In ILS mode, what does the CDI left/right needle indicate?

In ILS (Instrument Landing System) mode, the Course Deviation Indicator (CDI) is a critical instrument that guides a pilot during the approach phase of flight. The left/right needle of the CDI specifically indicates the aircraft's lateral position in relation to the localizer beam.

When the aircraft is aligned with the localizer, the needle centers. If the needle deflects to the left, it signals that the aircraft is to the left of the localizer course, and if it deflects to the right, it indicates that the aircraft is to the right. Thus, the CDI functions as a visual reference to help the pilot maneuver the aircraft back towards the correct centerline of the runway approach, ensuring a safe and precise landing.

The other options do not relate directly to the function of the CDI in ILS mode. Airspeed, groundspeed, and altitude are measured by different instruments and do not provide lateral positioning information regarding the localizer. Therefore, understanding the role of the CDI in localizer alignment is essential for effective navigation during instrument approaches.