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Flight Planning

Aviation & Time, I

Time. Usually taken for granted but it is of the utmost essence in aviation. For time and speed together relates to distance and we can calculate how long a flight will take and estimate our time of arrival and it can also provide us with our fuel consumption during the flight.

Do not forget that someone will start looking for us if we do not arrive on time, when we have filed a flight plan.

Precise time is measured with sophisticated atomic clocks with an accuracy of a couple of seconds in a million years or so. Which is just about good enough for us pilots.

Aviation regulations also require pilots to have a accurate time indicator, either as wrist watch or as a clock in the aircraft.

Time is the main ingredient used in navigation in, for example GPS devices. Which relies on exact time to calculate precise speed and position. Some countries only allow VFR during daylight so the pilot must know the exact moment of the end of daylight to make sure he lands before that. Flight plans also use UTC for time registration.

Determining Time

To measure time we need a certain accurate reference, the sun is a good candidate as it crosses our skies pretty much every day. This way we can divide the day into minutes and hours and the days will rollover into weeks, months and seasons until years have gone by.

Earths rotation

Rotation of the Earth

In 365 days, the Earth rotates around the sun and this period is called a year. At the same time the planet rotates around its own axis in a certain time period, which we called a day. The daily rotation is to the east and this makes the sun rise in the east and setting in the west.

Leap year

The orbit of the Earth around the sun is not exactly 365 days, it is 5 hours, 48 minutes and 45 seconds longer and slowing down. A correction is made every four years to enter a leap year of 366 days (usually at the end of February (2016)). With a final correction of removing 3 days every 400 years, the next one is coming up in 2100.


The axis of the Earth is tilted about 23°27'. Having the effect of seasons when the Earth moves around the sun. The tilt is also not quite stable, it wobbles a bit thus causing slight variations in the climate throughout the seasons. Even the distance from the Earth to the sun is not the same throughout the year. This creates more variations in the climate in those seasons.

Aphelion & Perihelion

The greatest Earth-Sun distance is during the beginning of July, and it is called Aphelion. The shortest distance is during the first days of January, called Perihelion. And this too is causing all kinds of seasonal variations to global weather/climate. But don't worry about it, this is quite normal.

Days and hours

As said before, the Earth rotates around its axis and it does that in 24 hours, that is 15° per hour. Each hour is divided into 60 minutes from 00 to 59 and hours are numbered from 00 to 23. Worldwide aviation uses a 24 hour clock and this is based on UTC, Coordinated Universal Time. In the old days this used to be called GMT, Greenwich Mean Time.

Time zones

The Earth is also divided into time zones and each zone is roughly 15° apart, equal to an hour. This way each zone has its own local time, although local mean time (see below) is different for each of the 15° meridians of longitude in that zone. Greenwich (UTC) is in the Z (Zulu) time zone, this notation is sometimes used in meteorological forecasts to indicate UTC.


At midday (noon), the sun is at its highest point in the sky and this will happen every day over each and every meridian, and again 24 hours later. Also, a day starts at 00 hours at the anti-meridian (opposite) of the sun position.

As you can see time is important to not only our personal flying but it is intertwined throughout our lives and we make good use of it during navigation. Our next article will discuss daylight and time definitions as they are used in aviation.

Written by EAI.

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