What must be done when the red ALT flag is displayed in flight?

When the red ALT flag is displayed in flight, it indicates a significant discrepancy in altitude data, often resulting from failure or inconsistency between the aircraft's electronic systems. The appropriate action in this scenario is to switch the Primary Flight Display (PFD) that shows the largest discrepancy from Interconnected Electronic Standby System (IESS) to the ADS 3 data. This step is taken to ensure that the flight crew is assessing the most reliable altitude information at that moment, thereby enhancing situational awareness and maintaining safety.

When faced with conflicting data on altitude, it's imperative for pilots to utilize the most accurate readings available to them. Thus, by addressing the issue in a systematic manner—switching to the alternate source of altitude data—pilots can make informed decisions based on the most reliable information to manage the aircraft's altitude safely.