Day-Night Sequence
Night time must be recorded in the pilot's logbook. EASA FCL.050 states that each logbook must include the operational conditions time, of which night time is one. However, this can sometimes be difficult because during a flight, there may be several periods when the flight changes from day to night and vice versa, so that there are several periods of night. But how exactly is night defined, and what are some of the possible factors that can affect night time?
Definition of night time
EASA defines night in FCL.010 as follows:
"Night" means the period between the end of evening civil twilight and the beginning of morning civil twilight or such other period between sunset and sunrise as may be prescribed by the appropriate authority.
The FAA has a very similar definition of night in its Airplane Flying Handbook as the period between the end of evening civil twilight and the beginning of morning civil twilight.
This means that, contrary to what some people think, night is not simply defined as the period between sunset and sunrise. What this civil twilight actually refers to is discussed in the next section.
It is important to note that day and night may alternate during a flight, and that several sequences of day and night flight time may be part of the same flight. The night time to be recorded is the sum of all these sequences of night flight time. These sequences occur, for example, when flying from very high (polar) latitudes to lower latitudes.
Definition of civil twilight
The National Weather Service of the United States of America defines twilight as the time before sunrise and after sunset when the sky is partially illuminated by the sun, neither completely dark nor fully lit, and distinguishes between three types of twilight: civil, nautical and astronomical. Morning civil twilight begins when the geometric center of the sun is six degrees below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun is six degrees below the horizon. Both typically last about 30 minutes, but there are seasonal variations.
The legitimate question now is which horizon to use, as this can vary considerably depending on position and altitude of the aircraft. In practice, for the sake of simplicity, a specific position and altitude are used to determine this twilight. In Switzerland, for example, Skyguide provides a list of all civil morning and civil evening twilight times in the VFR manual, which uses the Berne observatory as reference. This practice is useful for VFR flights within Switzerland. For longer flights, especially long-haul flights over the oceans, other simplifications or assumptions have to be made. One possible approach with good practical results is to treat the flight path as a great circle route and then to use a certain number of coordinates from this route to determine the corresponding position of the sun in relation to the horizon at a given time.
What is a great circle route?
A great circle route is the shortest path between two points on a sphere such as the Earth, often resulting in an arc-like path rather than a straight (rhumb) line on most commonly used chart projections.
Polar twilight and polar night
As a significant number of flights, especially intercontinental ones between Europe and the US cross polar regions (see illustration below), the phenomenons of polar twilight and polar night are also a factor for night time calculation.
Polar twilight only happens in regions that are in the inner borders of the polar circle and are related to the winter solstice when the sun is below the horizon all day. During this time there is no true daylight and thus no true sunset and sunrise but only (polar) civil twilight. This phenomenon can be observed at latitudes between 67°24‘ and 72°34‘ North or South.
During a (civil) polar night, only a faint glow of light can be observed at noon. This phenomenon occurs when there is no civil twilight but only nautical twilight (geometric center of sun between 12° and 6° below the horizon). As a consequence, this is limited to latitudes above 72°34‘ (exactly 6° inside the polar circle).
Both phenomena add another level of complexity to calculate day and night sequences on routes which cross the polar circle at any time as both can either be very long or very short.
Day-night sequence changes
We have already mentioned several factors that can have a major influence on the day-night sequence. But other factors, such as seasons, also play an important role. In February in Europe, for example, flights with three or more changes between day and night can be observed when flying from the far north, such as Finnish Lapland (above the Polar Circle), to more southerly regions, such as Switzerland. There was one flight where this actually happened. It took off from EFKT at 14:33 UTC on 29 January 2022 and landed at LSZH at 17:58 UTC. It was still day when the plane took off. At 14:36 UTC it became night at its current position, at 16:03 UTC it became day again, and finally at 17:24 UTC it became night again. This means that a total of three changes were recorded on this single flight, despite it following a mostly north-south line.
A combination of other factors can add to the complexity of determining day and night changes, such as the speed of the aircraft. For example, on the route from the above example, the faster the aircraft travels, the more changes are possible.
Whenever a large number of longitudes are crossed, i.e. either with or against the twilight zone, changes in the day and night sequences are to be expected. But it is not only when flying at high altitudes that confusion can arise in relation to the position of the horizon, as mentioned in the definition of civil twilight above. For example, during taxiing (within the block time) there may be a change from day to night or vice versa. As block time is the flight time to be recorded according to EASA FCL.050, such changes must be taken into account.
Did you know?
If you wish to record a night landing, it must take place within the night part of the flight in accordance with EASA FCL.010 or a similar requirement of your competent authority. The same applies to take-offs and touch-and-goes.