GPS data stops satisfying the FMGS's availability and integrity tests.
- Loss of usable satellite data: poor reception, satellite geometry, masking on the ground or an integrity failure can make GPS unacceptable even when a receiver is still powered.
- Radio-frequency interference: GPS jamming commonly causes loss of reception. Spoofing is more difficult because it can create a plausible but false position and may not produce a clean GPS-loss warning.
- Equipment or electrical failure: receiver, antenna, power or data-bus faults can remove one or both GPS inputs.
- Temporary data rejection: the FMGS can reject GPS when it detects excessive disagreement between available position sources.
Why does it happen in a flight simulator?
In a simulator, configuration and state-management faults are more common than a realistically modelled satellite outage.
- An injected failure: a GPS, receiver or electrical failure may be active in the simulator or aircraft's own failure system.
- Incomplete ADIRS set-up: loading an unusual panel state, leaving an ADIRS selector out of NAV or failing to complete a modelled alignment can prevent the add-on from building a valid navigation solution.
- Slew or instant repositioning: teleporting the aircraft, loading some saved flights, changing location externally or aggressive time acceleration can leave the add-on's inertial and GPS positions out of agreement.
- Add-on conflicts or stale state data: incompatible avionics modifications and badly restored aircraft states can interrupt the simulated GPS data path.
- Limited system modelling: simpler A320 simulations may display the warning without reproducing the real aircraft's source selection and recovery logic accurately.
A malformed flight plan, route discontinuity or old navigation database does not by itself cause a genuine GPS PRIMARY LOST condition. Those faults can corrupt waypoints or procedures, but re-entering the route will not restore a missing GPS source. This is a common misdiagnosis when both problems appear after importing or changing an approach.
What should you do after GPS PRIMARY LOST?
The correct response is to preserve a verified flight path, assess actual navigation accuracy and then apply the limits for the procedure being flown.
- Fly and stabilise the aircraft. If managed NAV is wandering or cannot be verified, use a safe selected heading and altitude. Our explanation of how to select and monitor heading modes on the FCU covers the relevant controls.
- Check the complete navigation status. Review the PROG page's required and estimated accuracy, GPS-monitor information and position-monitor pages available on that aircraft. Determine whether one receiver, both receivers or only GPS integrity has been lost.
- Cross-check the position independently. Compare the displayed route with identified VOR/DME information, localiser indications, other inertial positions and visual references where appropriate. Simmers who need a refresher can use our practical guide to using conventional radio-navigation aids and raw data.
- Match the response to the operation. Check whether the active SID, STAR or approach requires GNSS, and whether the remaining sensors satisfy its navigation specification. Real crews must follow the aircraft procedure, operator SOP and ATC instructions.
- Avoid destructive resets. Do not cycle the ADIRS selectors or enter an arbitrary position correction in flight merely to clear the message. That can turn a GPS degradation into a wider loss of inertial navigation.
- Change the plan when necessary. Advise ATC, use heading guidance, select a conventional approach or go around if the required navigation capability cannot be confirmed.
Can you continue an approach after GPS PRIMARY LOST?
Sometimes, but the decision is based on required navigation performance and independent guidance, not the continued presence of a magenta line.
| Operation | Normal decision basis |
|---|---|
| En-route or terminal RNAV | Continuation may be possible when the remaining sensors are authorised for the navigation specification, accuracy remains HIGH and position is independently verified. |
| RNAV SID or STAR requiring GNSS | Do not rely on the procedure after the required GNSS capability is lost. Obtain heading guidance or another clearance. |
| RNP, RNAV(GNSS) or GPS approach | Do not commence or continue when the procedure requires GNSS integrity and that capability has been lost. Go around if the loss occurs after committing to the approach, unless the approved aircraft and operator procedures explicitly provide another response. |
| ILS or localiser approach | Normally remains available if the aid is identified and valid because localiser and glideslope guidance are independent of GPS. Monitor the raw radio indications rather than trusting only the FMS path. |
| VOR/DME or NDB approach | May remain available where the aircraft is equipped and the aid is serviceable. Fly and monitor the published raw-data procedure. |
| Visual approach | May continue when visual conditions, clearance and position awareness are adequate. |
The term RNAV alone does not always mean GPS-only; some RNAV operations permit DME/DME and inertial updating. The chart, required navigation specification and approved aircraft capability decide the issue. Our explanation of how GPS/RNAV approach guidance is built and monitored in a simulator gives the practical context.
What if the aircraft still follows the route?
A route that still tracks normally only shows that the FMGS has a usable position of some kind. It may be operating on radio-updated inertial data, or on inertial position alone while the error grows gradually.
The displayed route line is database geometry, so seeing it does not prove that the aeroplane is in the correct place. If accuracy remains HIGH and the remaining sensors satisfy the operation, managed navigation may still be usable. If accuracy is LOW, the track becomes erratic or the position cannot be verified, use selected guidance and abandon any procedure that depends on the suspect position.
Known or suspected GPS spoofing needs particular caution: a plausible track and a HIGH estimated-accuracy indication do not prove that the GPS-derived position is correct. Independent radio, inertial, visual and ATC cross-checks become decisive.
How do you fix GPS PRIMARY LOST in a simulator?
There is no universal reset: isolate whether the simulated loss comes from an injected failure, incomplete aircraft set-up or a broken session.
- Inspect the failure controls. Check both the simulator and the aircraft add-on for an active GPS, receiver, antenna or electrical failure. Clear it only if it was not part of the intended scenario.
- Correct the ground set-up. With the aircraft on the ground, confirm normal electrical power, all required ADIRS selectors in NAV and completion of the modelled alignment. Enter or confirm the reference position if that add-on requires it.
- Allow normal reacquisition. After a brief interruption, maintain a stable aircraft state and give the model time to accept valid GPS data again. Avoid repeated switch cycling.
- Recover from repositioning problems. If the warning followed slew, teleporting or a saved-flight load, restart from a clean gate state when practical. Some complex aircraft cannot restore every inertial and GPS variable reliably from a saved session.
- Test without conflicting modifications. Temporarily remove recently added aircraft or avionics modifications and repeat the same start. Use mutually compatible simulator and aircraft builds rather than mixing packages intended for different releases.
- Treat route faults separately. Repair discontinuities, missing procedures or incorrect RNP entries only after the position source is healthy. Updating or rebuilding the flight plan does not repair GPS reception.
Will GPS PRIMARY LOST clear itself?
It can clear without intervention when valid GPS data and integrity return. The FMGS can then reaccept GPS as primary, although crews should confirm the position and accuracy rather than assuming that disappearance of the message proves every earlier discrepancy has vanished.
A persistent message after a clean ground start usually points to an active failure, incomplete system configuration or an add-on problem. No MCDU key provides a universal cure, and resetting the whole navigation system in flight is the wrong response.