Skid Plan

Following an increase in the number of incidents in which plans left the runway on landing due to reduced runway friction caused by bad weather in winter a Norwegian safety body has published a report with questions traditional thinking on how to prevent such accidents. Taking off and landing in wintry conditions is hazardous in a number of aspects but one of the major factors relating to accidents and serious incidents in such conditions is reduced runway friction. Due to the country’s geography and winter climate Norway’s airports have a broad experience of dealing with such conditions while the regulatory authorities have conducted detailed studies of the phenomenon. Last year the Accident Investigation Board Norway (AIBN) published a detailed study focusing on incidents and accidents involving runway surface friction.The recommendations in the report are far reaching and have a far wider application than Norway’s specific meteorological environment. Over a 10-year period the AIBN has received 30 reports of accidents and investigations related to operations on contaminated and slippery runways. During the same time-frime it has published 12 investigation reports of its own and issued 36 safety recommendations. In the 30 investigated occurences the AIBN found that the aircraft braking coefficent (ABC) was not matched with the measured or estimated runway friction coefficients (FC). It concluded that this and a number of factors relating to weather, runway threatment and friction measurements uncertainty were reducing safety margins. ‘The AIBN believes that incidents relating to slippery runways occur because the involved parties do not realise that existing rules and regulations are based on a simplification of the actual conditions.’ It adds that the measured or estimated friction values are used as “scientific truths” and do not take into account other meteorological conditions. ‘The safety margins are reduced by operational procedures which to a limited degree take into account the uncertainties connected to input parameters used for landing distance calculations,’ the report adds. Ice cold in Oslo The climate in Norway is clearly a major factor affecting runway friction although the report notes that climate changes over the last 10 years have increased problems with unpredictable weather as rising average temperatures during the winter months have increased the temperature fluctuations. Yet frequent snowfalls and frost combined with the difficulties in keeping the runways free from snow and ice have forced the operators to allow routine operations on contaminated runways. Most Norwegian airports are located along hilly coastline terrain which in winter months fluctuate between frost and thaw and are subject to turbulent wind conditions and strong crosswinds . Since 1999 the AIBN has received 30 report of incidents involving slippery runways during winter operations – roughly three a year. The majority of these events occurred at six airports – Svalbard, Harstad/Narvik, Tromso, OsloGardermoen, Bardufoss and Sandefjord/Torp. Of these nine were regarded as serious but the majority were less serious whereby the pilot was able to recover control of the aircraft quickly or the aircraft ran off the runway at low speed resulting in little or no damage to the plane or injury to passengers Internationally, runway excursions are on the increase and are the cause of many fatalities. According to the European safety body Nationaal Lucht -en Ruimtevaartlaboratorium (NLR) nearly 59 per cent of runway excursions occur on wet or contaminated runways. Other studies have shown that the risk of having a runway excursion are 13 times higher when landing on a wet or contaminated runway. In the face of these statistics the AIBN report has made public a number of findings that should contribute towards a re-assessment of what is considered good practice. The following is a list of the main findings from the report
Friction measurement uncertainty Moisture in combination with contaminated runway plays a more significant role in relation to ‘slipperiness’ than previously understood.

The difference between measured/estimated runway friction coefficients (FC) and airplane braking coefficients (ABC) is particularly great under certain meteorological conditions. Layered contaminants, wet and moist conditions, air temperature, dewpoint temperature, sanding and strong crosswinds are important factors. The correlation, when measured on ‘dry’ compact snow or ice, between measured friction coefficient (FC) and experienced airplane braking coefficient (ABC) is in the order of 0.5 of measured FC. On all other types of contaminations there is no consistent correlation.

▪ 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 improved friction measuring devices are reliable on all types of contaminations.

There is an apparent correlation between the observed meteorological conditions and runway slipperiness. The measured friction coefficient should be considered on the basis of temperature, dew point, precipitation and the history of these parameter values. These factors can be used as practical ‘Safety indicators’ for assessing runway friction. Runway Treatment

There has been limited scientific research and inadequate approval by the authorities concerning friction-improving means – both related to sanding and the use of chemicals. Sanding on wet and compact snow or ice, and sanding of loose layers of material in the form of slush, wet or dry snow on top of compact snow or ice, is not very effective. Friction measuring devices measure friction values that are too high when used on such surfaces. A challenge associated with the use of chemicals is that melting snow and ice results in wet and mixed contamination so that friction is reduced until the contaminant is fully melted. In addition water from melted snow and ice dilute the chemical liquid, so that it can freeze and form invisible ice (’black ice’).

