What does light characteristic 'Fl' indicate?

• A. Isophase light
• B. Flashing light
• C. Occulting light
• D. Fixed light

Answer: (B)

Light patterns describe how a navigation light changes over time to help you identify it. Fl indicates a flashing light: it emits a brief, bright flash, then stays dark for most of the cycle, and the flash repeats at a steady interval. The important point is that the light is not continuous; the flash is short compared to the interval between flashes. This makes the light stand out and lets you distinguish it from other patterns: isophase lights have equal light and dark periods; occulting lights are mostly on with short eclipses; fixed lights stay on continuously without dark periods. If you see Fl followed by a number (for example, Fl 5s), that means a flash every 5 seconds.