How can you identify a potential data block discrepancy in an FMS during flight?

• A. Reboot the FMS and continue
• B. Ignore small deviations and proceed
• C. Look for missing waypoints, incorrect routing, speed or altitude constraints, or unexpected deviations; verify with raw data
• D. Use only GPS data and skip the FMS

Answer: (C)

The main idea is spotting when the FMS plan doesn’t match the actual navigation data by checking for discrepancies inside the data block that feeds the flight plan. The FMS relies on a validated database of waypoints, routes, and constraints, so if any piece of that data is missing, wrong, or out of sequence, the aircraft can end up following an incorrect path or violating speed or altitude limits. The best way to confirm a potential discrepancy is to look for missing or misnamed waypoints, an incorrect route, speed or altitude constraints that don’t align with the clearance or the filed plan, or any unexpected deviations in the aircraft’s actual track. Then verify these findings against raw data from the navigation system—the raw FMS data, GPS positions, and leg-by-leg entries—to determine where the mismatch lies and take corrective action.

Rebooting the FMS and continuing can mask a data integrity problem and may lead to persisting wrong guidance. Ignoring deviations is unsafe because even small inconsistencies can accumulate into significant navigation errors. Relying solely on GPS and skipping the FMS removes a structured, validated navigation source and can cause the route to diverge from the intended plan.