The issue is specific to the large Boeing airframes, 747 and 777. These planes use the radio altimeter to monitor all landings, not just CAT 1 low vis landings. The large planes have to be kept within a narrow range on flare to avoid tail strikes. Interference could cause spurious cockpit alerts or implement flight control corrections during this critical phase of flight and was considered and unacceptable risk. If Boeing can demonstrate it has a fix or some other procedure to mitigate the risk the FAA will lift the AD.
“These planes use the radio altimeter to monitor all landings, not just CAT 1 low vis landings.”
Can the planes be landed without the electronic assists? Are pilot trained on how to fly the planes without these assists?