Learning hub · General Navigation · Lesson 3

Time, speed, distance and magnetic variation

Almost all of VFR navigation is two skills: knowing how long a leg will take, and knowing which direction to steer and how to check it. This lesson covers the time, speed and distance sums, a quick way to correct a track error, the step from true to compass directions, and the time zone trap.

About 12 minutes, plus the quiz. Part of General Navigation. It builds on charts, latitude, longitude and distance and on the wind triangle.

1. Time, speed and distance

Speed is distance divided by time, so the three quantities are tied together:

distance = groundspeed × time    time = distance ÷ groundspeed

With the speed in knots and the distance in nautical miles, the time comes out in hours. For a flight you want minutes, so multiply by 60. Use groundspeed, which is your true airspeed corrected for the wind (see the wind triangle). Using TAS will make every leg look faster than it is when you have a headwind.

Two shortcuts help in the air. First, groundspeed divided by 60 is the distance you cover each minute: at 90 kt that is 1.5 NM per minute, and at 120 kt it is 2 NM per minute. Second, 60 minutes at 105 kt is 105 NM, so 20 minutes is a third of that, or 35 NM.

Worked example (invented numbers): a leg of 66 NM with a groundspeed of 88 kt. Time = 66 ÷ 88 = 0.75 h, and 0.75 × 60 = 45 minutes. If you take off at 0920 UTC and pass the start of the leg at 0925, you expect to be at the end at 1010.

2. The 1 in 60 rule

This is a rule of thumb. If you are one degree off course and you fly 60 NM, you end up about 1 NM off to the side. The same ratio holds at other distances, so you can work in proportion. It is accurate enough for small angles, up to roughly 20°, and it is for quick checks, not for precise plotting.

angle ≈ (distance off track ÷ distance flown) × 60

Two angles solve a track error. The track error angle is the angle between the track you have made and the track you wanted, from the distance already flown. The closing angle is the extra angle you need to get back to the line by the destination, from the distance still to go.

Worked example (invented): after 40 NM you find you are 3 NM left of your intended track, and 80 NM remain.

1. Track error angle = 3 ÷ 40 × 60 = 4.5° (you have drifted left, so you have been making a track 4.5° to the left of the plan).
2. Closing angle = 3 ÷ 80 × 60 = 2.25°.
3. Turn right by 4.5 + 2.25 ≈ 7°. The first part makes you parallel to the plan, and the second brings you back to it at the destination.

3. True, magnetic and compass

The tracks you measure on the chart are measured from true north. The compass points to magnetic north. The angle between them is the variation, and it depends on where you are. It is printed on the chart, often as lines of equal variation called isogonals, and it changes slowly over the years, so use a current chart. Variation is east when magnetic north lies to the east of true north, and west when it lies to the west.

The aeroplane has its own small magnetic fields from the radio, the electrics and the metal. They push the compass off by a small angle called deviation. It depends on the heading and is shown on a compass card near the compass.

The chain, from chart to cockpit, is True, Variation, Magnetic, Deviation, Compass. A silly sentence helps, such as Tall Vikings Make Delicious Cakes. Invent your own. Working left to right:

• True to magnetic: east is least, west is best. Subtract easterly variation, add westerly variation.
• Magnetic to compass: apply the deviation from the card.
• Compass to true: go backwards and reverse every sign.

Worked example (invented values): true track 120°, wind correction 6° right, so true heading 126°. Variation is 6°E: magnetic heading = 126 − 6 = 120°. The card says to steer 122° on the compass for a magnetic heading of 120°, so you fly 122° compass. Always read the real variation from your chart; 6°E is only an example.

One more trap: the wind in a METAR or TAF is a true direction, while runway numbers and the wind given by ATIS or the tower are magnetic.

4. UTC and local time

Aviation runs on UTC (Coordinated Universal Time), written with four digits, for example 0920 or 0920Z. Weather reports, NOTAMs and flight plan times are all in UTC. The clock on your wall is not. Romania uses UTC+2 in winter and UTC+3 in summer, so to change local summer time to UTC you subtract 3 hours. For example, 14:30 local summer time is 11:30 UTC.

Sunrise and sunset times are listed in the AIP, an almanac or your flight planning app, and may be shown in UTC or local time, so check which. The rules for what counts as day and night, and for flying at night, are written into regulations and national rules, so look them up and do not guess from a sunset time. A good plan puts you on the ground well before the light fades, with a margin for delay.

All of these are planning aids. In flight, check your position against landmarks, and for real flights use official data and the aeroplane’s flight manual and your school’s procedures.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. A leg is 72 NM long and you expect a groundspeed of 96 kt. How long does it take? If you pass the start of the leg at 0915 UTC, when do you expect to arrive at the end?
   Show the answer

   1. Time = 72 ÷ 96 = 0.75 hours.
   2. 0.75 × 60 = 45 minutes.
   3. 0915 + 45 minutes = 1000 UTC.
2. After 30 NM you are 2 NM right of track, and 60 NM remain. By how many degrees and in which direction should you change heading?
   Show the answer

   1. Track error angle = 2 ÷ 30 × 60 = 4°.
   2. Closing angle = 2 ÷ 60 × 60 = 2°.
   3. Total = 4 + 2 = 6°.
   4. You are to the right of track, so turn left by 6°.
3. (a) True track 255°, wind correction 8° left, variation 7°W, deviation card says to steer 2° less than the magnetic heading. What heading do you steer on the compass? (b) You plan to depart at 16:10 local summer time (UTC+3). What is that in UTC?
   Show the answer

   1. Wind correction left means subtract: true heading = 255 − 8 = 247°.
   2. Westerly variation is added: magnetic heading = 247 + 7 = 254°.
   3. Apply the card: 254 − 2 = 252° compass.

   (b) 16:10 − 3 hours = 13:10 UTC.

6. Watch

This video from Aviation Theory goes through true and magnetic courses and headings, which is the third part of this lesson.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

The same channel explains variation and deviation as compass errors.

Open on YouTube if the player does not load. Not made by VectorEFB.

A short video on the 1 in 60 rule from a different channel.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: filling in an ICAO flight plan. Back to the learning hub.