Learning hub · General Navigation · Lesson 7

Diversions and the one-in-sixty rule

Sooner or later the plan stops working: the weather ahead closes, the fuel is less than you hoped, or someone in the cabin is unwell. A diversion is a navigation problem you have to solve quickly, with a pencil, a chart and your head. This lesson shows a method that fits on one page.

About 12 minutes, plus the quiz. Part of General Navigation.

1. Decide early

The best diversion is the one you start before you need it. Every minute you spend hoping the weather will improve is fuel and daylight you do not have on the diversion. Set yourself a decision point on the ground, for example “if the cloud base at the next town is below 2,500 ft, I turn there”, and keep to it.

Choose the alternate before you need it as well. A good one is close, has a runway into the wind, is outside controlled airspace you cannot enter, and is ahead of the weather rather than behind it. Your pre-flight planning should already have named one or two.

2. The one-in-sixty rule

The rule comes from a piece of geometry. At 60 nautical miles, one degree of angle is almost exactly one nautical mile sideways. So for a small angle:

angle in degrees ≈ distance across ÷ distance along × 60

“Across” is how far the alternate lies to one side of your present track. “Along” is how far it lies ahead, measured parallel to that track. Both are read from the chart with a ruler or the scale on your plotter, so they do not need to be exact.

An alternate 8 nm to the right and 40 nm ahead needs 8 ÷ 40 × 60 = 12° to the right of your present track. The exact answer is 11.3°. The rule is good to about 20°. Beyond that it over-reads, and at 45° it gives 60° where the true angle is 45°. For large angles measure with the plotter instead.

3. Distance across and distance along

Do not try to measure the distance to the alternate with a straight ruler and the angle with a protractor in the cockpit. Split it into two easy measurements instead:

• the distance along: draw a line on the chart from your position forward, parallel to your present track, until you are level with the alternate;
• the distance across: the gap between that point and the alternate.

The distance to fly is then close to the longer of the two numbers, a little more when they are similar. If the two are equal the angle is 45° and the distance is about 1.4 times either of them.

Try it: move the diversion point along the planned leg, then move the alternate and change the wind. The blue line is the plan. The dashed lines are the “along” and “across” distances. The coloured line is the diversion track, and the little aeroplane flies it with its nose on the heading that holds it. The bar at the bottom is your fuel: orange is what the diversion burns, red marks the final reserve.

4. A quick estimate: split the angle

Before you have a plotter in your hand, you can still get a track good enough to turn onto. Chart grid lines run at known angles: a meridian is 360° and a parallel is 090°. If the alternate lies between a meridian and a parallel, the track is between 360° and 090°. Halve that to 045° and ask whether the alternate is nearer to that line or to one of the edges. Halve again if you need to: 022° or 067°.

This gets you within about 10°. Turn onto it, then refine the track with the one-in-sixty rule as soon as you have time to measure.

5. Apply the wind, then check the clock and the fuel

Once you have the track and the distance, work like any other leg:

1. Take the true track to the alternate and apply the wind, as in the wind triangle lesson, to get the heading and the groundspeed.
2. Convert true to magnetic with the variation, then read the compass or the heading indicator.
3. Divide the distance by the groundspeed for the time. At 100 kt, one nautical mile takes 36 seconds.
4. Multiply the time by the fuel flow and compare it with the fuel on board. You must arrive with at least the final reserve, which for a day VFR flight in an aeroplane is 30 minutes at normal cruise under EASA rules.

If the answer is below the final reserve, the diversion is wrong, not the arithmetic. Go to a nearer aerodrome, or turn earlier next time.

6. Commit and tell someone

Turn onto the new heading first, then tell ATC. The usual order is aviate, navigate, communicate. Say what you are doing and why, give the new destination and your intentions, and ask for what you need: a frequency change, a transponder code, a service. If you are in trouble and the diversion is urgent, say so with the proper call. The radio lessons cover Mayday and Pan Pan.

If you are flying with a flight plan, the diversion should be reported to ATC or the aerodrome of arrival so that nobody starts a search for an aeroplane that is on the ground somewhere else.

7. Keep a position check going

A diversion goes wrong when you stop navigating. Note the time of the turn and write it down with the heading. Then pick the first feature you should cross, such as a railway, a river or a town, and the time you expect it. If it does not arrive when it should, stop and work out why before you fly further: check the heading indicator against the compass, check the wind, and use the one-in-sixty rule again from your actual position.

A diversion is a plan made in a hurry. Keep it simple, write the heading and the time on the chart, and fly it carefully. Not certified for navigation: use official charts and your instructor’s method.

8. A worked example

All the numbers are invented. You are on a track of 360° true at 100 kt TAS. The wind is from 270° at 15 kt. You decide to divert to an aerodrome 8 nm to the right of your track and 40 nm ahead. You have 40 litres and burn 30 litres an hour.

1. Angle off the track: 8 ÷ 40 × 60 = 12°, so the track is about 012° (the exact value is 011°). The distance is about 41 nm.
2. The wind is from the left of the new track, almost a full crosswind. Crosswind about 15 kt, so the correction is 15 ÷ 100 × 60 = 9° to the left. The heading is about 003°. Groundspeed is about 102 kt.
3. Time: 41 ÷ 102 × 60 = 24 minutes. Fuel: 24 minutes at 30 litres an hour is 12 litres.
4. You arrive with 40 − 12 = 28 litres. The final reserve is 15 litres (30 minutes at 30 litres an hour), so the diversion works with room to spare.

9. Try it yourself

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

1. You are tracking 090°. The alternate is 6 nm to the right of your track and 30 nm ahead. What heading change do you make by the one-in-sixty rule, and what is the new track?
   Show the answer

   Angle = across ÷ along × 60 = 6 ÷ 30 × 60 = 12°. The alternate is on the right, so turn right: the new track is 090 + 12 = 102°.

   The distance is a little more than 30 nm: about 31 nm.

2. The alternate is 15 nm to the left and 10 nm ahead. Why should you not trust one-in-sixty here, and what would you do?
   Show the answer

   The rule would give 15 ÷ 10 × 60 = 90°, but the true angle is about 56°. The angle is far beyond the 20° where the rule works.

   Measure the track on the chart with the plotter, or use the split-the-angle estimate: the alternate is between 360° and 270°, nearer the middle, so about 304° (the exact track is 304°).

3. You have 30 litres on board, burn 30 litres an hour, and the diversion is 36 nm at a groundspeed of 90 kt. Can you go?
   Show the answer

   Time = 36 ÷ 90 × 60 = 24 minutes. Fuel burned = 12 litres, so you arrive with 18 litres.

   The final reserve is 30 minutes at 30 litres an hour, which is 15 litres. You have 3 litres above it, so it works, but with little margin. Divert now, not later, and do not plan on anything going slower than the figures.

10. Check yourself

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

Back to the learning hub.