ICAO Annex 3 (Met) provides guidance on the format for METAR (Appx 3). A METAR may provide runway state under supplementary information. This is reported as either friction coefficient or braking action, expressed as nn or //.
The numerical format appears to be as used in a SNOWTAM – ICAO Annex 15 (AIS), where runway friction is reported in either a two digit or single digit format (measured/calculated vs estimated friction; Good - Poor, 5 - 1). The decode for this is in Appx 2, or ICAO Annex 14 (Aero) Vol 1, att A, para 6.
Thus it is possible for METAR “braking action” - estimated friction (single digit), to originate from a PIREP, but as noted in reports below, ICAO does not define braking action or its origin, which adds to any subjective inaccuracy of a PIREP.
A two digit report would indicate measured friction, but this also is subject to many inaccuracies.
The ICAO Cir 329 ‘Runway Surface Condition Assessment, Measurement and Reporting’ (
http://www.iata.org/iata/RERR-toolki..._Reporting.pdf) provides the most recent guidance, which aims to improve understanding of surface friction characteristics and their contribution to landing safety. Recommendations as to measurement and reporting will be considered by ICAO.
In particular, note the significant misgivings about operational friction measurement which more often does not relate to ‘braking action’.
In parallel with the ICAO work, the FAA Aviation Rulemaking Committee (ARC) working on Takeoff and Landing Performance Assessment (TALPA) has made several recommendations (
www.skybrary.aero/bookshelf/books/1508.pdf).
Note the move away from subjective assessment of braking action (PIREPS) and cautions about friction measurements.
Re the minimum friction standard for a wet runway (i.e. runway maintenance; surface deterioration - #8)
The recommendation is to reducing the effective braking action for a wet runway from “good” to “medium” when the runway is designated as “slippery when wet.”