Which of the following is NOT a component of the approach light system?

• A. PAPI
• B. Threshold lights
• C. Crossbar lights
• D. ILS

Answer: (D)

The approach light system provides visual cues along the approach path to help a pilot visually acquire and align with the runway, especially in low visibility. It includes lights arranged on the approach path, such as threshold lights and crossbar lights, and systems like PAPI that support visual slope information.

The instrument landing system, however, is an electronic navigation aid that guides the aircraft via radio signals for the localizer and glide slope. It does not rely on runway lights or approach lighting. Since the question asks which item is not a component of the approach light system, the instrument landing system is the correct choice because it serves instrument-based guidance rather than visual lighting cues along the approach.