The system will revert to Dead Reckoning (DR) Mode if the system is no longer fully using any GPS sensor or DME / DME for position fixing, even if airspeed and heading data are unavailable. In DR Mode, the system will use its last-known position combined with continuously updated airspeed and heading data (when available) to calculate and display the aircraft’s current estimated position. It is important to note that estimated navigation data supplied by the system in DR mode will become increasingly unreliable and must not be used as a sole means of navigation.

DR mode is inherently less accurate due to the lack of measurements otherwise provided by GPS or DME/ DME. Changes in wind speed and/or wind direction compound the relative inaccuracy of DR mode. Because of this degraded accuracy, other navigation equipment must be relied upon for position awareness until GPS or DME/DME position data is restored.

DR mode will be indicated on the system by the appearance of the letters “DR” displayed in amber on the HSI. The CDI deviation bar will be displayed in amber. The autopilot can be coupled in DR mode, as long as the lateral deviation guidance is available.

As a result of operating in DR mode, all data that is dependent upon FMS position data is displayed as amber text to denote degraded navigation information. Airspace alerts continue to function, but with degraded accuracy. Also, while the system is in DR mode, SVT and forward looking terrain alerting functions are disabled.