Calculating Crosswinds & Headwinds

The Clock Code: a quick and easy way to calculate headwind and crosswind components

The Clock Code is a practical method used to mentally calculate headwinds, tailwinds and crosswinds for:

Take-off and landing

In-flight diversions

Holding patterns

Any other time you need to quickly and easily calculate a crosswind or headwind/tailwind component!

Calculating Crosswinds & Headwinds

When the wind is at an angle (other than 90°) to the aircraft’s heading, there will be both a headwind/tailwind component and a crosswind component. We can use something known as the ‘clock code‘ to help with this. In this guide, we’ll use examples to explain how to:

Calculate Your Crosswind

Calculate Your Drift Angle

Calculate Your Headwind or Tailwind

1. Calculating Your Crosswind

You are taking off from runway 09 and the surface wind is 045° at 20 knots. What will the crosswind be for your departure and is it below your aircraft’s maximum demonstrated crosswind component of 15 knots?

1 First, find the angle between the wind and your heading

In this case, the wind is 45° left of your nose.

2 Next, use the ‘clock code’. Take this number of degrees and think of it as minutes on a clock:

15 is ¼ of an hour, so when the wind is 15° off your heading, the crosswind is ¼ of the wind strength

30 is ½ of an hour, so when the wind is 30° off your heading, the crosswind is ½ of the wind strength

45 is ¾ of an hour, so when the wind is 45° off your heading, the crosswind is ¾ of the wind strength

60 is a full hour, so when the wind is 60° or more off your heading, the crosswind is equal to the full wind strength

Today, the wind is 45° left of your nose, so the crosswind is ¾ of the wind strength.

3 In this case, the wind strength is 20 knots so the crosswind component is 15 knots

The crosswind for your departure will be 15 knots – equal to this aircraft’s maximum demonstrated crosswind component.

2. Calculating Your Drift Angle

The clock code can also be used to calculate your drift (i.e. how far you will be blown ‘off course’ by the wind). Let’s use the same example to see how the clock code can be used to calculate the drift:

After taking off from runway 09, you will be flying on a heading of 090° with a true airspeed (TAS) of 90 knots. The wind is 045/20. What drift will you experience?

1 First, find the angle between the wind and your heading

In this case it is 45°

2 Next, determine the max drift for this TAS and wind speed

See our article on Maximum Drift for more information on this

3 Again, use the clock code to determine how much of the max drift needs to be applied

15 is ¼ of an hour, so when the wind is 15° off your heading, your drift will be ¼ of the max drift

30 is ½ of an hour, so when the wind is 30° off your heading, your drift will be ½ of the max drift

45 is ¾ of an hour, so when the wind is 45° off your heading, your drift will be ¾ of the max drift

60 is a full hour, so when the wind is 60° or more off your heading, your drift will be equal to the max drift

Today, the wind is 45° off your heading. 45 is ¾ of an hour so your drift will be ¾ of the max drift:

Max Drift = 13°
Drift Experienced = ¾ of 13° = 10° (approx.)

3. Calculating Your Headwind or Tailwind

Your headwind or tailwind can also be calculated with the clock code. This can then be used to work out your ground speed by adding or subtracting the headwind or tailwind from your true airspeed.

This calculation can be particularly critical when taking off or landing with a tailwind. A tailwind that increases your ground speed by 10% will cause your take-off and landing distances to increase by 20%!

If the wind is directly on your nose or tail, the headwind/tailwind component will be the full wind speed.

If the wind is 90° to your nose, it will be all crosswind and our groundspeed will not change.

For any angle in between, the ‘clock code’ method can be used. However, this time we need to see how far the angle differs from a direct crosswind. Let’s look at an example:

You are flying on a heading of 320° with a true airspeed (TAS) of 90 knots. The wind is 170/20. What is your groundspeed?

1 Calculate the shortest angle between a direct crosswind and the actual wind:

Your heading is 320° so a direct crosswind would be 230° (320° – 90°)

The angle between 230° (a direct crosswind) and 170° is 60°

2 Use the clock code to determine how much of the wind is headwind or tailwind

The wind is 60° off a direct crosswind, so the headwind/tailwind component is the full 20 knot wind speed.

From the diagram, we can see the wind is coming from behind the aircraft, so we have a 20 knot tailwind.

3 Add the tailwind to or subtract the headwind from the true airspeed (TAS) to get the groundspeed

TAS = 90 knots
Tailwind = 20 knots
Groundspeed = 90 + 20 = 110 knots

To remind yourself that you need to find how much the wind differs from a direct crosswind, it may help you to remember that it can be either a headwind or a tailwind – the only way to know which is by starting in the middle!

Interested in other rules of thumb like the clock code? Check out Skybrary’s article on aviation Rules of Thumb: https://skybrary.aero/articles/rules-thumb