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The Max Drift Navigation Technique

We can get the forecast wind from the Met Office and ATC but how can we apply it to our tracks to find initial headings to fly without using a computer? We can use the worst-case drift for the heading based on the wind and the 1 in 60 Rule – this is the basic Maximum Drift Technique. To explain how we do this read on..

Well if we were flying at 60 knots we would cover 60 nautical miles in an hour wouldn’t we? If the wind were 10 kts we would be blown 10 miles in an hour. Therefore, if we flew 90 degrees to the wind we would have 10 degrees drift as we would be 10 miles downwind and we would have covered 60 miles (1 in 60 gives 10º drift). This figure is our maximum drift because a wind at less than 90º off the track reduces the crosswind component and therefore the drift - it becoming a head or tail wind component.

What about ac speed?

If we were flying at 90 kts we would travel 1.5 times the 1 in 60 benchmark distance so our drift angle would be reduced (10 divided by 1.5 equals 7º give or take). It is easier to convert your speed to Nm per minute.

We then apply the very simple calculation of:

Why max drift, well because the wind is all across and can only diminish in strength, head or tail components, and therefore we know that our calculated heading will put us up-wind.

The wind is rarely on the beam, so what next?

We could fly the whole route using maximum drift but we should improve the accuracy by factoring the drift for each track or leg. We can work out the drift applicable to each track if we know the angle of the wind off the beam, ie the amount of the drift, head and tail wind components for each leg.

Trigonometry dictates that between 60 and 90º the sin of an angle approximates 1 (60º = 0.87). At 45º ¾ (0.7), 30º ½ (0.5) and finally at 15º ¼ (0.26).

If we relate this to the clock code we have an easy way to remember how much of the max drift value to apply on a leg.
15º = quarter past therefore ¼ the drift, 30º = half past therefore half the drift 45º ¼ to therefore ¾ the drift, 60º or more off apply all the drift.

For example:

Clever so far, don’t be put off by the approximations after all how accurate is the wind forecast at the height you are flying and how accurate is your compass/DI or flying?

Timing with Max Drift Technique

The easiest way to run timing on a leg planned max drift is to plan still air times.  At planned "fix" points a third or a quarter of the way along the leg make an accumulative correction ie:

If the first fix is a quarter of the way down the route and expected at 5:30 in still air, then if we arrive at the fix at 6 mins we are 30 seconds late after ¼ of the leg.  Therefore you would be 1 min late at halfway, 1:30 secs at ¾ and 2 minutes at the end of the leg.

However, you can help your flying by making a guestimate of the ground speed during planning by using simple maths and the clock code once again.  If the wind was from the 12 o’clock then ground speed = airspeed – wind speed.

If the wind is from the beam then the head or tail wind effect is zero.  Therefore, using the clock code we can work from the beam and factor the head or tail component. Eg

The Max Drift technique and the groundspeed rules based on trigonometry and the clock code are workable approximations for navigation.  Given that we are limited in the visibilities we fly VFR, the errors are far less than the limits of the VFR visibility and so, provided you look out, you will see the turning point/destination/fix.