1.1 Dissemination of RWY State Information
SNOWTAMs and RWY reports are only provided for winter conditions and not for other hazardous RWY conditions e.g. RWY slippery when wet or combined with dust. Observations and measurements are not made at routine intervals. Information on RWY state and braking conditions as contained in SNOWTAM are available from ATC and are usually broadcasted on ATIS. When giving landing information, the thirds of the RWY are referred to as first, second, and third part. Other information e.g. state of TWY, apron etc. are added.
1.2 SNOWTAM
The SNOWTAM is a specialized NOTAM notifying the presence of hazardous RWY condition due to snow, ice, etc. by using a specified ICAO format. It is available on the NOTAM or at the AIS office as soon as the presence of contamination is considered to be operationally significant. The closure of a RWY for clearance and the subsequent reopening may be notified by a NOTAM.
⇒ Meteorology General Information 8.5 SNOWTAM Decoding
Issue/Validity of SNOWTAMs
A new SNOWTAM should normally be issued every 6 HRs. For aerodromes with no night operation or closed at night, a new SNOWTAM should be promulgated 2 HRs before the aerodrome is reopened for operation. The validity of SNOWTAMs is maximum 24 HRs. If the validity expires, a new observation, measurement should be made even if conditions have not changed and a new SNOWTAM should be issued.
1.3 METAR Runway Report
RWY reports are provided in the abbreviated form of 8-figure groups appended to weather reports (METAR). These reports are derived from the SNOWTAM. For obvious reasons e.g. traffic density the half hourly rhythm of METARs is not practicable for RWY condition observations. Thus a repetition of the previous RWY report may mean that no significant change has occurred. If an aerodrome is closed due to snow or removal of snow and ice, the term SNOCLO may replace the 8-figure code group.
⇒ Meteorology General Information 8.1.1 Runway Report Decoding
1.4 Issue/Validity of RWY Reports.
RWY reports are based on the same observation/measurements as the SNOWTAM and are repeated with every subsequent weather report until a new RWY report is made.
Assessment of Deposit Water on the RWY
Whenever water is present on RWYs description of the conditions on the center half of the RWY should be made using the following terms:
• DAMP – the surface shows a change of color due to moisture.
• WET – the surface is soaked but there is no standing water.
• WATER PATCHES – significant patches of standing water are visible.
• STANDING WATER – standing water of significant depth and area which affects significantly the braking action (aquaplaning).
• FLOODED – extensive standing water is visible during heavy rainfall.
Snow and Slush on the RWY
• DRY SNOW – snow which can be blown if loose or, if compacted by hand, will fall apart again upon release.
• WET SNOW – snow which, if compacted by hand will stick together and tend to form a snowball.
• SLUSH – water saturated snow which with a heel and toe slap down motion against the ground will be displaced with a splatter.
• COMPACTED SNOW – snow which has been compressed into a solid mass that resists further compression and will hold together or break up into lumps if picked up.
Combination of Ice, Snow, and Standing Water
Such combinations, especially when rain sleet or snow is falling, may have a rather transparent appearance due to their high water / ice content, shall be considered as slush. Accuracy of Depth of Deposit
When dry or wet snow or slush is present on a RWY, an assessment of the mean depth should be made to an accuracy of approximately 2CM for dry snow, 1CM for wet snow and 3MM for slush. The depth of deposit of compacted / solid contaminants such as ice, compacted snow or frozen ruts or ridges which do not cause a drag (retarding rolling) is not considered to be operationally significant and needs not to be assessed and reported. The depth of standing water is very difficult to assess. Hence it is often reported as “not measurable”.
⇒ Meteorology General Information 8.1.1 Runway Report Decoding
Assessment of Extent of Runway Coverage
The extent of the coverage of the RWY reported in percent is the best possible estimate but shall not be understood to be an accurate measurement.
1- Assessment of Braking Condition
Operation considerations require reliable and uniform reports on braking conditions whenever RWYs are contaminated and subsequent updating when conditions significantly change. The reported Friction Coefficient (FC) is a relative measure for the achievable friction of the tire to surface interface of a braked wheel and should be the maximum which occurs when a wheel is slipping but still rolling. Measurements should be made at a distance of 3m or the distance from the centerline where most operations take place. The reported Braking Action (BA) may reflect the FC converted to BA as well as subjective pilot reports or assessments by empirical methods. Therefore, for operation applications the FC is generally preferable to the BA which often lacks a proper standardized reference basis. If BA is reported it is recommended to request the FC if measured or, if the BA stems from pilot reports, to be informed on the aircraft type involved. The FC and especially the BA should not be taken as absolute due to the many methods of assessment used and the different variables such as aircraft weight, speed, landing technique, braking performance, tires, and undercarriage characteristics, etc.
2- Measuring Devices
All types of measuring equipment do not enable measurements with satisfactory reliability in aquaplaning conditions (standing water, slush, and wet snow) or if more than a thin layer of loose snow is present. In such cases the braking conditions should be reported as “unreliable”.
When considering Friction Coefficients (FC) of wet RWYs, especially with smooth surface, it should be noted that the friction may drop markedly with increasing speed.
Equipment Permitting Continuous Measurement. e.g:
• Surface Friction Tester (SFT)
• Skiddometer SKH/SKL (high/low pressure)
• Mu-Meter (MUM)
• Grip Tester (GRT)
• RWY Friction Tester (RFT)
can be used for friction values on compacted snow and ice covered RWYs.
Decelerometers, e.g. Tapley Meter (TAP) and Dynometer may be used on certain surface condition (compacted snow, ice, and very thin layers of dry snow). Decelerometers should not be used in loose snow or slush. Subjective methods permitting assessment of Braking Action (BA) only: Truck, car or pilot‘s report. The reliability of such reports is questionable.
3- Relationship Between Friction Coefficient (FC) and Braking Action (BA)
The table below was developed by ICAO based on measurements on ice and compacted snow. It states the relationship between FC and BA. It should not be taken to be absolute values and applicable in all conditions and should not be used to convert BA into FC. The relation between FC and empirically assessed BA is rather loose due to the lack of a proper standardized reference basis referring e.g. to the type of vehicle and its tires used, the pilot‘s experience, the aircraft type, and its performance in such conditions. Therefore the table should not be used to convert BA into FC.
For operational purposes:
Friction Coefficient Estimated Braking Action
0.40 and above GOOD
0.39 – 0.36 MEDIUM TO GOOD
0.35 – 0.30 MEDIUM
0.29 – 0.26 MEDIUM TO POOR
0.25 and below POOR
4- Differences from ICAO Standard
I.e. Canada and Russia apply procedures different from ICAO Standard. For further details refer to respective RSIs and CRARs.
Source: FCRG
See also: