For an aircraft with two FMCs accessed via two EADIs and two EHSIs, which action should pilots avoid?

• A. Simultaneously enter data into the FMCs
• B. Enter data sequentially
• C. Turn off one FMC during use
• D. Input data through CDU only

Answer: (A)

Entering data into both flight management computers at the same time is something you should avoid because it can create conflicting navigation data between the two systems. With dual FMCs, each computer can hold its own route and waypoint edits, and the active navigation you rely on for the autopilot and flight director is expected to be consistent. If you edit both simultaneously, you risk one FMC loading a different leg or waypoint than the other, which can lead to mismatched guidance on the EADI/EHSI and confusion about which route the aircraft is actually following. Keeping data entry to one FMC at a time (and then, if needed, duplicating the data or rechecking for consistency) ensures a single, coherent flight plan for the flight management system to use.

Sequential entry is appropriate because it provides a controlled update, reduces the chance of data conflicts, and preserves navigation integrity. Keeping both FMCs on is preferred for redundancy, so turning one off during use would unnecessarily degrade that redundancy. CDU input is the normal interface for loading data into the FMCs, so using the CDU is not something to avoid.