Civil Twilight Is 30 Minutes Under Part 107

Outside Alaska, Part 107 defines civil twilight as a fixed 30 minutes: the half hour before official sunrise and the half hour after official sunset. Weather and photography apps usually show a different civil twilight, based on the sun's angle, and its length changes with place and season. When they disagree, the 30-minute window is what Part 107 uses.

How does Part 107 define civil twilight?

14 CFR §107.29(c) gives three definitions:

• except in Alaska, the period that begins 30 minutes before official sunrise and ends at official sunrise;
• except in Alaska, the period that begins at official sunset and ends 30 minutes after official sunset;
• in Alaska, civil twilight as defined in the Air Almanac, which the Alaska post explains.

Why does your app show something different?

Because it is measuring something different. The U.S. Naval Observatory and NOAA define civil twilight by geometry: it ends in the evening, and begins in the morning, when the center of the sun is 6 degrees below the horizon. How long that takes depends on how steeply the sun crosses the horizon. It is shorter near the equator, longer toward the poles, and at high latitudes in summer the sun may never get 6 degrees below the horizon, so civil twilight can last all night.

The FAA chose a fixed number instead. Outside Alaska, the Part 107 window does not stretch in June or shrink in December.

What changes when the 30 minutes end?

After the 30-minute mark, the flight is a night flight under Part 107, whatever your app says about the sky. Two separate sets of requirements are involved:

In both periods the pilot may dim the light if conditions call for it, but may not turn it off.

A worked example

Here is an illustration, not real data for any city. Suppose official sunset is at 7:00 p.m. and your sky app says civil twilight ends at 7:38 p.m. Under Part 107, civil twilight ends at 7:30 p.m. From 7:30 onward you are flying at night. If your last training predates April 6, 2021, or you have never done the updated recurrent training, the extra eight minutes are not covered.

The reverse can happen in places where the sun sets steeply: your app may show civil twilight ending before 30 minutes, while the regulation's window still runs. The lighting rule applies for the full 30 minutes either way.

Where do you get official sunrise and sunset times?

The rule refers to official sunrise and sunset, and the Naval Observatory publishes those for any date and location. A sky or photography app is a reasonable way to see sunset, but check which twilight it is showing before you use its end time.

What this means for the exam

Questions on this topic tend to give you a sunset time and a flight time and ask what is required. Count 30 minutes from sunset. Inside that window, the question is about the anti-collision light. After it, the question is also about night training. The night operations post covers the night rules in more detail, and physiological effects covers what dusk does to your eyes.

The handbook covers civil twilight under UA.I.B.K24 and night operations under UA.I.B.K25.