The POSITION UNCERTAIN alert is an action‑required message that appears when the radio signals received by the system typically from VOR/DME or VLF/OMEGA stations don't match the position computed by the Flight Management Computer, indicating an inconsistency that requires the pilot to manually update the aircraft's position in the system.

The FMS/VLF POS DISAGREE alert is a more specific message related to very low‑frequency radio navigation. It illuminates when the difference between the FMS‑calculated position and the position determined by the VLF/OMEGA sensor exceeds a threshold of 2.3 nautical miles. It remains active until that discrepancy is reduced below that value, signaling that the long‑range radio system is providing a fix that doesn't converge with the rest of the navigation solution.

Finally, the FMS IN DR MODE message (as I explained above) is a status message indicating the worst‑case scenario for navigation. It means the FMS is not receiving adequate radio signals from any source whether VOR/DME or VLF/OMEGA forcing the system into Dead Reckoning mode, where it relies solely on the last known position, combined with magnetic heading and true airspeed, to project the current location. This degrades accuracy and causes the computed wind vector to freeze on the display until radio signals are restored and the system automatically reverts to primary mode.

The first two messages are alerts that call for corrective action by the pilot (updating the position in the system), while the third is a confirmation that the system has lost all external references and is operating only on internal dead‑reckoning data.

Hi!

If you're flying using the FMS rather than the GNS430, you might run into Dead Reckoning (DR) mode which is a degraded state of the FMS navigation system that occurs when the multi‑sensor navigation unit loses enough valid sensor inputs to accurately compute position. When the FMS is in DR, the aircraft's position is calculated by dead reckoning, based on heading and true airspeed (TAS) data, along with the last known position, without continuous updates from external sources like VOR, DME, or VLF/Omega.

During DR operations, the system should be used with caution. You'll need to cross‑check and update the system's position visually by referencing a ground point or using other installed navigation equipment such as VOR and DME as soon as possible. This condition is indicated on the navigation display (ND) by the yellow 'DR' message, which appears whenever the FMS is in this mode, alerting the pilot to the reduced accuracy.

For IFR operations using the FMS, the minimum equipment typically includes, for example, 1 VOR, 1 DME, and valid heading and TAS inputs or 3 VLF/Omega stations along with the same heading and TAS data. The loss of these sensors is the most likely cause for DR mode to activate, which degrades navigation to the point where you can't fully rely on the indicated position without frequent cross‑checks.

This is simulated in the BS Starship. Hope that helps!

Alan / From Brazil.