Saturday, 30 August 2008

PPL Ex 18 (x): NAVIGATION: (MAX DRIFT & Cross Wind Component XWC)

(Remember - Click on an image to ENLARGE.)


Q. What IS Maximum Drift?
A. It is the maximum drift possible with a given wind strength at a given TAS. The Drift Angle that the aircraft would experience IF the wind was directly ACROSS i.e. at 90-degrees to the Track.

Q. How is this useful to ME?
A. Having calculated the Maximum Drift, it is easy to calculate the Drift Angle for a given Wind Direction relative to the aircraft track.

Fraction of Max Drift: We only consider ANGLES OFF from 0 to 60 and this is done by using 1/6ths Rule:

0-degrees OFF = 0/6
10-degrees OFF = 1/6
20-degrees OFF = 2/6
30-degrees OFF = 3/6
40-degrees OFF = 4/6
50-degrees OFF = 5/6
60-degrees OFF = 6/6
Example 1:
True Air Speed (TAS) = 90kts.
Wind Speed = 27kts.
Max Drift = 60/90 x 27 = 18-degrees.

Assume Wind Direction relative to the Aircraft Track is 20-degrees OFF.
Therefore Drift Angle = 18-degrees X 2/6 = 6-degrees

Example 2:
True Air Speed (TAS) = 100kts.
Wind Speed = 40kts.
Max Drift = 60/100 x 40 = 24-degrees

Assume Wind Direction relative to the Aircraft Track is 30-degrees OFF.
Therefore Drift Angle = 24-degrees X 3/6 = 12-degrees.
Now apply the Drift Angle to the Track to get the Heading.

Example 3: (Illustrated on the 1:500,000 ICAO Aeronautical Chart above)
True Air Speed (TAS) = 90.
True Track = 030
W/V = 050/27
Max Drift = 18
Actual Drift = 6

The wind is blowing from the Right, therefore the Heading is 036-degrees.

Example 4:
True Air Speed = 100
True Track = 140
W/V = 350/40
Max Drift = 24
Actual Drift = 12

The wind is blowing from the Left, therefore the Heading is 128-degrees.

Cross Wind Component (XWC) :
MAX DRIFT - which is an 'angle' has been addressed above. Let's now have a look at CROSS WIND COMPONENT (XWC) which is a 90-degree vector:

The 1/6ths Rule above applies.

Assume that the aircraft that you are flying today has a 12 knot Cross Wind Limit (XWC). We will consider only winds up to 60-degrees OFF the runway (RW) Track:
0-degrees OFF RW = 0/6ths of Wind Strength
10-degrees OFF RW = 1/6th of Wind Strength
20 = 2/6
30 = 3/6.......etc to >>>> 60 = 6/6ths of Wind Strength

Example 1: If the Runway is RW23 [(say) 233-deg Magnetic = 230-deg True (Var 3-deg West)], and the Wind Vector is 260/22 i.e. 30-degrees OFF, then the XWC is 3/6 X 22 = 11kts.

Example 2: If the Runway is RW35 [(say) 348-deg Magnetic = 351-deg True (Var 3-deg West)], and the Wind Vector is 310/18 i.e. 40-degrees OFF, then the XWC is 4/6 X 18 = 12kts.

I think that is the MAX DRIFT & XWC dealt with!

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