Operate the Approach Light System (ALS) between sunrise and sunset when the prevailing visibility is 5 miles or less, or when the ceiling is less than ______ feet.

• A. 4,000
• B. 3,000
• C. 2,000
• D. 1,000

Answer: (D)

The ALS is used in daylight to provide a clear visual path for pilots when weather reduces the amount of natural visual cues. If the visibility is 5 miles or less, or if the ceiling drops below a certain level, turning on the ALS helps pilots identify and maintain the correct approach path to the runway.

That ceiling threshold is 1,000 feet. Below this height, the visual environment is sufficiently limited that extra lighting on the approach becomes important to help the pilot establish and maintain proper descent and alignment. Higher ceilings mean pilots usually have enough daylight and horizon references that the ALS isn’t required for the approach.

So, the correct threshold is 1,000 feet.