Braking Action Reports (NIL)

[Abovo]

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?

[Alex Whittingham]

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.