Training Tip: Aviation's twilight zone

Sure, it will be rather dark before 2202Z, but we’ll be in a regulatory twilight zone, flying in nocturnal conditions, with the airport looking the way airports look at night, but with the night-flight columns of our logbooks embargoed until the mandated moment.

When defining “night” for aviation purposes—for example, for keeping track of the night flying experience you need for private pilot training—night is “the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the Air Almanac, converted to local time.”

That’s a pretty busy definition. If you wonder why the word “civil’ is included in the regulation’s reference to twilight, it’s because civil twilight is one of three commonly noted phases of twilight.

During civil twilight, “the geometric center of the Sun's disk is at most 6 degrees below the horizon.” In “nautical twilight, the geometric center of the Sun's disk is between 6 and 12 degrees below the horizon.” “Astronomical twilight” is when the sun’s geometric center “is between 12 and 18 degrees below the horizon.”

If you haven’t seen too many pilots standing on a dusky ramp attempting to calculate the position of the sun’s disk lately, it may be because this is where the Air Almanac (and thankfully, the internet) takes over, providing data you’ll need to figure out your local civil twilight intervals (or find a webpage where the information is a click away).

If you left the iPad at home, you can eyeball the sky and guesstimate the end of evening civil twilight. It’s when “the brightest stars are just visible, and terrestrial objects can be easily distinguished,” assuming good atmospheric conditions and no light pollution.

Here’s another way to think of civil twilight’s endpoint: It’s when artificial lighting helps you continue outdoor activities.