Norwegian investigators believe that winter weather measurements give a false impression of accuracy, and that this misunderstanding contributes to incidents and accidents on slippery runways.

Assessment of 30 such events, nine of them serious, in a period of 10 years has led the accident investigation agency SHT to conclude that those involved "do not realise" that regulations are "based on a simplification of the actual physical conditions".

Runway friction values, in particular, are "used as scientific truths" but are not compared with other meteorological conditions, and uncertainty is not sufficiently taken into account.

In the 30 occurrences examined, the SHT found that the aircraft-braking coefficient was not in line with measured or estimated runway friction coefficients, and that discrepancies were particularly wide under certain weather conditions.

It added that friction-measuring devices are not reliable for all types of runway contamination, while moisture plays a "more significant" role than has previously been understood. Uncertainty over friction measurement is greater under humid conditions, particularly when the difference between air temperature and dew point is less than 3°C.

Friction values are also "reported, trusted and used" to a higher accuracy than that recommended by Norway's aeronautical information publication, the SHT said.

Combining uncertain friction data with similar inaccuracies in wind data - particularly if instantaneous wind readings, rather than averages, are used - can "give pilots a false feeling that they are using scientific data".

Nineteen of the 30 incidents occurred in crosswind conditions in combination with a slippery runway.

The SHT also highlights possible hazards associated with runway treatment, pointing out that sanding can be ineffective and result in misleading friction data. Norway should consider following other countries by introducing national limitations for winter operations, it said.

The agency said that regular winter operations require an equivalent level of safety to summer operations, but that the number of incidents indicates this is not being achieved.

"National regulations governing operations on contaminated and slippery runways are less strict than those that govern operations in summer conditions," it added.

Norway's civil aviation administration said it will "read the report thoroughly with a special view on its conclusions and recommendations", adding: "We will then consider any necessary measures."

Source: Flight International