Winter Operations, Friction Measurements and Conditions for Friction Predictions

The ICAO Airport Services Manual is generally outdated and not very appropriate as support for todays winter operations. The manual should describe in more detail the newer types of friction measuring devices, the limitations that apply to measurement on moist contamination, requirements for sand, sand application, requirements for de-ice and anti-ice chemicals and the use of chemicals, and updated information on expected friction on different types and depths of contamination.

The AIBN recommends that ICAO initiate an updating and revision of the Airport Services Manual on the basis of the results of investigations of runway excursions and recent research findings.

EASA?s certification requirements use default friction values for various contaminants, irrespective of temperature and dew point, and permit conversion between various types and depths of contamination on the basis of ?water equivalent depth? (WED) using a speed-based formula. EASA?s certification requirements are optimistic and not in accordance with the findings of the AIBN?s investigations.

The AIBN recommends that EASA considers a more conservative determination of friction values on various types and depths of contamination.

In 19 of 30 incidents, conditions of slippery runways combined with crosswind were present. The aircraft manufacturers have defined recommended crosswind limits which are not included in the basis for certification of the respective aircrafts. Transport Canada?s table of crosswind versus friction values is far more conservative than the tables used by Norwegian airlines.

The AIBN recommends that FAA, EASA and CAA Norway evaluate the airlines? crosswind limits in relation to friction values and consider whether they should be subject to separate approval by the authorities.

Reverse thrust represents approximately 20 % of the total available braking force when braking on a slippery runway. Operations on contaminated runways are permitted on the basis of „advisory? (not „certified?) friction data and the use of thrust reversers. EASA has regulated that consideration of engine failure during landing should be considered, but this is not adhered to. Hence, the extra safety margin that the reverse thrust would constitute is not available.

The AIBN recommends that FAA, EASA and CAA Norway consider, on the basis of risk assessments, whether all available reverse thrust should continue to be included in part or in whole when calculating the required landing distance on contaminated and slippery runways.

The figures in the ICAO SNOWTAM table showing measured friction values are in hundredths (1/100) and independent of the type of friction measuring device that is used. AIP Norway describes the use of friction measuring devices in general and warns that the measurements are associated with such a high degree of uncertainty that the figures should not be reported to more than one decimal place (one tenth, 1/10). The figures from the SNOWTAM table are used in flight operations through the airlines? individual correlation curves/tables which further increases the uncertainty.

Based on the above, the AIBN recommends that ICAO, FAA, EASA and CAA Norway consider revising the
SNOWTAM table to reduce the degree of friction uncertainty.

In the investigated occurrences, the AIBN found that the aircraft braking coefficients were not in accordance with the measured and reported values. Validity ranges for friction measuring devices lack the necessary scientific basis. The various types of friction measuring devices measure different friction values when used on the same surface. None of the internationally approved friction measurements devices are reliable on all types of contaminations. In particular, moisture and less than 3 K dew point spread and loose/layered contaminations increase the friction measurement uncertainty.

The AIBN recommends that ICAO, FAA, EASA and CAA Norway review and validate the permitted measuring (validity) ranges for approved friction measuring devices.

The many incidents and accidents relating to contaminated and slippery winter runways, reveal that an ?equivalent level of safety? to summer conditions is not maintained in connection with Norwegian winter operations. The CAA Norway seems to lack an overall risk assessment of winter operations as part of the State Safety Program (SSP) and establishment of an Acceptable Level of Safety (ALoS).

The AIBN recommends that the CAA Norway carries out risk assessments and considers introducing national limitations of winter operations in order to ensure an „equivalent level of safety?.