The Wind Effect on vectoring 

(Rules include both ACC and APP considerations)

 

The purpose of the present article is to inform young controllers on the factors affecting vectoring under wind and help them with some very simple rules to correct for the wind effect . We assume that basic principles and terms are already known by the reader .Some standard situations are considered which will provide the basics for a further deeper understanding if properly examined . 

The "Wind Ratio"

 The wind effect is not as directly related as one should expect with the wind intensity itself but by the relative speed of the aircraft – that includes performance at certain altitude of a specific type . It is the relation between wind speed and aircraft speed that counts more . For example a wind of say 50 knots is almost unimportant for a B744 at high levels ( say FL 370 ) where this aircraft would normally run at a speed of 500 Kt . You may say that the ratio of (wind speed / aircraft speed) = 10% . During the final approach phase the same aircraft reduces down to around 200 Kt and then the same ratio becomes 200/500 = 40% and the wind effect becomes then extremely important .

Take a AT72 at FL 250 where the wind is say 40 Kt . For a B744 it is practically still a no wind case while at the same conditions the AT72 has a ratio of (wind speed / aircraft speed) = 15% which can not be ignored

So it is the Ratio = (wind speed / aircraft speed)  that makes the difference . This ratio tells us if the wind effect has to be considered or not , on the first place . 

What values of Wind Ratio may be considered ? 

Well here it is necessary to focus on practical examples . Use your reason . If you talk for a speed of wind that compares with ˝ or 1/3 of the speed of the aircraft itself you face an extreme case of wind effect . At high altitudes jet aircraft making 450-500 Kt may not even be able to stand a wind of 200-300 Kt . They will not even be able to continue their flight and they will probably ask for a large level change to avoid those ‘jet streams’ . Accordingly during the approach phase an aircraft making 160-200 Kt would not even think of a wind of say 70-100 Kt . Such a ground wind is similar to a typhoon , producing extreme turbulence . In both cases we are talking about an unusual situation . 

What values are then more reasonable ? By experience and for any practical purpose we classify the ratio according to the following table :

  

Ratio = (wind speed / aircraft speed)

Classification

0.25

Very Strong

0.20

Strong

0.15

Important

  

The Drift Angle

It is the drift angle imposed to the pilot on keeping a track that is important to vectoring since we deal with turns and headings . We will study then what wind directions and at what Wind Ratio , affect the drift . 

 

  

 

Ratio Wind-Aircraft speed

 

Very Strong

Strong

Important

 

0,25

0,2

0,15

Wind angle

Drift

Drift

Drift

10

14

11

8

20

13

11

8

30

12

10

7

40

11

9

7

50

9

7

6

60

7

6

4

70

5

4

3

80

2

2

1

90

0

0

0

 The table indicates that :

 

1.       Since the headings given are chosen from multiples of 5 , if the drift angle is <= 5 degrees the wind effect itself is not clearly important for the vectoring

2.       The directions ‘hitting’ the aircraft more ‘head-on’ or ‘tail’ between 60-90 degrees laterally are not  significant for the drift at any ratio

3.       The maximum drift angle for Very Strong lateral wind is 15 degrees , in all other cases ranges between 7-10 degrees

4.      The drift angle changes with heading changes at a rate of about : 3 degrees for every 20 degrees of heading in Very Strong wind , 2  for the Strong and 1 for the Important . This is the most important observation to help us corrections

 How to detect the wind component

There is a simple answer : Ask the pilot what is his present heading and then compare the track he does with what he told you . The result is the Drift Angle . Pilots using an FMS can directly tell you the drift angle itself . Bearing in mind the previous table we can end up with some very simple rules :

 If the Drift Angle ( = Difference between Track observed and Heading reported ) is :

 How do we include a wind correction to our headings for separation ? 

There are 2 cases here : 

  1. 'Short Vectors' :  +/- 20,30 degrees from present track , a typical case for normal track separations both in ACC and APP

  2. 'Large Vectors' : between 50-90 degrees away from present track . It is typical for APP maneuvers and positioning for the ILS and ...avoiding actions both for ACC,APP 

Let us consider the 'Short vectors' case : 

By simply :

The rounding up with the 5 degrees is enough for all wind ratios !  Just look at our table : For any change of only +/- 20,30 degrees , the change in  drift is in the area of 3-4 degrees . Have a look ! Even in the strongest case the wind angle at 40 degrees produces a drift of 11 while if it becomes 10 degrees it becomes 14 . That is for 30 degrees the correction due to drift needed is just 3 degrees !

