Operate Sequenced Flashing Lights (SFL) when the visibility is less than ___ mile(s).

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (C)

Sequenced Flashing Lights are used to help pilots locate and align with the runway when visibility is poor. The rule is to operate SFL whenever the prevailing visibility is less than 3 miles. That 3-mile threshold is chosen because, below it, pilots benefit from an obvious on-ground cue to find the runway environment, reducing the risk of misidentification or misalignment in low visibility. If visibility is at or above 3 miles, pilots typically have enough sight to locate the runway without this aid, so the SFL remains off. Choosing a higher trigger, like 4 miles, would turn on the lights too early, while a lower trigger, like 1 or 2 miles, would activate them only at stricter conditions than required by standard procedures.