LHR Friction Analysis

Project summary

The resurfacing of the southern runway at London Heathrow Airport in 2013 offered an opportunity to gather data about a consistently difficult physical behaviour to quantify and assess – runway friction. During the nightly closures, the airport’s operations team carried out friction runs over the full length of the runway encompassing: ungrooved and newly laid Marshall asphalt, recently grooved Marshall asphalt and existing wearing course. The aim of the assessment was to ascertain whether there was consistent and measurable friction performance on the various runway surfaces; if volatile compounds from newly laid asphalt had a detrimental friction performance; and whether any relaxation could be applied to the established and time-consuming practice of multiple runs over the full runway length.

Scope of assessment

Friction testing is highly dependent on the behavioural routine of the operator undertaking the runs with inevitable variances in driving style, calibration, equipment set-up and logging of the data. The initial activities consisted of base-lining the supplied information into a pre-determined format before beginning any analysis of the data itself. This baselining took the mean value of the initial recorded figures for each monthly period and normalising the data sets to a common starting point. The resultant graphs had discernible trends with drops in friction noticeable at runway chainages associated with the touchdown points in particular. The data also showed observable dips in regions that had been recently laid with Marshall asphalt. Further examination of these areas at a later date showed friction values had recovered once the surface course had been grooved.

Ungrooved asphalt section seen on approach

Localised reduction in friction (centre)

While these were expected observations, what was of most interest was to what extent the friction values dropped to. This is particularly important to an airport operator in that should a 100m rolling average of the friction values recorded fall below the Minimum Friction Level (MFL) then the airport was obliged to NOTAM that region as “may be slippery when wet”. A further statistical analysis of the 100m rolling average over the entire runway length showed that the probability of obtaining a value below the MFL was in the region of 1%.

Benefit

The investigation showed that noticeable drops in runway friction values could be expected in regions with newly laid and ungrooved wearing course. The data showed however that despite this, the likelihood of a 100m rolling average falling below the MFL was minimal. The report also showed that for the future northern runway project, consideration could be given to reducing the airport’s established practice of carrying out time-consuming multiple friction runs per night over the full paved length provided the material specification and mix design remained largely the same.