Final answer:
When a blue G/S unexpectedly becomes armed during cruise, the flight crew should verify autopilot modes, check navigation systems, and possibly disengage and re-engage the auto flight system as per standard procedures to resolve the issue.
Step-by-step explanation:
When a blue G/S unexpectedly becomes armed on the Flight Mode Annunciator (FMA) at cruise, it indicates that the aircraft's auto flight system is preparing for a glide slope interception. This is typically not expected in cruise phase and suggests that there may be an issue with the automation or that an incorrect mode has been engaged.
Action Steps for Flight Crew
First, the flight crew should verify the current autopilot modes and settings to ensure that they are appropriate for the cruise phase of flight.
Next, they should cross-check the onboard navigation systems, including the Flight Management System (FMS) and the Instrument Landing System (ILS) frequency, if applicable, to ensure they do not inadvertently have an approach mode active or selected.
If the unintended armament of the G/S mode persists, the crew should follow standard operating procedures, which may include disengaging the auto flight system and re-engaging it after ensuring all settings are correct, or troubleshooting as per the Aircraft Flight Manual (AFM) or Quick Reference Handbook (QRH).
Communicating with Air Traffic Control (ATC) to confirm assigned altitudes and routes can also be part of the corrective action.
It's critical that the flight crew remains vigilant and ready to take manual control of the aircraft should the need arise, to maintain safe flight operations.