Guidance for aircraft operators relating to winter operations
Aircraft operators are reminded that it is their responsibility to ensure that the correct data is used by flight crew. Furthermore service providers should be included in the operator’s quality assurance programme and the performance data production process audited as necessary.
Flight crews should be aware that in changing winter conditions, performance calculations carried out at the planning stage may no longer be appropriate at the time of intended use. Further calculation based on the latest prevailing conditions may be needed.
Movement areas covered by slush, snow, water and ice create additional hazards. Typical reported incidents include vehicles colliding with parked aeroplanes, tugs sliding during pushback and aeroplanes encountering control difficulties during taxi. Operators should make their flight crews’ aware of these potential hazards and crews should report any hazardous conditions encountered to the aerodrome operator via Air Traffic Control (ATC).
UK AIP AD 1.2.2 Snow Plan requires runway conditions to be reported every 30 minutes for as long as such conditions prevail, catering for a scenario of changing weather. In addition, the AIP requires the contaminant depth and type measurements to be carried out every 30 minutes. However, flight crew should be extremely cautious in changing conditions. Snow depth can increase quickly and in typical UK conditions a slight thaw can also turn wet snow rapidly to slush. Flight crews should use the most adverse report available in such conditions.
The provision of performance information for contaminated runways should not be taken as implying that ground handling characteristics on these surfaces will be as good as can be achieved on dry or wet runways (in particular, in crosswinds when using reverse thrust).
A runway is considered to be contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width being used is covered by the following:
(i) surface water more than 3 mm (0.125 in) deep, or by slush, or loose snow;
(ii) snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up (compacted snow); or
(iii) ice, including wet ice.
Where an aerodrome operator is unable to clear contaminants from a runway, the depth and type of contaminant remaining on the runway surface will be reported.
Continuous Friction Measuring Equipment (CFME) does not provide a reliable correlation between the readings off a contaminated runway and aeroplane braking performance. Performance calculations must not be based on such readings.
Braking action will not be measured or reported at UK licensed aerodromes on contaminated runways except on those runways covered in compacted snow and ice as described in the UK AIP AD 1.2.2 Snow Plan. Under these circumstances CFME readings may be used for performance calculations.
Aerodromes are required to conduct periodic surveys of runway surface friction characteristics using calibrated equipment operated within the manufacturer’s technical criteria. If a survey indicates that the runway surface friction characteristics have deteriorated below a specified Minimum Friction Level, that runway will be notified via NOTAM as ‘may be slippery when wet’. Aerodrome operators are encouraged to be specific with regards to the location and size of any areas of reduced grip to help enable operators better calculate performance limitations. Braking action is assumed to be poor on a wet runway that is notified as one that may be slippery when wet. The term slippery should not be confused with the term icy. Runways that are dry or wet with less than 3 mm of water will normally provide good braking.
The ‘3 Kelvin Spread Rule’ is not an absolute rule but may be used as an indicator that runway surface conditions might be more slippery than anticipated. The rule states that at air temperatures of +3°C and below, with a dew point spread of 3°C or less, the runway surface condition may be more slippery than anticipated on snow and ice. The narrow dew point spread indicates that the air mass is relatively close to saturation which is often associated with actual precipitation, intermittent precipitation, nearby precipitation or fog. The validity of the rule may depend on its correlation with precipitation but it may also, at least in part, depend on the exchange of water at the air-ice interface. The rule was observed in 21 out of the 30 incidents related to braking action on ice and snow investigated by Accident Investigation Board Norway (AIBN). Due to the other variables involved (such as surface temperature, solar heating and ground cooling or heating) a small spread does not always mean that the braking action will be poor. The rule may be used as an indicator of slippery conditions but not as an absolute. When these conditions exist it may be appropriate to factor the landing distance.