When is the FMS not to be used for navigation?

The FMS (Flight Management System) should not be used for navigation during periods of dead reckoning because dead reckoning involves estimating one's current position based on previously determined positions. This method relies on a combination of course, speed, time, and environmental factors, such as wind. The FMS typically utilizes inputs from various navigational aids including GPS, waypoints, and other navigational information to provide more precise navigation and guidance. In situations where you are relying solely on dead reckoning, the precision and accuracy offered by the FMS may not be utilized adequately, making manual navigation and assessment more appropriate in those circumstances.

In contrast, there are scenarios where the FMS can still play a supporting role even if weather conditions are poor, during emergency landings when other navigation aids may be unreliable, or when GPS is malfunctioning and alternative navigation strategies can be employed. Thus, the reliance on the FMS to navigate during dead reckoning is less effective than using it alongside more established methods.