Braking Action Reports (NIL)
OK here is what the FAA has to say about Braking Action Reports and Mu values.
"A precise correlation has not been established between the MU numbers provided by the friction measuring devices and an airplane's actual stopping performance. However, airplane manufacturers can typically provide additional data for operators to use as guidance in assessing the performance effects of slippery runways and for developing a policy for such operations. Further research is planned to investigate the correlation between the friction measuring device MU numbers and airplane stopping performance. In the interim, pilots should use the MU information together with other knowledge, including the operator's policy for slippery runway operations, recent pilot braking action reports, airplane performance characteristics, type, and weight, previous experience, and wind conditions for determining runway suitability. A report of MU 40 is commonly understood to mean that runway friction is good, but that should not be misunderstood to mean that braking action will be as good as on a clean, dry pavement. It is generally agreed that a friction report of MU 25 or less means that an aircraft may experience directional control and/or braking difficulties. Although ICAO has published a table comparing friction values to subjective terms, there is no recognized or appropriate engineering correlation between MU values and the subjective pilot braking descriptors of "good, "fair," "poor," and "nil." " So let me set up a scenario.... Single runway airport Part 121 air carrier reports braking action NIL at 0800 local which basically closes the airport to any air carriers until a subsequent report comes out. Two hours later the airport puts out an ATIS advertising a Mu value of .29 which indicates a braking action of POOR according to the ICAO table. So when would you depart for your destination? And when could you do so legally? The main confusion relates to the air carrier reporting NIL braking action, do you have to wait until someone else decides to break the reg and land anyway? |
The FAA statement:
Although ICAO has published a table comparing friction values to subjective terms, there is no recognized or appropriate engineering correlation between MU values and the subjective pilot braking descriptors of "good, "fair," "poor," and "nil." " ...appears fairly meaningless. The ICAO table has been around for years and represents a well accepted correlation between subjective and objective assessments. It is certainly 'recognised' by any normal definition. For your scenario you have to rely on your operations manual. If you are prohibited from planning to land at a destination that has poor or medium to poor (0.29) braking action expected at your time of landing thank the foresight of your predecessors. If not, you're a bit more on your own. As an example of the regulatory confusion you might encounter the UK CAA have an AIC extant which states that pilots should avoid operating on contaminated runways wherever possible which conflicts rather awkwardly with JAA guidance which allows ops from contaminated runways. I have to say, I'm with the AIC. Contaminated runway ops are extremely dangerous. |
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