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From the Norwegian AIP:
A numerical expression regarding the quality of the friction levels reported in the SNOWTAM can not be provided. Tests show that the accuracy indicated in the table can not be provided using todays friction measuring devices. While the table use numbers with two digits, the tests show that only numbers with one digit can be of operational value. Utmost caution should therefore be taken when using the reported friction levels, and the use of the table must be based upon the aircraft operators own experience.
In this context:
Friction measuring devices and acceptable conditions
2.6.1 The following friction measuring devices are accepted for use at Norwegian aerodromes:
GRT - Grip Tester
SFH - Surface Friction Tester, High pressure tyre
SKH - Skiddometer BV 11, High pressure tyre
RUN - Runar
VIN - Vertec Inspector
TAP - Tapleymeter
2.6.2 In general there is great uncertainty related to measurement carried out under wet conditions. The snow and ice is then at its melting point. For instance is TAP not accepted under wet conditions. Ref. is made to item 2.7 below for more information.
2.6.3 A measured friction level is associated with the measuring device and can not be used as an isolated number. The acceptable conditions for the measuring devices are:
SKH/SFH:
- Dry snow up to 25 mm.
- Dry compact snow - any thickness
- Dry ice - any thickness
- Slush up to 3 mm.
- Wet snow up to 3 mm.
- Wet ice.
GRT/RUN/VIN:
- Dry snow up to 25 mm.
- Dry compact snow - any thickness
- Dry ice - any thickness
- Slush up to 3 mm.
- Wet snow up to 3 mm.
TAP:
- Dry snow up to 5 mm.
- Dry compact snow - any thickness.
- Dry ice - any thickness.
2.7 SNOWTAM format item H
The table used under item H, with associated descriptions, was developed in the early 1950’s from friction data collected only on compact snow and ice. The friction levels should not be regarded as absolute values and they are generally not valid for other
surfaces than compact snow or ice. Nevertheless it is accepted that friction level may be reported when conditions with wet snow or slush up to 3 mm depth are present and a continuous measuring device is being used. A numerical expression regarding the quality of the friction levels reported in the SNOWTAM can not be provided. Tests show that the accuracy indicated in the table can not be provided using todays friction measuring devices. While the table use numbers with two digits, the tests show that
only numbers with one digit can be of operational value. Utmost caution should therefore be taken when using the reported friction levels, and the use of the table must be based upon the aircraft operators own experience.