Any system that relies on a radio altimeter to function will be impacted. Both the equipment and the pilot’s operating it must be certified to land in low visibility conditions. We are essentially taking the certification of the equipment away. For example, in the Q400 our HGS (heads up guidance system) will be impacted, this is required for any Category 3 approach. The autonomous flare cue may indicate improper flare symbology or no symbology at all.
Other systems in the Q400 that are affected that rely on radio altimeter are configuration systems (warnings for improper gear or flap settings), our TCAS (Traffic Collision Avoidance System) system could give us a “descend” command while at low altitude whereas before that would be inhibited, and most worrisome is the stall protection equipment, which includes the stick pusher, could be triggered at low altitude when normally inhibited by the radio altimeter. I have spoken with other pilots and on the Embraer E175, the auto land system will be unavailable in addition to similar TCAS warnings, configuration warnings, and autothrottle issues. Although I can’t comment for certain, I have heard that Airbus aircraft rely on the radio altimeter for many functions and will be impacted more than Boeing.
Uncertainty is the one word that describes it the best. Those of us not scheduled to fly on the 18th are glad because we honestly don’t know what’s going to happen from a flight deck point of view. We understand the operational aspects (i.e. canceled flights and diversions) but in regard to how the plane will react, we have only been provided cursory guidance.