Operational considerations
▪ The airport owner, pilots, airport staff and the CAA Norway, who approve the airlines’ and airports’ procedures, do not take into account the uncertainty attached to the use of friction measurements and estimation of friction on contaminated runways. Independent of the friction measuring device used, included in wet/moist conditions, measured friction values are reported, trusted and used to an accuracy of one hundredths (1/100). This is in conflict with AIP Norway AD 1.2 which 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 combined use of two very uncertain parameters (uncertain friction values stated in hundredths (1/100) and wind direction and wind force) when calculating landing distances by means of cockpit performance computers (CPCs) could cause aircraft to land in too strong crosswinds in relation to the available friction. The use of measured friction values and CPCs tends to give pilots a false feeling that they are using scientific data.

▪ In five of 30 incidents investigated by the AIBN, the aircraft crew based their landing calculations on the TWR’s instant wind speed readings (average 2-minute or 3 sec wind speed), which was more favourable for landing than the relevant METAR wind (average 10-minute wind) . During the landing, the actual wind was similar to the reported and stronger METAR wind. This resulted in loss of directional control. Instantaneous wind data should not be used for landing calculations, but should be monitored during the approach to ensure that the wind speed does not exceed the basis for the landing calculations.The report found that 19 of 30 investigated incidents occurred in conditions of crosswind in combination with slippery runways. Crosswind has a major impact on directional stability during the landing roll. The aircraft manufacturers have defined recommended crosswind limits which are not included in the basis for the certification of the respective aircraft.

The various aircraft manufacturers have different policies for operations on contaminated runways and therefore the airlines use different correlation curves/tables. In several instances the curves/tables have an uncertain basis and result in highly unreliable braking coefficients for the relevant type of aircraft. Boeing’s method, which is based on conservative use of airplane braking coefficients (ABC), provides the greatest safety margin compared with the methods of Bombardier and Airbus.

Regulatory conditions
ICAO and EASA’ guidelines include assumptions that are too optimistic and only to a limited degree founded on scientific evidence. International guidelines do not take into account the Norwegian climatic conditions. Norway should consider introducing national limitations for winter operations, just as USA, Canada and UK have done.

▪ Reverse thrust represents approximately 20 per cent of the total available braking force when braking on a slippery runway. The international guidelines for operation on contaminated runways are not in accordance with the strict requirements for certification of aircraft which are based on documented performance on dry runways without the use of thrust reversers. Nevertheless, 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 ICAO Safety Managemement Manual, gives advice regarding the development of national safety standards. In this respect ICAO recommends that each State define an ‘acceptable level of safety bassed on experience and knowledge gained from own investigations AIBN has concluded that the Norwegian climate and operating conditions requires adjustments to the general ICAO framework. Hence, Norway is required to establish national ALoS. Such a safety level should be based on a general safety analysis/assessment of routine operations on contaminated and slippery runways. A consequence from this may be that special measures must be taken in order to achieve ‘an equivalent level of safety’ as with ‘summer’ operations. The CAA Norway seems to lack an overall risk assessment of winter operations as part of the State Safety Program (SSP).

▪ The ICAO Airport Service Manual, on which the Norwegian rules relating to friction measurements, reporting and the use of friction data are based, is generelly outdated and not very appropriate as support for today’s 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 uncertainty in predicting the correct friction level is also applicable to the estimation of the friction category from 1 to 5 (in) the ICAO SNOWTAM format. The figures in the ICAO SNOWTAM table showing measured friction values are in hundredths (1/100) and are 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.

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

The AIBN findings show that the national regulations governing operations on contaminated and slippery runways are less strict than those that govern operations in summer conditions. This is in spite of the ICAO and EASA guidelines and regulations which prescribe that if winter operations are to be performed on a regular basis, the authorities require the operators to take special measures in order to attain an ‘equivalent level of safety’ to summer conditions. The many incidents and accidents relating to contaminated and slippery winter runways, reveal that an ’equivalent level of safety’ is not achieved in connection with Norwegian winter operations. The CAA Norway seems to lack an overall risk assessment quantifying the level of safety of winter operations as part of the State Safety Program SSP and establishment of an Acceptable Level of Safety (ALoS).

Safety Recommendations
The AIBN report issued the following Safety Recommendations:

▪ The CAA Norway carries out risk assessments and considers introducing national limitations of winter operations in order to ensure an ‘equivalent level of safety’.

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

▪ ICAO, FAA, EASA and CAA Norway consider revising the SNOWTAM table to reduce the degree of friction uncertainty.

▪ 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.

▪ 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.

▪ EASA considers a more conservative determination of friction values on various types and depths of contamination.

▪ 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.

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