Definition
A series of high-intensity white lights installed along the approach lighting system of a runway that flash in rapid sequence from the outermost light toward the runway threshold, producing a visible 'ball of light' that appears to travel toward the runway. SFLs help pilots identify the approach path and the direction of the runway during night operations and low-visibility approaches.
Plain English
Bright white lights along the approach to a runway that flash one after another, very quickly, so it looks like a ball of light is racing toward the runway. The motion shows the pilot which way the runway is and helps line up for landing.
Context Anchor
A pilot may encounter SFL in airport lighting information, runway approach descriptions, or visually during a night or low-visibility approach.
Derivation
Sequence' comes from the Latin sequi, meaning 'to follow.' The lights are named for the way each one follows the previous one in rapid order, creating the apparent motion toward the runway.
Why Pilots Care
Provides positive visual identification of the runway and helps maintain alignment during the final stages of landing in marginal weather.
Intuition Check
Do not read “sequence flashing lights” as just several lights blinking at random. The key idea is that they flash in order, leading the pilot’s attention toward the runway.
Example Sentence 1
As we broke out of the cloud at minimums, the sequence flashing lights pointed straight at the threshold and made the runway easy to identify.
Example Sentence 2
With the SFL guiding the way, the approach remained stable until the runway edge lights came into view.