FMS DR RNAV

When an aircraft enters dead reckoning mode then external sources of data, such as VORVOR —VHF Omnidirectional Range and DMEDME —Distance Measuring Equipment, are lost. It therefore has to rely on internal information or last computer information. Airspeed with still be calculated by the Air Data computer using the pitot tubes, and Magnetic Heading can still be taken directly from the compass. However, as the aircraft had lost reliable external navigation, it cannot tell exactly where it is and so a comparison cannot be made between air position and actual position to derive wind. Consequently, the last known wind is used.