Final answer:
The sub mode of IAS is ILS - Instrument Landing System, which is used to guide aircraft for safe landings during low visibility conditions.
Step-by-step explanation:
The sub mode of IAS, or Instrument Approach System, is ILS - Instrument Landing System. ILS is a navigational aid system used by pilots to guide their aircraft to safely land at an airport during low visibility conditions, such as fog or heavy rain. It provides precise alignment and descent guidance to the runway through radio signals, enabling pilots to make a safe approach and landing.