Final answer:
The 'FD NAV DATA INVLD' message warns pilots of invalid or untrustworthy navigation data in the flight director system, requiring them to verify data integrity or use alternative methods.
Step-by-step explanation:
The message FD NAV DATA INVLD on the Flight Mode Annunciator (FMA) typically indicates that there is an issue with the flight director's navigation data which is deemed invalid or untrustworthy. This could be due to a variety of reasons such as a discrepancy between onboard databases, corrupted data, sensor malfunctions, or a misalignment in the navigation systems. Pilots encountering this message would need to verify the integrity of their navigation data, cross-check against other sources, and potentially revert to alternative navigation methods if necessary to maintain safe operation of the aircraft.