In an ILS approach, which needle indicates whether the aircraft is right or left of the centerline?

In an Instrument Landing System (ILS) approach, the needle that indicates whether the aircraft is to the right or left of the centerline is the localizer needle. The localizer system provides lateral guidance to the pilot, helping them align with the runway during the final approach. It transmits a signal that defines the centerline of the runway, allowing the aircraft to determine its position relative to that centerline.

When the aircraft is off-center, the localizer needle will deflect to either the left or the right, informing the pilot how to adjust the aircraft's heading to stay on the correct approach path. This precise lateral guidance is critical for a successful and safe landing, particularly in low visibility conditions.

The other needles mentioned, such as the glide slope needle, provide information regarding vertical positioning, while the altitude needle relates to the aircraft's current altitude, and the vertical speed needle indicates the rate of climb or descent. However, it is the localizer needle that focuses specifically on lateral alignment with the runway during an ILS approach.