Invalid navigation data

A deviation display is useful only if the underlying signal is valid. FSS describes an X when a required lateral or vertical deviation is unavailable. The example above shows valid indications, not an invalid-data X.

Flight path

Use the aircraft’s attitude and selected guidance as appropriate while checking the indication.

Identify which source is invalid

Check the primary navigation source, preview receiver and the associated source label.

Check setup

For an ILS, verify frequency and course against the chart and check that the signal is available at the aircraft position.

Check the FMS

For FMS deviation, review the active flight plan, selected approach and the phase of flight.

Invalid-data indication

If the display is invalid or the aircraft is not following the intended path, discontinue the approach as appropriate and reassess. ^[200]