When receiving an ILS signal, the CDI needle indicates a maximum of how many degrees either side of the center-line?

The correct choice indicates that the Course Deviation Indicator (CDI) needle for an Instrument Landing System (ILS) signal can move a maximum of 2.5 degrees either side of the center line. This specification is important for pilots as it helps them understand the precision with which they are approaching the runway during an ILS approach.

A 2.5-degree deviation means that if the needle is centered, the aircraft is on the correct approach path. If it indicates a deviation to the left or right, it emphasizes the need for corrective action to maintain alignment with the runway center line. Being aware of this range is crucial for ensuring a stable and safe approach, particularly in low visibility conditions where ILS is typically used.

Understanding the limits of the CDI also emphasizes the need for pilots to be proficient in interpreting instrument signals and maintaining situational awareness during an ILS approach.