Getting to Grips with Approach-and-Landing Accidents Reducti(166)

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Statistical Data
Adverse wind conditions (i.e., tail wind component, high crosswind component, wind gustiness or low level wind shear) are involved in more than 30 % of landing incidents and accidents.
Because wind sensors often are distant from the touchdown zone, wind conditions at touchdown frequently differ from conditions reported by ATC.
ICAO Standards
Recommendations for measuring and reporting wind information are defined by the International Civil Aviation Organization (ICAO) and:
.  
Relayed to the World Meteorological Organization (WMO);

.  
Implemented by the member states’ National Weather Services (NWS);

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Through the local Airport Weather Services (AWS).

Defining Average-Wind and Gust
Wind direction and velocity are sampled every second.
The wind profile is averaged over the last 2-minute period to provide the ATIS or tower-reported average-wind.
The average wind is available to the controller on a display terminal ( some control towers, however, can also provide instantaneous indications of wind direction and velocity ).
The wind profile is also observed over the last 10-minute period, the maximum ( peak ) wind value recorded during this period defines the gust value.
ICAO considers that the wind is gusty only if the 10-minute peak value exceeds the 2-minute average-wind by 10 kt or more, however gust values lower than 10 kt often are provided by airport weather services.
Figure 1 shows a 10-minute wind profile featuring:
.  
A 2-minute average wind of 15kt; and,

.  
A 10-minute gust of 10 kt (i.e., a 25 kt peak wind velocity during the 10-minute period).

Note: The ATIS and tower winds are referenced to the magnetic north (unless all airport directions are referenced to the true north, for example in regions with large magnetic variation).
What’s Your Current Wind ?
Page 1

Wind ( kt )
Gust
25
20

Average
15

Wind
10 5 0 012 3456 78910Minutes

Wind 15 kt Gusting 25 kt
Figure 1

If the wind peak value is observed during the last 2-minute period, the gust becomes part of the average wind, as illustrated by Figure 2.
Wind ( kt )
25

Average
Wind

20
15
10
5
0

012 3456 78910Minutes
Wind 20 kt Gusting 25 kt ( * )
Figure 2

( * ) : or no reference to gust if the 5-kt gust is not           accounted for.
Average-wind and gust values displayed to the controller are refreshed every minute.
The 2-minute average-wind and the 10-minute gust are used by ATC for:
.  
ATIS messages;

.  
Wind information on GND, TWR, APP and INFO frequencies.

METAR observation messages include a 10-minute average-wind and the 10-minute gust, as illustrated by Figure 3 ( XXX is the wind direction, referenced to the true north).
Wind ( kt )
Gust
25
20

Average Wind 15
10
5
0 0123 4567 8910

Minutes
METAR -Wind XXX15G23KT
Figure 3

Maximum Demonstrated Crosswind
The maximum demonstrated crosswind, published in the Performance section of the approved Airplane Flight Manual (AFM), is the maximum crosswind component that has been encountered and documented during certification flight tests or subsequently.
The wind value is recorded during a period bracketing the touchdown time ( typically from 100 ft above airfield elevation down to taxi speed ).
What’s Your Current Wind ?
Page 2

For some aircraft models, if a significant gust could be recorded during this period, a demonstrated gust value is also published in the AFM and FCOM.
The maximum demonstrated crosswind:
　
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