@Zajcek100 To keep this from getting too long, I've picked out a few relevant FMS or EICAS messages that may appear regarding degraded navigation modes.
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.
