The FAA has locked in a two-track retrofit mandate for radio altimeters, forcing operators to fit interference-tolerant systems before expanded 5G Upper C-band use reaches the fleet. Part 121 airlines, and some Part 129 aircraft, face a 30 December 2030 deadline; most other affected civil and public aircraft have until 31 October 2034, turning altimeter compliance into a long-cycle procurement and STC programme across transport, business aviation and general aviation. The rule covers operations in the contiguous United States and the District of Columbia, replacing temporary restrictions with a permanent performance standard. FAA analysis puts the retrofit burden at roughly 58,500 units on 40,700 aircraft, with costs running into billions and rebates available to qualifying domestic operators.