The only thing to consider is during a very strong wind to rather round up against the wind 

 

See an example here :

 

Some examples with the simulator

 

The B734 (EIN3344) is experiencing a wind of 060/60 and is on a track of 149 ( Track magnetic ) on a TAS of 437 Kt . The simulator indicates that the pilot is on a heading of 141 (Compass heading )  .  The drift is 8 degrees . Wind is coming from the left . In this case the ratio is 060/437=0.15 , an inportant wind and our table does predict exactly for a full lateral wind an 8 degrees drift !! You may notice that the ground speed is 432 which for a lateral wind differs only by 5 Kt in 437 , that is less than 1.5% 

What will be the effect on vectoring ? 

Say you aim at a lateral deviation of about 5 NM to the left at about 30 NM from a point . The rule '1 in 60' predicts that in 60 NM you need 5 degrees of vectoring angle for 5 NM deviation but 10 for 30 . If you use the reported heading of 141 and you subtract 10 for the deviation , without wind this is practically 130 degrees . However you need to adjust for another 5 degrees against the wind so more to the left or 130-5 = 125 degrees . In the picture below the thin line represents the heading and the thick the track while you read at the end of the heading the value you need for the deviation and the distance to that point ( = 125 degrees in 33 NM ) 

 

On the following picture the wind is 340/60  ,the wind is practically on the tail , it is only the ground speed that makes a difference and not the drift .  This increases the ground speed to 494 , a very high value for a B734  but the associated drift is merely 4 degrees (< 5 ) .

Let us consider the 'Large vectors' case : 

The turns here are well away from the present track and the vectoring angles may even be considered  up to 90 degrees ! In such a case the correction must be full because we turn the aircraft in a totally different situation and the drift correction of the pilot has no longer a meaning , it is the controller who will estimate  now all the drift ! He has to realise whether the wind is lateral or not . We again have a look at the table . Remember the maximum deviation ? 

In the case of very strong wind we use the 15 degrees in all other cases the 10 degrees , which have to added to the track we want the pilot to fly , disregarding his own present heading as the drift on present track is here useless as a value ....but ...we suppose we do know where the wind is coming from . The later is easy for the APP as the wind indication is constantly displayed in front of the controller but in the ACC if the controller is not aware of it already , he will have to ask for the present heading to understand it . Is it now a head wind or a lateral one ? Corrections are needed only on the lateral case

See some examples here :

 

 

Some examples with the simulator

In the picture below an AT42 (BAW403) turns on to downwind (heading 240) for RWY06 . The wind is 080/040 . The aircraft speed is about 200 Kt and as compared with 40 Kt the Wind Ratio is 'Very Strong' . Yet , the wind is practically on the tail (wind angle = 20 dgrees) and despite the high ratio , the drift is few degrees < 5 , actually 3 . So turning on to downwind wind corrections are not important . You may assign heading 240  

When , however , turning to base the wind is all lateral and since the ratio is 'Very Strong' our study suggests a drift of 15 degrees . The simulator gives 13 !! An amazing accuracy for a 'rule-of-thumb' !  Check :  the Compass heading = 150 , the Track = 163 . 

When the final heading for the ILS interception is given to make a track of 090 ( for a 30 degrees interception angle) then the wind becomes 'head-wind' which , like the tail one, will not contribute to drift despite the strong wind . So your heading may just be equal to the track of 090 for interception . 

In actual fact , as you see in the picture below , there is a drift of 3 degrees as Compass heading =090 and Track = 093 , the same as with the downwind ...but ...not to worry with your vectors . 

You may notice that the ground speed (Gnd Spd) falls to practically 131 Kt with a TAS of 171 ( or better say indicated 160 ) . This means that on the ILS with strong head-wind , as expected , the speed is immediately reduced .  

 

 

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